cryptol/docs/ProgrammingCryptol/crashCourse/CrashCourse.tex

\commentout{
\begin{code}
  module CrashCourse where
\end{code}
}

%#####################################################################
\chapter{A Crash Course in Cryptol}
\label{cha:crash-course-cryptol}

Before we can delve into cryptography, we have to get familiar with
Cryptol.  This chapter provides an introduction to Cryptol, just to
get you started. The exposition is not meant to be comprehensive, but
rather as an overview to give you a feel of the most important tools
available.  If a particular topic appears hard to approach, feel free
to skim it over for future reference.

A full language reference is beyond the scope of this document at this
time.
% The language features not covered in this document at this time
% include:
% \begin{itemize}
% \item feature one
% \item feature two
% \end{itemize}
% \todo[inline]{2.1: Add list here of language features not yet
%   covered.}

% TODO The full grammar for Cryptol is included
% in~\autoref{cha:cryptol-grammar}.

%=====================================================================
\section{Basic data types}
\label{sec:basic-data-types}

Cryptol provides five basic data types: bits, sequences, integers,
tuples, and records. Words (i.e., $n$-bit numbers) are a special case
of sequences. Note that, aside from bits and integers, all other
Cryptol types can be nested as deep as you like. That is, we can have
records of sequences containing tuples made up of other records, etc.,
giving us a rich type-system for precisely describing the shapes of
data our programs manipulate.

While Cryptol is statically typed, it uses type inference to supply
unspecified types.  That is, the user does {\em not} have to write the
types of all expressions; they will be automatically inferred by the
type-inference engine.  Of course, in certain contexts the user might
choose to supply a type explicitly.  The notation is simple: we simply
put the expression, followed by {\tt :} and the type. For instance,
\begin{Verbatim}
   12 : [8]
\end{Verbatim}
means the value {\tt 12} has type {\tt [8]}, i.e., it is an 8-bit
word. We can also supply partial types. For example,
\begin{Verbatim}
   12 : [_]
\end{Verbatim}
means that \texttt{12} has a word type with an unspecified number of
bits. Cryptol will infer the unspecified part of the type in this
case. We shall see many other examples of typed expressions in the
following discussion.

%=====================================================================
\sectionWithAnswers{Bits: Booleans}{sec:bits}

The type {\tt Bit}\indTheBitType represents a single bit of
information. There are precisely two values of this type: {\tt
  True}\indTrue and {\tt False}\indFalse. Bit values play an important
role in Cryptol, as we shall see in detail shortly. In particular, the
test expression in an {\tt if-then-else} statement must have the type
{\tt Bit}.  The logical operators {\tt \&\&}\indAnd (and), {\tt
  ||}\indOr (or), {\tt \Verb|^|}\indXOr (xor), and {\tt
  \Verb|~|}\indComplement (complement) provide the basic operators
that act on bit values.

\begin{Exercise}\label{ex:dataBit}
  Type in the following expressions at the Cryptol prompt, and observe
  the output:
\begin{Verbatim}
  True
  false
  False : Bit
  if True && False then 0x3 else 0x4
  False || True
  (True && False) ^ True
  ~False
  ~(False || True)
\end{Verbatim}
Remember that Cryptol is case sensitive, and hence {\tt false} is
different from {\tt False}.\indCaseSensitive
\end{Exercise}
\begin{Answer}\ansref{ex:dataBit}
Here is the response from Cryptol, in order:
\begin{small}
\begin{Verbatim}
  True
  [error] at <interactive>:1:1--1:6 Value not in scope: false
  False
  0x4
  True
  True
  True
  False
\end{Verbatim}
\end{small}
\end{Answer}

\begin{tip}
  Cryptol provides extensive command line/tab completion; use
  up/down-arrow to see your previous commands, hit tab to complete
  identifier names, etc.
\end{tip}

%=====================================================================
\sectionWithAnswers{Words: Numbers}{sec:words}

A word is simply a numeric value with some number of bits,
corresponding to the usual notion of binary numbers.\indTheWordType To
match our observation of how cryptographers use binary numbers,
Cryptol only supports non-negative ($\geq 0$) word values (i.e., no
floating point or negative numbers).\indFloatingPoint However, words
can be arbitrarily large: There is no predefined maximum number of
bits that we are limited to.\indArbitraryPrecision The type of $n$-bit
words is written \texttt{[n]}; e.g. \texttt{[8]} is the 8-bit word type.

By default, Cryptol prints words in base 16. You might find it useful
to set the output base to be 10 while working on the following
example. To do so, use the command:\indSettingBase
\begin{Verbatim}
  :set base=10
\end{Verbatim}
The supported values for this setting are 2 (binary), 8 (octal), 10
(decimal), and 16 (hexadecimal).  Conversely, we can \emph{write}
numbers in these bases in Cryptol programs too:
\begin{Verbatim}
  0b11111011110    // binary
  0o3736           // octal
  2014             // decimal
  0x7de            // hexadecimal
\end{Verbatim}

Decimal numbers pose a problem in a bit-precise language like Cryptol.
Numbers represented in a base that is a power of two unambiguously
specify the number of bits required to store each digit. For example
\texttt{0b101} takes three bits to store, so its type is \texttt{[3]}.
A hexadecimal digit takes 4 bits to store, so \texttt{0xabc} needs 12
bits, and its type is \texttt{[12]}. On the other hand, a decimal
digit could require anywhere from 1 to 4 bits to represent, so the
number of digits does not determine the type. Decimal numbers may
assume any of a variety of types in Cryptol; that is, they are
\emph{polymorphic}.\indPolymorphism For example, \texttt{19} could
have the unbounded type \texttt{Integer} (see~\autoref{sec:integer}),
type \texttt{[8]}, type \texttt{[5]}, or any other word type with at
least 5 bits.

\paragraph*{Defaulting and explicit types}\indDefaulting
Polymorphic values in a Cryptol program are subject to
\emph{defaulting}, meaning that Cryptol will choose specific types to
represent them. Cryptol usually prints a message whenever defaulting
occurs, showing which type was chosen. For numbers, Cryptol usually
chooses either \texttt{Integer} or the word type with the
\emph{smallest} number of bits required to represent them. Users can
override this by giving an explicit type signature.\indSignature

\todo[inline]{2.1: Make decision about
  \href{https://www.galois.com/cryptol/ticket/217}{ticket \#217} for
  the suppression of bitwidth assumption messages.}
%% If you get tired of these messages, you can suppress them with the
%% \texttt{:set reportAssuming=off} command.  Alternatively, you can
%% help Cryptol avoid from having to assume anything by using binary,
%% octal, or hexadecimal constants exclusively.  Many cryptographic
%% algorithms use decimal for their constants, but it is important to
%% keep track of how many bits they need, so the current behavior was
%% deemed a good compromise.

\begin{Exercise}\label{ex:decimalType}
  Try writing some decimal numbers at different types, like
  \texttt{12:[4]} and \texttt{50:[8]}. How does Cryptol respond if
  you write \texttt{19:[4]}, and why? What is the largest decimal
  number you can write at type \texttt{[8]}? What is the smallest
  allowable bit size for \texttt{32}? What is the smallest allowable
  bit size for \texttt{0}?
\end{Exercise}
\begin{Answer}\ansref{ex:decimalType}
  Cryptol prints the error ``\texttt{Unsolvable constraint: 4 >= 5}'',
  because \texttt{19} requires at least 5 bits to represent.
  \texttt{255:[8]} is the largest value of type \texttt{[8]}.
  \texttt{[6]} is the smallest type for \texttt{32}. \texttt{[0]} is
  the smallest type for \texttt{0}.
\end{Answer}

\begin{Exercise}\label{ex:setBase}
  Experiment with different output bases by issuing {\tt :set
    base=10}, and other base values. Also try writing numbers directly
  in different bases at the command line, such as {\tt 0o1237}.  Feel
  free to try other bases.  What is the hexadecimal version of the
  octal number {\tt 0o7756677263570015}?
\end{Exercise}
\begin{Answer}\ansref{ex:setBase}
{\tt 0xfeedfacef00d}.
\end{Answer}

\note{We will revisit the notion of words in \autoref{sec:words2},
  after we learn about sequences.}

%=====================================================================
\sectionWithAnswers{Integers: Unbounded numbers}{sec:integer}

The type \texttt{Integer} represents mathematical integers of
unlimited size. To write an integer value in Cryptol, simply write the
number in decimal, optionally annotated with \texttt{:Integer} to
disambiguate the type. Numbers written in base 2, 8, or 16 are always
$n$-bit words, and never have type \texttt{Integer}. However, they can
be converted to unbounded integers using \texttt{toInteger}. For
example, \texttt{toInteger 0xff} represents the integer 255. To write
a negative integer, simply negate the result: For example,
\texttt{-12} and \texttt{- toInteger 0xc} represent the integer $-12$.

\begin{Exercise}\label{ex:int:1}
  Compare the results of evaluating the expressions \texttt{0x8 + 0x9}
  and \texttt{toInteger 0x8 + toInteger 0x9}. Also try evaluating
  \texttt{toInteger (- 0x9)}. Can you explain the results?
\end{Exercise}
\begin{Answer}\ansref{ex:int:1}
Here are Cryptol's responses:
\begin{Verbatim}
  0x1
  17
  11
\end{Verbatim}
\texttt{0x8 + 0x9} evaluates to \texttt{0x1} because addition on 4-bit
words wraps around when the sum is 16 or more and it overflows. On the
other hand, \texttt{Integer} addition never overflows, so the answer
is 17. The final result is also due to wraparound: \texttt{- 0x5}
evaluates to \texttt{0xb} or 11, which is $-5 + 16$.
\end{Answer}

%=====================================================================
% \section{Tuples: Heterogeneous collections}
% \label{sec:tuple}
\sectionWithAnswers{Tuples: Heterogeneous collections}{sec:tuple}

A tuple is a simple ordered collection of arbitrary values with
arbitrary types.\indTheTupleType Tuples are written with parentheses
surrounding two or more comma-separated elements. Cryptol also
supports a special zero-element tuple, written
\texttt{()}.\footnote{There is no syntax for tuples with exactly one
  element. Writing parentheses around a single value does not make a
  tuple; it denotes the same value with the same type.}

Tuple types have syntax similar to tuple values. They are written with
parentheses around two or more comma-separated types. Values of a
tuple type are tuples of the same length, where the value in each
position has the corresponding type. For example, type \texttt{([8],
  Bit)} comprises pairs whose first element is an 8-bit word, and whose
second element is a bit. The empty tuple \texttt{()} is the only value
of type \texttt{()}.

\begin{Exercise}\label{ex:tup:1}
Try out the following tuples:
\begin{Verbatim}
  (1, 2+4)
  (True, False, True ^ False)
  ((1, 2), False, (3-1, (4, True)))
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:tup:1}
Here are Cryptol's responses:
\begin{Verbatim}
  (1, 6)
  (True, False, True)
  ((1, 2), False, (2, (4, True)))
\end{Verbatim}
\end{Answer}

\todo[inline]{Reflect upon these computed expressions.  What is the
  operational semantics of computation of constant-valued arithmetic
  expressions?  There seems to be differing behavior between
  object-level and type-level expressions, but also differing
  semantics at the object level in different contexts.}

\paragraph*{Projecting values from tuples} Use a \texttt{.} followed by
$n$ to project the $n+1$-th component of a tuple. Nested projections
are also supported. Note that projections cannot be used with the
empty tuple, as it has no components to project.

\begin{Exercise}\label{ex:tup:2}
Try out the following examples:
\begin{Verbatim}
  (1, 2+4).0
  (1, 2+4).1
  ((1, 2), False, (3-1, (4, True))).2.1
\end{Verbatim}
Write a nested projection to extract the value \texttt{True} from the
expression:
\begin{Verbatim}
  ((1, 2), (2, 6, (True, 4)), False)
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:tup:2}
Here are Cryptol's responses:
\begin{Verbatim}
  Cryptol> (1, 2+4).0
  1
  Cryptol> (1, 2+4).1
  6
  Cryptol> ((1, 2), False, (3-1, (4, True))).2.1
  (4, True)
\end{Verbatim}
The required expression would be:
\begin{Verbatim}
  ((1, 2), (2, 6, (True, 4)), False).1.2.0
\end{Verbatim}
\end{Answer}

\begin{tip}
  While tuple projections can come in handy, we rarely see them used
  in Cryptol programs. As we shall see later, Cryptol's powerful
  pattern-matching mechanism provides a much nicer and usable
  alternative for extracting parts of tuples and other composite data
  values.
\end{tip}

%=====================================================================
% \section{Sequences: Homogeneous collections}
% \label{sec:sequences}
\sectionWithAnswers{Sequences: Homogeneous collections}{sec:sequences}

While tuples contain heterogeneous data, sequences are used for
homogeneous collections of values, akin to value arrays in more
traditional languages.  A sequence contains elements of any
\emph{single} type, even sequences themselves, arbitrarily nested.  We
simply write a sequence by enclosing it within square brackets with
comma-separated elements.\indTheSequenceType

\todo[inline]{Mention sequence types, syntax}

\begin{Exercise}\label{ex:seq:1}
Try out the following sequences:
\begin{Verbatim}
  [1, 2]
  [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
\end{Verbatim}
Note how the latter example can be used as the representation of a
$3\times3$ matrix.
\end{Exercise}

\begin{tip}
  The most important thing to remember about a sequence is that its
  elements must be of exactly the same type.
\end{tip}

\begin{Exercise}\label{ex:seq:2}
  Type in the following expressions to Cryptol and observe the
  type errors:
\begin{Verbatim}
  [True, [True]]
  [[1, 2, 3], [4, 5]]
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:seq:2}
In each case we get a type error:
\begin{Verbatim}
  Cryptol> [True, [True]]
  [error] at <interactive>:1:8--1:14:
    Type mismatch:
      Expected type: Bit
      Inferred type: [1]
  Cryptol> [[1, 2, 3], [4, 5]]
  [error] at <interactive>:1:13--1:19:
    Type mismatch:
      Expected type: 3
      Inferred type: 2
\end{Verbatim}
In the first case, we are trying to put a bit (\texttt{True}) and a
singleton sequence containing a bit (\texttt{[True]}) in the same
sequence, which have different types. In the second case, we are
trying to put two sequences of different lengths into a sequence,
which again breaks the homogeneity requirement.
\end{Answer}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Enumerations}\indEnum
\label{sec:enumerations}

Cryptol enumerations allow us to write sequences more compactly,
instead of listing the elements individually.  An enumeration is a
means of writing a sequence by providing a (possibly infinite) range.
Cryptol enumerations are not equivalent to mainstream programming
languages' notions of enumeration types, other than both kinds of
constructs guarantee that enumeration elements are distinct.

\begin{Exercise}\label{ex:seq:3}
  Explore various ways of constructing enumerations in Cryptol, by
  using the following expressions:
\begin{Verbatim}
  [1 .. 10]       // increment with step 1
  [1, 3 .. 10]    // increment with step 2 (= 3-1)
  [10, 9 .. 1]    // decrement with step 1 (= 10-9)
  [10, 9 .. 20]   // decrement with step 1 (= 10-9)
  [10, 7 .. 1]    // decrement with step 3 (= 10-7)
  [10, 11 .. 1]   // increment with step 1
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:seq:3}
Here are the responses from Cryptol:
\begin{Verbatim}
  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  [1, 3, 5, 7, 9]
  [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
  []
  [10, 7, 4, 1]
  []
\end{Verbatim}
Note how \texttt{[10, 11 ..\ 1]} and \texttt{[10, 9 ..\ 20]} give us empty
sequences, since the upper bound is smaller than the lower bound in
the former, and larger in the latter.
\end{Answer}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Comprehensions}\indComp
\label{sec:comprehensions}

A Cryptol comprehension is a way of programmatically computing the
elements of a new sequence, out of the elements of existing
ones.\indComp  The syntax is reminiscent of the set comprehension
notation from ordinary mathematics, generalized to cover parallel
branches (as explained in the exercises below).  Note that Cryptol
comprehensions are not generalized numeric comprehensions (like
summation, product, maximum, or minimum), though such comprehensions
can certainly be defined using Cryptol comprehensions.

\begin{Exercise}\label{ex:seq:4}
  The components of a Cryptol sequence comprehension are an expression
  of one or more variables (which defines each element of the
  sequence), followed by one or more \emph{branches}, each preceded by
  a vertical bar, which define how the variables' values are
  generated. A comprehension with a single branch is called a
  \emph{cartesian comprehension}. We can have one or more components
  in a cartesian comprehension. Experiment with the following
  expressions:\indComp\indCartesian
\begin{Verbatim}
   [ (x, y) | x <- [1 .. 3], y <- [4, 5] ]
   [ x + y  | x <- [1 .. 3], y <- [] ]
   [ (x + y, z) | x <- [1, 2], y <- [1], z <- [3, 4] ]
\end{Verbatim}
What is the number of elements in the resulting sequence, with respect
to the sizes of components?

\end{Exercise}
\begin{Answer}\ansref{ex:seq:4}
Here are the responses from Cryptol:
\begin{Verbatim}
  [(1, 4) (1, 5) (2, 4) (2, 5) (3, 4) (3, 5)]
  []
  [(2, 3) (2, 4) (3, 3) (3, 4)]
\end{Verbatim}
The size of the result will be the sizes of the components
multiplied. For instance, in the first example, the generator
\texttt{x <- [1 ..\ 3]} assigns 3 values to \texttt{x}, and the generator
\texttt{y <- [4, 5]} assigns 2 values to \texttt{y}; and hence the result has
$2\times 3 = 6$ elements.
\end{Answer}

\begin{Exercise}\label{ex:seq:5}\indParallel\indComp
  A comprehension with multiple branchess is called a \emph{parallel
    comprehension}. We can have any number of parallel branches. The
  contents of each branch will be \emph{zipped} to obtain the results.
  Experiment with the following expressions:
\begin{Verbatim}
   [ (x, y) | x <- [1 .. 3] | y <- [4, 5] ]
   [ x + y  | x <- [1 .. 3] | y <- [] ]
   [ (x + y, z)  | x <- [1, 2] | y <- [1] | z <- [3, 4] ]
\end{Verbatim}
What is the number of elements in the resulting sequence, with respect
to the sizes of the parallel branches?
\end{Exercise}
\begin{Answer}\ansref{ex:seq:5}
Here are the responses from Cryptol:
\begin{Verbatim}
  [(1, 4) (2, 5)]
  []
  [(2, 3)]
\end{Verbatim}
In this case, the size of the result will be the minimum of the
component sizes. For the first example, the generator
\texttt{x <- [1 ..\ 3]} assigns 3 values to \texttt{x}, and the generator
\texttt{y <- [4, 5]} assigns 2 values to \texttt{y}; and hence the result has
$\min(2,3) = 2$ elements.
\end{Answer}

\begin{tip}
  One can mix parallel and cartesian comprehensions, where each
  parallel branch can contain multiple cartesian
  generators.\indComp\indCartesian\indParallel
\end{tip}

\begin{tip}
  While Cryptol comprehensions \emph{look} like standard mathematical
  comprehensions, one must remember that the codomain of Cryptol
  comprehensions is a sequence type of some kind, \emph{not} a set.
\end{tip}

Comprehensions may be nested.\indNestedComp In this pattern, the
element value expression of the outer nesting is a sequence
comprehension (which may refer to values generated by the outer
generator). The pattern looks like this:

\begin{minipage}{\textwidth}  %% trying to avoid splitting this across pages
\begin{Verbatim}
[  [ <expr with x & y>  //                   \
     | y <- [1 .. 5]    // inner generator    -- outer
   ]                                         /  elements
   | x <- [1 .. 5]      // outer generator
]
\end{Verbatim}
\end{minipage}

\begin{Exercise}\label{ex:seq:6}
  Use a nested comprehension to write an expression to produce a
  $3\times3$ matrix (as a sequence of sequences), such that the $ij$th
  entry contains the value {\tt (i, j)}.
\end{Exercise}
\begin{Answer}\ansref{ex:seq:6}
  Here is one way of writing such an expression, laid out in multiple
  lines to show the structure:
\begin{Verbatim}
  [ [ (i, j) | j <- [1 .. 3] ]
     | i <- [1 .. 3]
  ]
  produces:
  [[(1, 1), (1, 2), (1, 3)],
   [(2, 1), (2, 2), (2, 3)],
   [(3, 1), (3, 2), (3, 3)]]
\end{Verbatim}
The outer comprehension is a comprehension (and hence is nested). In
particular the expression is:
\begin{Verbatim}
     [ (i, j) | j <- [1 .. 3] ]
\end{Verbatim}
You can enter the whole expression in Cryptol all in one line, or
recall that you can put \texttt{\textbackslash} at line ends to continue to
the next line.\indLineCont If you are writing such an expression in a
program file, then you can lay it out as shown above or however most
makes sense to you.
\end{Answer}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Appending and indexing}
\label{sec:appending-indexing}

For sequences, the two basic operations are appending\indAppend
(\texttt{\#}) and selecting\indIndex elements out (\texttt{@},
\texttt{@@}, \texttt{!}, and \texttt{!!}). The forward selection
operator (\texttt{@}) starts counting from the beginning, while the
backward selection operator\indRIndex (\texttt{!}) starts from the
end. Indexing always starts at zero: that is, \texttt{xs @ 0} is the
first element of \texttt{xs}, while \texttt{xs !\ 0} is the last. The
permutation\indIndexs versions\indRIndexs (\texttt{@@} and
\texttt{!!}, respectively) allow us to concisely select multiple
elements: they allow us to extract elements in any order (which makes
them very useful for permuting sequences).

\todo[inline]{Cite other languages that express permutations like this?}

\begin{Exercise}\label{ex:seq:7}
Try out the following Cryptol expressions:
\begin{Verbatim}
  [] # [1, 2]
  [1, 2] # []
  [1 .. 5] # [3, 6, 8]
  [0 .. 9] @ 0
  [0 .. 9] @ 5
  [0 .. 9] @ 10
  [0 .. 9] @@ [3, 4]
  [0 .. 9] @@ []
  [0 .. 9] @@ [9, 12]
  [0 .. 9] @@ [9, 8 .. 0]
  [0 .. 9] ! 0
  [0 .. 9] ! 3
  [0 .. 9] !! [3, 6]
  [0 .. 9] !! [0 .. 9]
  [0 .. 9] ! 12
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:seq:7}
Here are Cryptol's responses:
\begin{Verbatim}
  Cryptol> :set warnDefaulting=off
  Cryptol> [] # [1, 2]
  [1, 2]
  Cryptol> [1, 2] # []
  [1, 2]
  Cryptol> [1 .. 5] # [3, 6, 8]
  [1, 2, 3, 4, 5, 3, 6, 8]
  Cryptol> [0 .. 9] @ 0
  0
  Cryptol> [0 .. 9] @ 5
  5
  Cryptol> [0 .. 9] @ 10
  invalid sequence index: 10
  Cryptol> [0 .. 9] @@ [3, 4]
  [3, 4]
  Cryptol> [0 .. 9] @@ []
  []
  Cryptol> [0 .. 9] @@ [9, 12]
  invalid sequence index: 12
  Cryptol> [0 .. 9] @@ [9, 8 .. 0]
  [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
  Cryptol> [0 .. 9] ! 0
  9
  Cryptol> [0 .. 9] ! 3
  6
  Cryptol> [0 .. 9] !! [3, 6]
  [6, 3]
  Cryptol> [0 .. 9] !! [0 .. 9]
  [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
  Cryptol> [0 .. 9] ! 12
  invalid sequence index: 12
\end{Verbatim}
\end{Answer}

\begin{Exercise}\label{ex:seq:8}
  The permutation operators ({\tt @@} and {\tt !!}) can be defined
  using sequence comprehensions.  Write an expression that selects the
  even indexed elements out of the sequence \texttt{[0 ..\ 10]} first
  using {\tt @@}, and then using a sequence comprehension.
\end{Exercise}
\begin{Answer}\ansref{ex:seq:8}
Using a permutation operator, we can simply write:
\begin{Verbatim}
  [0 .. 10] @@ [0, 2 .. 10]
\end{Verbatim}
Using a comprehension, we can express the same idea using:
\begin{Verbatim}
  [ [0 .. 10] @ i | i <- [0, 2 .. 10] ]
\end{Verbatim}
Strictly speaking, permutation operations are indeed redundant.
However, they lead to more concise and easier-to-read expressions.
\end{Answer}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Finite and infinite sequences}\indFiniteSeq\indInfSeq
\label{sec:finite-infin-sequ}

So far we have only seen finite sequences. An infinite sequence is one
that has an infinite number of elements, corresponding to
streams.\indStream Cryptol supports infinite sequences, where the
elements are accessed \emph{on demand}. This implies that Cryptol will
\emph{not} go into an infinite loop just because you have created an
infinite sequence: it will lazily construct the sequence and make its
elements available as demanded by the program.

\begin{Exercise}\label{ex:seq:9}
Try the following infinite enumerations:
\begin{Verbatim}
  [1:[32] ...]
  [1:[32], 3 ...]
  [1:[32] ...] @ 2000
  [1:[32], 3 ...] @@ [300, 500, 700]
  [100, 102 ...]
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:seq:9}
  When you type in an infinite sequence, Cryptol will only print the
  first 5 elements of it and will indicate that it is an infinite value
  by putting $\ldots$ at the end.\footnote{You can change this behavior
    by setting the {\tt infLength} variable, like so: {\tt Cryptol>
      :set infLength=10} will show the first 10 elements of infinite
    sequences} Here are the responses:
\begin{Verbatim}
  [1, 2, 3, 4, 5, ...]
  [1, 3, 5, 7, 9, ...]
  2001
  [601, 1001, 1401]
  [100, 102, 104, 106, 108, ...]
\end{Verbatim}
\end{Answer}
\note{We are explicitly telling Cryptol to use 32-bit words
  as the elements. The reason for doing so will become clear when we
  study arithmetic shortly.}
\begin{Exercise}\label{ex:seq:10}
  What happens if you use the reverse index operator ({\tt !}) on an
  infinite sequence? Why?
\end{Exercise}
\begin{Answer}\ansref{ex:seq:10}
Here is a simple test case:
\begin{Verbatim}
  Cryptol> [1:[32] ...] ! 3

  [error] at <interactive>:1:1--1:17:
    Expected a finite type, but found `inf`.
\end{Verbatim}
The error message is telling us that we \emph{cannot} apply the reverse
index operator ({\tt !}) on an infinite sequence (\texttt{inf}).  This
is a natural consequence of the fact that one can never reach the end
of an infinite sequence to count backwards.  It is important to
emphasize that this is a \emph{type error}, i.e., the user gets this
message at compile time; instead of Cryptol going into an infinite
loop to reach the end of an infinite sequence.
\end{Answer}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Manipulating sequences}
\label{sec:manip-sequ}

Sequences are at the heart of Cryptol, and there are a number of
built-in functions for manipulating them in various ways.  It is
worthwhile to try the following exercises to gain basic familiarity
with the basic operations.

\begin{Exercise}\label{ex:seq:11}
Try the following expressions:\indTake\indDrop\indSplit\indGroup\indJoin\indTranspose
\begin{Verbatim}
  take`{3} [1 .. 12]
  drop`{3} [1 .. 12]
  split`{3} [1 .. 12]
  groupBy`{3} [1 .. 12]
  join [[1 .. 4], [5 .. 8], [9 .. 12]]
  join [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
  transpose [[1, 2, 3, 4], [5, 6, 7, 8]]
  transpose [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
\end{Verbatim}
And for fun, think about what these should produce:
\begin{Verbatim}
  join [1,1]
  transpose [1,2]
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:seq:11}
Here are Cryptol's responses:
\begin{Verbatim}
  Cryptol> take`{3} [1 .. 12]
  [1, 2, 3]
  Cryptol> drop`{3} [1 .. 12]
  [4, 5, 6, 7, 8, 9, 10, 11, 12]
  Cryptol> split`{3}[1 .. 12]
  [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
  Cryptol> groupBy`{3} [1 .. 12]
  [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
  Cryptol> join [[1 .. 4], [5 .. 8], [9 .. 12]]
  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
  Cryptol> join [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
  Cryptol> transpose [[1, 2, 3, 4], [5, 6, 7, 8]]
  [[1, 5], [2, 6], [3, 7], [4, 8]]
  Cryptol> transpose [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
  [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
\end{Verbatim}
And for the fun bonus problems:
\begin{Verbatim}
  Cryptol> join [1,1]
  Showing a specific instance of polymorphic result:
    * Using '1' for type argument 'each' of 'Cryptol::join'
  3
  Cryptol> transpose [1,2]
  Showing a specific instance of polymorphic result:
    * Using '2' for type argument 'cols' of 'Cryptol::transpose'
  [1, 2]
\end{Verbatim}
The argument to \texttt{join} must be a sequence of sequences, so each
\texttt{1} must have a bitvector type. Cryptol defaults to the
smallest possible type, \texttt{[1]}. So \texttt{join [1,1]} becomes
\texttt{join [[True], [True]]}, which equals \texttt{[True, True]}, or
\texttt{3}.

With \texttt{transpose}, Cryptol similarly chooses the smallest
possible bitvector type \texttt{[2]} for the sequence elements. Then
\texttt{[1,2]} becomes \texttt{[[False, True], [True, False]]}, which
transposes to the same value.
\end{Answer}

\begin{Exercise}\label{ex:seq:12}
  Based on your intuitions from the previous exercise, derive laws
  between the following pairs of functions: {\tt take} and {\tt drop};
  {\tt join} and {\tt split}; {\tt join} and {\tt groupBy}; {\tt
    split} and {\tt groupBy} and {\tt transpose} and itself.  For
  instance, {\tt take} and {\tt drop} satisfy the following
  equality:\indTake\indDrop\indJoin\indSplit\indGroup\indTranspose
\begin{Verbatim}
   (take`{n} xs) # (drop`{n} xs) == xs
\end{Verbatim}
whenever {\tt n} is between {\tt 0} and the length of the sequence
{\tt xs}. Note that there might be multiple laws these functions
satisfy.
\end{Exercise}
\begin{Answer}\ansref{ex:seq:12}
  The following equalities are the simplest
  candidates:\indJoin\indSplit\indGroup\indTranspose
\begin{Verbatim}
  join (split`{parts=n} xs) == xs
  join (groupBy`{each=n} xs) == xs
  split`{parts=n} xs == groupBy`{each=m} xs
  transpose (transpose xs) == xs
\end{Verbatim}
In the first two equalities {\tt n} must be a divisor of the length of
the sequence {\tt xs}.  In the third equation, {\tt n} $\times$ {\tt m}
must equal the length of the sequence {\tt xs}.
\todo[inline]{These are theorems that should be part of the prelude!}
\end{Answer}

\begin{Exercise}\label{ex:seq:13}
What is the relationship between the append operator {\tt \#} and {\tt
  join}?\indAppend\indJoin
\end{Exercise}
\begin{Answer}\ansref{ex:seq:13}
Append ({\tt \#})\indAppend\indJoin joins two sequences of arbitrary
length, while {\tt join} appends a sequence of equal length
sequences. In particular, the equality:
\begin{Verbatim}
   join [xs0, xs1, xs2, .. xsN] = xs0 # xs1 # xs2 ... # xsN
\end{Verbatim}
holds for all equal length sequences {\tt xs0}, {\tt xs1}, $\ldots$,
{\tt xsN}.
\end{Answer}

\paragraph*{Type-directed splits} The Cryptol primitive function {\tt
  split}\indSplit splits a sequence into any number of equal-length
parts. An explicit result type is often used with {\tt split}, since
the number of parts and the number of elements in each part are not
given as arguments, but are determined by the type of the argument
sequence and the result context.
\begin{Verbatim}
  Cryptol> split [1..12] : [1][12][8]
  [[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]]
  Cryptol> split [1..12] : [2][6][8]
  [[1, 2, 3, 4, 5, 6], [7, 8, 9, 10, 11, 12]]
  Cryptol> split [1..12] : [3][4][8]
  [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
\end{Verbatim}
Here is what happens if we do {\em not} give an explicit signature on
the result:\indSignature
\begin{Verbatim}
  Cryptol> split [1..12]
  Cannot evaluate polymorphic value.
  Type: {n, m, a} (n * m == 12, Literal 12 a, fin m) => [n][m]a
  Cryptol> :t split [1..12]
  split [1 .. 12] : {n, m, a} (n * m == 12, Literal 12 a, fin m) =>
                      [n][m]a
\end{Verbatim}
%% cryptol 1 said: : {a b c} (fin c,c >= 4,a*b == 12) => [a][b][c]

A complex type signature like this one first defines a set of type
variables \verb|{n, m, a}|, a set of constraints on those variables
\verb|(n * m == 12, Literal 12 a, fin m)|, a \texttt{=>} and finally
the shape description. In this case, Cryptol's \texttt{[n][m]a} is
telling us that the result will be a sequence of \texttt{n} things,
each of which is a sequence of \texttt{m} things, each of which is a
value of type \texttt{a}. The \texttt{Literal} constraint tells us
that \texttt{a} is a type that can represent numbers (at least) up to
12. The other constraints are that \texttt{n * m == 12}, which means
we should completely cover the entire input, and that the length
\texttt{m} needs to be finite. As you can see, \texttt{split} is a
very powerful function. The flexibility afforded by \texttt{split}
comes in very handy in Cryptol. We shall see one example of its usage
later in \autoref{sec:scytale}.

\begin{Exercise}\label{ex:split:0}
  With a sequence of length 12, as in the above example, there are
  precisely 6 ways of splitting it: 1 $\times$ 12, 2 $\times$ 6, 3
  $\times$ 4, 4 $\times$ 3, 6 $\times$ 2, and 12 $\times$ 1. We have
  seen the first three splits above. Write the expressions
  corresponding to the latter three.\indSplit
\end{Exercise}
\begin{Answer}\ansref{ex:split:0}
Here they are:\indSplit
\begin{Verbatim}
  Cryptol> split [1..12] : [4][3][8]
  [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
  Cryptol> split [1..12] : [6][2][8]
  [[1, 2], [3, 4], [5, 6], [7, 8], [9, 10], [11, 12]]
  Cryptol> split [1..12] : [12][1][8]
  [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]
\end{Verbatim}
\end{Answer}

\begin{Exercise}\label{ex:split:1}
  What happens when you type
  \texttt{split [1 ..\ 12] :\ [5][2][8]}?\indSplit
\end{Exercise}
\begin{Answer}\ansref{ex:split:1}
Cryptol will issue a type error:\indSplit
\begin{Verbatim}
  Cryptol> split [1..12] : [5][2][8]
  [error] at <interactive>:1:7--1:14:
    Type mismatch:
      Expected type: 10
      Inferred type: 12
\end{Verbatim}
Cryptol is telling us that we have requested 10 elements in the final
result (5 $\times$ 2), but the input has 12.
\end{Answer}

\begin{Exercise}\label{ex:split:2}
  Write a \texttt{split} expression to turn the sequence \texttt{[1 ..\ 120]
    :\ [120][8]} into a nested sequence with type {\tt [3][4][10][8]},
  keeping the elements in the same order.\indSplit \lhint{Use nested
    comprehensions.}  \indComp
\end{Exercise}
\begin{Answer}\ansref{ex:split:2}
  We can split\indSplit 120 elements first into 3 $\times$ 40, splitting each
  of the the elements ({\tt level1} below) into 4 $\times$ 10. A nested
  comprehension fits the bill:\indComp
\begin{Verbatim}
  [ split level1 : [4][10][8]
  | level1 <- split ([1 .. 120] : [120][8]) : [3][40][8]
  ]
\end{Verbatim}
(Note again that you can enter the above in the command line all in
one line, or by putting the line continuation character
\texttt{\textbackslash} at the end of the first two
lines.)\indLineCont \todo[inline]{You don't need nested
  comprehensions: \texttt{split (split [1..120]) :\ [3][4][10][8]}
  also works.}
\end{Answer}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Shifts and rotates}
\label{sec:shifts-rotates}

Common operations on sequences include shifting and rotating them.
Cryptol supports both versions with left/right
variants.\indShiftLeft\indShiftRight\indRotLeft\indRotRight
\begin{Exercise}\label{ex:seq:14}
Experiment with the following expressions:
\begin{Verbatim}
   [1, 2, 3, 4, 5] >> 2
   [1, 2, 3, 4, 5] >> 10
   [1, 2, 3, 4, 5] << 2
   [1, 2, 3, 4, 5] << 10
   [1, 2, 3, 4, 5] >>> 2
   [1, 2, 3, 4, 5] >>> 10
   [1, 2, 3, 4, 5] <<< 2
   [1, 2, 3, 4, 5] <<< 10
\end{Verbatim}
\noindent Notice that shifting/rotating always returns a sequence
precisely the same size as the original.
\end{Exercise}
\begin{Answer}\ansref{ex:seq:14}
Here are Cryptol's responses:
\begin{Verbatim}
  [0, 0, 1, 2, 3]
  [0, 0, 0, 0, 0]
  [3, 4, 5, 0, 0]
  [0, 0, 0, 0, 0]
  [4, 5, 1, 2, 3]
  [1, 2, 3, 4, 5]
  [3, 4, 5, 1, 2]
  [1, 2, 3, 4, 5]
\end{Verbatim}
\todo[inline]{Reflections on this exercise?}
\end{Answer}
\begin{Exercise}\label{ex:seq:15}
  Let {\tt xs} be a sequence of length $n$. What is the result of
  rotating {\tt xs} left or right by a multiple of $n$?
\end{Exercise}
\begin{Answer}\ansref{ex:seq:15}
  Rotating (left or right) by a multiple of the size of a sequence
  will leave it unchanged.
\end{Answer}

%=====================================================================
% \section{Words revisited}
% \label{sec:words2}
\sectionWithAnswers{Words as sequences}{sec:words2}

In \autoref{sec:words} we have introduced numbers as a distinct value
type in Cryptol. In fact, a number in Cryptol is nothing but a finite
sequence of bits, so words are not a separate type. For instance, the
literal expression \texttt{42:[6]} is precisely the same as the
bit-sequence \texttt{[True, False, True, False, True,
    False]}.\indTheWordType The type \texttt{[6]} is really just an
abbreviation for \texttt{[6]Bit}.

\begin{Exercise}\label{ex:words:0}
  Explain why {\tt 42} is the same as {\tt [True, False, True, False,
    True, False]}.  Is Cryptol little-endian, or
  big-endian?\indEndianness
\end{Exercise}
\begin{Answer}\ansref{ex:words:0}
  Cryptol is big-endian,\indEndianness meaning that the
  most-significant-bit comes first. In the sequence {\tt [True, False,
    True, False, True, False]}, the first element corresponds to the
  most-significant-digit, i.e., $2^5$, the next element corresponds to
  the coefficient of $2^4$, etc.  A {\tt False} bit yields a
  coefficient of $0$ and a {\tt True} bit gives $1$. Hence, we have:
$$1\times2^5 + 0\times2^4 + 1\times2^3 + 0\times2^2 + 1\times2^1 + 0\times2^0 = 32 + 0 + 8 + 0 + 2 + 0 = 42$$
\end{Answer}

\begin{Exercise}\label{ex:words:1}
  Try out the following words: \lhint{It might help to use {\tt :set
      base=2} to see the bit patterns.}\indSettingBase
\begin{Verbatim}
  12:[4]
  12 # [False]
  [False, False] # 12
  [True, False] # 12
  12 # [False, True]
  32:[6]
  12 # 32
  [True, False, True, False, True, False] == 42
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:words:1}
  After issuing {\tt :set base=2}, here are Cryptol's
  responses:\indSettingBase
\begin{Verbatim}
  0b1100
  0b11000
  0b1100
  0b101100
  0b110001
  0b100000
  0b1100100000
  True
\end{Verbatim}
\end{Answer}

\begin{Exercise}\label{ex:words:4}
Since words are sequences, the sequence functions from
\exerciseref{ex:seq:11} apply to words as well.
Try out the following examples and explain the outputs you
observe:\indTake\indDrop\indSplit\indGroup
\begin{Verbatim}
  take`{3} 0xFF
  take`{3} (12:[6])
  drop`{3} (12:[6])
  split`{3} (12:[6])
  groupBy`{3} (12:[6])
\end{Verbatim}
\end{Exercise}
\noindent Recall that the notation {\tt 12:[6]} means the constant 12
with the type precisely 6 bits wide.
\begin{Answer}\ansref{ex:words:4}
  Remember that Cryptol is big-endian\indEndianness and hence {\tt
    12:[6]} is precisely {\tt [False, False, True, True, False,
    False]}.  Here are Cryptol's responses:\indTake
\begin{Verbatim}
  Cryptol> take`{3} 0xFF
  7
  Cryptol> take`{3} (12:[6])
  1
  Cryptol> drop`{3} (12:[6])
  4
  Cryptol> split`{3} (12:[6])
  [0, 3, 0]
  Cryptol> groupBy`{3} (12:[6])
  [1, 4]
\end{Verbatim}
For instance, the expression {\tt take`\{3\} (12:[6])} evaluates as follows:
\begin{Verbatim}
  take`{3} (12:[6])
    = take (3, [False, False, True, True, False, False])
    = [False, False, True]
    = 1
\end{Verbatim}
Follow similar lines of reasoning to justify the results for the
remaining expressions.
\end{Answer}
\begin{Exercise}\label{ex:words:5}
  Try \exerciseref{ex:words:4}, this time with the constant {\tt
    12:[12]}. Do any of the results change? Why?
\end{Exercise}
\begin{Answer}\ansref{ex:words:5}
  Because of the leading zeros in {\tt 12:[12]}, they all produce
  different results:\indTake\indDrop\indSplit\indGroup
\begin{Verbatim}
  Cryptol> take`{3} (12:[12])
  0
  Cryptol> drop`{3} (12:[12])
  12
  Cryptol> split`{3} (12:[12])
  [0, 0, 12]
  Cryptol> groupBy`{3} (12:[12])
  [0, 0, 1, 4]
\end{Verbatim}
We will show the evaluation steps for {\tt groupBy} here, and urge the
reader to do the same for {\tt split}:
\begin{Verbatim}
  groupBy`{3} (12:[12])
    = groupBy`{3} [False, False, False, False, False, False,
                   False, False, True, True, False, False]
    = [[False, False, False], [False, False, False]
       [False, False, True], [True, False, False]]
    = [0, 0, 1, 4]
\end{Verbatim}
\end{Answer}

\paragraph*{Shifts and rotates on words} Consider what happens if we
shift a word, say {\tt 12:[6]} by one to the right:
\indShiftLeft\indShiftRight\indRotLeft\indRotRight
\begin{Verbatim}
  (12:[6]) >> 1
    = [False, False, True, True, False, False] >> 1
    = [False, False, False, True, True, False]
    = 6
\end{Verbatim}
That is shifting right by one effectively divides the word by 2. This
is due to Cryptol's ``big-endian'' representation of
numbers.\footnote{This is a significant change from Cryptol version 1,
  which interpreted the leftmost element of a sequence as the
  lowest-ordered bit (and thus shifting right was multiplying by 2,
  and shifting left was dividing by 2). The way it is handled now
  matches the traditional
  interpretation.}\indRotLeft\indRotRight\indShiftLeft\indShiftRight

\begin{Exercise}\label{ex:words:6}
Try the following examples of shifting/rotating words:
\begin{Verbatim}
  (12:[8]) >> 2
  (12:[8]) << 2
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:words:6}
Here are Cryptol's responses:
\begin{Verbatim}
  3
  48
\end{Verbatim}
\todo[inline]{Reflect upon these responses.}
\end{Answer}

\todo[inline]{2.0: This is \textbf{NOT} architecture endianness; we
  need a better discussion here.  Dylan will do some code-diving on
  Cryptol version 1.}

\paragraph*{Little-endian vs Big-endian} The discussion of endianness
comes up often in computer science, with no clear
winner.\indEndianness Since Cryptol allows indexing from the beginning
or the end of a (finite) sequence, you can access the 0th
(least-significant) bit of a sequence \texttt{k} with \texttt{k!0}, the 1st bit with
\texttt{k!1}, and so on.\indIndex

%=====================================================================
\section{Characters and strings}
\label{sec:charstring}

Strictly speaking Cryptol does {\em not} have characters and strings
as a separate type. However, Cryptol does allow characters in
programs, which simply correspond to their ASCII
equivalents. Similarly, strings are merely sequences of characters,
i.e., sequences of words.\indTheStringType\indTheCharType The
following examples illustrate:
\begin{Verbatim}
  Cryptol> :set base=10
  Cryptol> :set ascii=off
  Cryptol> 'A'
  65
  Cryptol> "ABC"
  [65, 66, 67]
  Cryptol> :set ascii=on
  Cryptol> "ABC"
  "ABC"
  Cryptol> :set ascii=off
  Cryptol> ['A' .. 'Z']
  [65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,
       80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90]
  Cryptol> :set ascii=on
  Cryptol> ['A' .. 'Z']
  "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
\end{Verbatim}
\note{This is the reason why we have to use the {\tt :set ascii=on}
  command to print ASCII strings.  Otherwise, Cryptol will not have
  enough information to tell numbers from
  characters.\indSettingASCII}

\noindent Since characters are simply 8-bit words, you can do word
operations on them; including arithmetic:
\begin{Verbatim}
  Cryptol> 'C' - 'A'
  2
\end{Verbatim}

%=====================================================================
% \section{Records: Named collections}
% \label{sec:records}
\sectionWithAnswers{Records: Named collections}{sec:records}

In Cryptol, records are simply collections of named fields. In this
sense, they are very similar to tuples (\autoref{sec:tuple}), which
can be thought of records without field names. Like a tuple, the
fields of a record can be of any type. We construct records by listing
the fields inside curly braces, separated by commas. We project fields
out of a record with the usual dot-notation, similar to tuple
projections. Note that the order of fields in a record is
immaterial.\indTheRecordType\indTheTupleType

Record equality is defined in the standard fashion.  Two records are
equal if they have the same number of fields, if their field names are
identical, if identically named fields are of comparable types and
have equal values.

\begin{Exercise}\label{ex:record:1}
Type in the following expressions and observe the output:
\begin{Verbatim}
  {xCoord = 12:[32], yCoord = 21:[32]}
  {xCoord = 12:[32], yCoord = 21:[32]}.yCoord
  {name = "Cryptol", address = "Galois"}
  {name = "Cryptol", address = "Galois"}.address
  {name = "test", coords = {xCoord = 3:[32], yCoord = 5:[32]}}
  {name = "test", coords = {xCoord = 3:[32], \
                            yCoord = 5:[32]}}.coords.yCoord
  {x=True, y=False} == {y=False, x=True}
\end{Verbatim}
\noindent You might find the command {\tt :set
  ascii=on}\indSettingASCII useful in viewing the output.
\end{Exercise}
\begin{Answer}\ansref{ex:record:1}
Here are Cryptol's responses:
\begin{small}
\begin{Verbatim}
  Cryptol> {xCoord = 12:[32], yCoord = 21:[32]}
  {xCoord = 12, yCoord = 21}
  Cryptol> {xCoord = 12:[32], yCoord = 21:[32]}.yCoord
  21
  Cryptol> {name = "Cryptol", address = "Galois"}
  {name = "Cryptol", address = "Galois"}
  Cryptol> {name = "Cryptol", address = "Galois"}.address
  "Galois"
  Cryptol> {name = "test", coords = {xCoord = 3:[32], yCoord = 5:[32]}}
  {name = "test", coords = {xCoord = 3, yCoord = 5}}
  Cryptol> {name = "test", coords = {xCoord = 3:[32], \
                                   yCoord = 5:[32]}}.coords.yCoord
  5
  Cryptol> {x=True, y=False} == {y=False, x=True}
  True
\end{Verbatim}
\end{small}
\end{Answer}

%=====================================================================
% \section{\texorpdfstring{The {\tt zero}}{The zero}}
% \label{sec:zero}
\sectionWithAnswers{\texorpdfstring{The {\tt zero}}{The zero}}{sec:zero}

Before proceeding further, we have to take a detour and talk briefly
about one of the most useful values in Cryptol: {\tt zero}.\indZero
The value {\tt zero} inhabits every type in Cryptol, and stands for
the value that consists of all {\tt False}\indFalse bits. The
following examples should illustrate the idea:
\begin{Verbatim}
  Cryptol> zero : Bit
  False
  Cryptol> zero : [8]
  0
  Cryptol> zero : ([8], Bit)
  (0, False)
  Cryptol> zero : [8][3]
  [0, 0, 0, 0, 0, 0, 0, 0]
  Cryptol> zero : [3](Bit, [4])
  [(False, 0), (False, 0), (False, 0)]
  Cryptol> zero : {xCoord : [12], yCoord : [5]}
  {xCoord=0, yCoord=0}
\end{Verbatim}

\noindent On the other extreme, the value {\tt zero} combined with the
complement operator {\tt \Verb|~|}\indComplement gives us values that
consist of all {\tt True}\indTrue bits:
\begin{Verbatim}
  Cryptol> ~zero : Bit
  True
  Cryptol> ~zero : [8]
  255
  Cryptol> ~zero : ([8], Bit)
  (255, True)
  Cryptol> ~zero : [8][3]
  [7, 7, 7, 7, 7, 7, 7, 7]
  Cryptol> ~zero : [3](Bit, [4])
  [(True, 15), (True, 15), (True, 15)]
  Cryptol> ~zero : {xCoord : [12], yCoord : [5]}
  {xCoord=4095, yCoord=31}
\end{Verbatim}

\begin{Exercise}\label{ex:zero:0}
  We said that {\tt zero} inhabits all types in Cryptol. This also
  includes functions. What do you think the appropriate {\tt zero}
  value for a function would be?  Try out the following examples:
\begin{Verbatim}
   (zero : ([8] -> [3])) 5
   (zero : Bit -> {xCoord : [12], yCoord : [5]}) True
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:zero:0}
Here are Cryptol's responses:\indZero
\begin{Verbatim}
  Cryptol> (zero : ([8] -> [3])) 5
  0
  Cryptol> (zero : Bit -> {xCoord : [12], yCoord : [5]}) True
  {xCoord=0, yCoord=0}
\end{Verbatim}
The {\tt zero} function returns {\tt 0}, ignoring its argument.
\end{Answer}

%=====================================================================
% \section{Arithmetic}
% \label{sec:arithmetic}
\sectionWithAnswers{Arithmetic}{sec:arithmetic}

Cryptol supports the usual binary arithmetic operators {\tt +}, {\tt
  -}, {\tt *}, {\tt \Verb|^^|} (exponentiation), {\tt /} (integer
division), {\tt \%} (integer modulus), along with \emph{ceiling}
base-2 logarithm {\tt lg2} and binary {\tt min} and {\tt max}.

The important thing to remember is that all arithmetic in Cryptol is
modular,\indModular with respect to the underlying word size.  As a
consequence, there is no such thing as an overflow/underflow in
Cryptol, as the result will be always guaranteed to fit in the
resulting word size.  While this is very handy for most applications of
Cryptol, it requires some care if overflow has to be treated
explicitly.\indOverflow\indUnderflow\indPlus\indMinus\indTimes\indDiv\indMod\indLg\indMin\indMax\indExponentiate

\begin{Exercise}\label{ex:arith:1}
What is the value of \texttt{1 + 1 :~[\textunderscore]}? Surprised?
\end{Exercise}
\begin{Answer}\ansref{ex:arith:1}
  Since {\tt 1} requires only 1 bit to represent, the result also has
  1 bit. In other words, the arithmetic is done modulo $2^1 =
  2$. Therefore, {\tt 1+1 = 0}.
\end{Answer}

\begin{Exercise}\label{ex:arith:2}
What is the value of \texttt{1 + 1 :~[8]}? Why?
\end{Exercise}
\begin{Answer}\ansref{ex:arith:2}
  Now we have 8 bits to work with, so the result is {\tt 2}. Since we
  have 8 bits to work with, overflow will not happen until we get a
  sum that is at least 256.
\end{Answer}

\begin{Exercise}\label{ex:arith:3}
What is the value of \texttt{3 - 5 :~[\textunderscore]}? How about \texttt{3 - 5 :~[8]}?
\end{Exercise}
\begin{Answer}\ansref{ex:arith:3}
  Recall from \autoref{sec:words} that there are no negative
  numbers in Cryptol. The values \texttt{3} and \texttt{5} can be
  represented in 3 bits, so Cryptol uses 3 bits to represent the
  result, so the arithmetic is done modulo $2^3=8$. Hence, the result
  is \texttt{6}.  In the second expression, we have 8 bits to work with,
  so the modulus is $2^8 = 256$; so the subtraction results in
  \texttt{254} (or \texttt{0xfe}).
\end{Answer}

\note{Cryptol supports subtraction both as a binary operator, and as a
  unary operator. When used in a unary fashion (a.k.a. unary
  minus),\indUnaryMinus it simply means subtraction from \texttt{0}.
  For instance, \texttt{-5} precisely means \texttt{0-5}, and is subject to the
  usual modular arithmetic rules.}\indModular\indMinus

\begin{Exercise}\label{ex:arith:4}
Try out the following expressions:\indEq\indNeq
\begin{Verbatim}
  2 / 0
  2 % 0
  3 + (if 3 == 2+1 then 12 else 2/0)
  3 + (if 3 != 2+1 then 12 else 2/0)
  lg2 (-25) : Integer
  lg2 (-25) : [_]
\end{Verbatim}
In the last expression, remember that unary minus will\indUnaryMinus
be done in a modular fashion. What is the modulus used for this
operation?
\end{Exercise}
\begin{Answer}\ansref{ex:arith:4}
  The first, second, and fourth examples give a ``division by 0''
  error message. In the third example, the division by zero occurs
  only in an unreachable branch, so the result is not an error. The
  logarithm at type \texttt{Integer} gives a ``logarithm of negative''
  error.

  In the last expression, the number \texttt{25} fits in 5 bits, so
  the modulus is $2^5 = 32$. The unary minus yields \texttt{7}, hence
  the result is \texttt{3}. Note that \texttt{lg2} is the \emph{floor
    log base 2} function. The \texttt{width} function is the
  \emph{ceiling log base 2} function.\indLg\indEq\indNeq
\end{Answer}

\begin{Exercise}\label{ex:arith:5:1}
Division truncates down. Try out the following expressions:\indDiv\indMod
\begin{Verbatim}
  (6 / 3, 6 % 3)
  (7 / 3, 7 % 3)
  (8 / 3, 8 % 3)
  (9 / 3, 9 % 3)
\end{Verbatim}
What is the relationship between {\tt /} and {\tt \%}?
\end{Exercise}
\begin{Answer}\ansref{ex:arith:5:1}
Here are Cryptol's answers:\indDiv\indMod
\begin{Verbatim}
  (2, 0)
  (2, 1)
  (2, 2)
  (3, 0)
\end{Verbatim}
The following equation holds regarding {\tt /} and {\tt \%}:
$$
  x = (x / y) * y + (x \% y)
$$
whenever $y \neq 0$.
\end{Answer}

\begin{Exercise}\label{ex:arith:5}
  What is the value of \texttt{min (5:[\textunderscore]) (-2)}? Why?
  Why are the parentheses around \texttt{-2}
  necessary?\indMin\indModular\indUnaryMinus
\end{Exercise}
\begin{Answer}\ansref{ex:arith:5}
  The bit-width in this case is 3 (to accommodate for the number 5),
  and hence arithmetic is done modulo $2^3 = 8$. Thus, {\tt -2}
  evaluates to {\tt 6}, leading to the result {\tt min 5, (-2) == 5}.
  The parentheses are necessary because unary negation is handled in
  Cryptol's parser, not in its lexer, because whitespace is ignored.
  If this were not the case, reflect upon how you would differentiate
  the expressions \texttt{min 5 - 2} and \texttt{min 5 -2}.
\end{Answer}

\begin{Exercise}\label{ex:arith:6}
How about {\tt max 5 (-2:[8])}? Why?\indMin\indModular\indUnaryMinus
\end{Exercise}
\begin{Answer}\ansref{ex:arith:6}
  This time we are telling Cryptol to use precisely 8 bits, so {\tt
    -2} is equivalent to {\tt 254}. Therefore the result is {\tt 254}.
\end{Answer}

\begin{Exercise}\label{ex:arith:7}
  Write an expression that computes the sum of two sequences
  \texttt{[1..\ 10]} and \texttt{[10, 9 ..\ 1]}.\indPlus
\end{Exercise}
\begin{Answer}\ansref{ex:arith:7}
  The idiomatic Cryptol way of summing two sequences is to use a
  comprehension:\indComp
\begin{Verbatim}
     [ i+j | i <- [1 .. 10]
           | j <- [10, 9 .. 1]
     ]
\end{Verbatim}
However, you will notice that the following will work as well:
\begin{Verbatim}
     [1 .. 10] + [10, 9 .. 1]
\end{Verbatim}
That is, Cryptol automatically lifts arithmetic operators to
sequences, element-wise. However, it is often best to keep the
explicit style of writing the comprehension, even though it is a bit
longer, since that makes it absolutely clear what the intention is and
how the new sequence is constructed, without depending implicitely
upon Cryptol's automatic lifting.\indArithLift
\end{Answer}

\paragraph*{Comparison operators} Cryptol supports the comparison
operators {\tt ==}, {\tt !=}, {\tt >}, {\tt >=}, {\tt <}, {\tt <=},
with their usual meanings.\indEq\indNeq\indGt\indGte\indLt\indLte

\begin{Exercise}\label{ex:arith:8}
Try out the following expressions:
\begin{Verbatim}
  ((2 >= 3) || (3 < 6)) && (4 == 5)
  if 3 >= 2 then True else 1 < 12
\end{Verbatim}
\end{Exercise}

\paragraph*{Enumerations, revisited} In
\exerciseref{ex:seq:9}, we wrote the infinite
enumeration{\indEnum\indInfSeq}starting at {\tt 1} using an explicit
type as follows:
\begin{Verbatim}
  [(1:[32]) ...]
\end{Verbatim}
As expected, Cryptol evaluates this expression to:
\begin{Verbatim}
  [1, 2, 3, 4, 5, ...]
\end{Verbatim}
However, while the output suggests that the numbers are increasing all
the time, that is just an illusion! Since the elements of this
sequence are 32-bit words, eventually they will wrap over, and go back
to 0. (In fact, this will happen precisely at the element $2^{32}-1$,
starting the count at 0 as usual.) We can observe this much more
simply, by using a smaller bit size for the constant {\tt 1}:
\begin{Verbatim}
  Cryptol> [(1:[2])...]
  [1, 2, 3, 0, 1 ...
  Cryptol> take`{20} [(1:[2])...]
  [1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0]
\end{Verbatim}
We still get an infinite sequence, but the numbers will repeat
themselves eventually. Note that this is a direct consequence of
Cryptol's modular arithmetic.\indModular

\todo[inline]{FIXME the next examples don't make sense any more.}

There is one more case to look at. What happens if we completely leave
out the bit-width?
\begin{Verbatim}
  Cryptol> [1:[_] ...]
  [1, 0, 1, 0, 1, ...]
\end{Verbatim}
In this case, Cryptol figured out that the number {\tt 1} requires
precisely 1 bit, and hence the arithmetic is done modulo $2^1 = 2$,
giving us the sequence $1,0,1,0,\ldots$ In particular, an
enumeration of the form:
\begin{Verbatim}
  [k ...]
\end{Verbatim}
will be treated as if the user has written:
\begin{Verbatim}
  [k, (k+1) ...]
\end{Verbatim}
and type inference will assign the smallest bit-size possible to
represent {\tt k}.

\begin{Exercise}\label{ex:arith:9}
  Remember from \exerciseref{ex:decimalType} that the word \texttt{0}
  requires 0 bits to represent. Based on this, what is the value of
  the enumeration \texttt{[0:[\textunderscore], 1...]}? How about
  \texttt{[0:[\textunderscore]...]}? Surprised?
\end{Exercise}
\begin{Answer}\ansref{ex:arith:9}
Here are Cryptol's responses:\indModular\indEnum\indInfSeq
\begin{Verbatim}
  [0, 1, 0, 1, 0, ...]
  [0, 0, 0, 0, 0, ...]
\end{Verbatim}
The first example gets an element type of \texttt{[1]}, because of the
explicit use of the numeral \texttt{1}, while the second example's
element type is \texttt{[0]}. This is one case where \texttt{[k...]}
is subtly different from \texttt{[k, k+1...]}.
\end{Answer}

\begin{Exercise}\label{ex:arith:10}
  What is the value of \texttt{[1 ..\ 10]}? Explain in terms of the above
  discussion on modular arithmetic.\indModular
\end{Exercise}
\begin{Answer}\ansref{ex:arith:10}
  The expression \texttt{[1 ..\ 10]} is equivalent to \texttt{[1, (1+1) ..\ 10]},
  and Cryptol knows that \texttt{10} requires at least 4 bits
  to represent and uses the minimum implied by all the available
  information. Hence we get: \texttt{[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]}.
  You can use the \texttt{:t} command to see the type Cryptol infers for
  this expression explicitly:
\begin{Verbatim}
    Cryptol> :t [1 .. 10]
    {a} (a >= 4, fin a) => [10][a]
\end{Verbatim}
Cryptol tells us that the sequence has precisely $10$ elements, and each
element is at least $4$ bits wide.
\todo[inline]{Reflect upon this ``at least'' a bit more.}
\end{Answer}

%=====================================================================
% \section{Types}
% \label{sec:types}
\sectionWithAnswers{Types}{sec:types}

Cryptol's type system is one of its key features.\footnote{The Cryptol
  type system is based on the traditional Hindley-Milner style,
  extended with size types and arithmetic
  predicates (for details, see~\cite{erkok-carlsson-wick-cryptolCoverification-09,
    erkok-matthews-cryptolEqChecking-09, Hin97})}  You have seen that
types can be used to specify the exact width of values, or shapes of
sequences using a rich yet concise notation.  In some cases, it may
make sense to omit a type signature and let Cryptol {\em infer} the
type for you. At the interpreter, you can check what type Cryptol
inferred with the \texttt{:t} command.

\todo[inline]{More structured discussion of top and bottom types, \texttt{inf}
  and \texttt{fin}, and the precise kinds of type constraints that are
  possible.  Go over index as well wrt type-related terms.}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Monomorphic types}\indMonomorphism
\label{sec:monomorphic-types}

A monomorphic type is one that represents a concrete value. Most of
the examples we have seen so far fall into this category. Below, we
review the basic Cryptol types that make up all the monomorphic values
in Cryptol.

\paragraph*{Bits}\indTheBitType\indTrue\indFalse There are precisely
two bit values in Cryptol: {\tt True} and {\tt
  False}. The type itself is written {\tt Bit}. When we want to be
explicit, we can write it as follows: \texttt{(2 >= 3) :\ Bit}. However,
with type inference\indTypeInference writing the {\tt Bit} type
explicitly is almost never needed.

\paragraph*{Words}\indTheWordType\indTheBitType A word type is written
{\tt [}$n${\tt ]}, where $n$ is a fixed
non-negative constant. The constant can be as large (or small) as you
like.  So, you can talk about 2-bit quantities {\tt [2]}, as well as
384-bit ones {\tt [384]}, or even odd sizes like 17 {\tt [17]},
depending on the needs of your application. When we want to be
explicit about the type of a value, we say {\tt 5:[8]}. If we do not
specify a size, Cryptol's type inference engine will pick the
appropriate value depending on the context.\indTypeInference
Recall from \autoref{sec:words2} that a word is, in fact, a
sequence of bits.  Hence, an equivalent (but verbose) way to write the
type {\tt [17]} is {\tt [17]Bit}, which we would say in English as
``a sequence of length 17, whose elements are Bits.''

\paragraph*{Tuples}\indTheTupleType A tuple is a heterogeneous
collection of arbitrary number of
elements. Just like we write a tuple value by enclosing it in
parentheses, we write the tuple type by enclosing the component types
in parentheses, separated by commas: \texttt{(3, 5, True) :\ ([8], [32],
  Bit)}. Tuples' types follow the same structure: \texttt{(2, (False, 3),
  5) :\ ([8], (Bit, [32]), [32])}.  A tuple component can be any type:
a word, another tuple, sequence, record, etc. Again, type inference
makes writing tuple types hardly ever necessary.\indTypeInference

\paragraph*{Sequences}\indTheSequenceType A sequence is simply a
collection of homogeneous elements. If the element type is $t$,
then we write the type of a sequence of $n$ elements as \texttt{[}$n$\texttt{]}$t$.
Note that $t$ can be a sequence type itself. For
instance, the type {\tt [12][3][6]} reads as follows: A sequence of
12 elements, each of which is a sequence of 3 elements, each of which
is a 6-bit-wide word.

The type of an infinite sequence is written \texttt{[inf]}$t$, where $t$
is the type of the elements.\indInfSeq \indInf

\begin{Exercise}\label{ex:types:1}
What is the total number of bits in the type {\tt [12][3][6]}?
\end{Exercise}
\begin{Answer}\ansref{ex:types:1}
  We have $12$ elements, each of which is a sequence of $3$ elements;
  so we have $12*3=36$ elements total. Each element is a 6-bit word;
  so the total number of bits is $36*6 = 216$.
\end{Answer}

\begin{Exercise}\label{ex:types:2}
  How would you write the type of an infinite sequence where each
  element itself is an infinite sequence of 32-bit words? What is the
  total bit size of this type?
\end{Exercise}
\begin{Answer}\ansref{ex:types:2} {\tt [inf][inf][32]}. The size of
  such a value would be infinite!
\end{Answer}

\paragraph{Records}\indTheRecordType
A record is a heterogeneous collection of arbitrary number of labeled
elements.  In a sense, they generalize tuples by allowing the
programmer to give explicit names to fields. The type of a record is
written by enclosing it in braces, separated by commas:
\texttt{\{x :\ [32], y :\ [32]\}}. Records can be nested and can contain arbitrary
types of elements (records, sequences, functions, etc.).

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Polymorphic types}\indPolymorphism
\label{sec:polymorphic-types}

Our focus so far has been on monomorphic types---the types that
concrete Cryptol values (such as {\tt True}, {\tt 3}, or {\tt [1, 2]})
can have.  If all we had were monomorphic types, however, Cryptol
would be a very verbose and boring language.  Instead, we would like
to be able to talk about collections of values, values whose types are
instances of a given polymorphic type. This facility is especially
important when we define functions, a topic we will get to shortly. In
the mean time, we will look at some of the polymorphic primitive
functions Cryptol provides to get a feeling for Cryptol's polymorphic
type system.

\paragraph{The tale of {\ttfamily{\textbf tail}}}\indTail
Cryptol's built in function {\tt tail} allows us to drop the first
element from a sequence, returning the remainder:
\begin{Verbatim}
  Cryptol> tail [1 .. 5]
  [2, 3, 4, 5]
  Cryptol> tail [(False, (1:[8])), (True, 12), (False, 3)]
  [(True, 12), (False, 3)]
  Cryptol> tail [ (1:[16])... ]
  [2, 3, 4, 5, 6, ...]
\end{Verbatim}
What exactly is the type of {\tt tail}? If we look at the first
example, one can deduce that {\tt tail} must have the type:
\begin{Verbatim}
  tail : [5][8] -> [4][8]
\end{Verbatim}
That is, it takes a sequence of length 5, containing 8-bit values, and
returns a sequence that has length 4, containing 8-bit values. (The
type {\tt a -> b} denotes a function that takes a value of type {\tt
  a} and delivers a value of type {\tt b}.)

However, the other example uses of {\tt tail} above suggest that it
must have the following types, respectively:
\begin{Verbatim}
  tail : [10][32] -> [9][32]
  tail : [3](Bit, [8]) -> [2](Bit, [8])
  tail : [inf][16] -> [inf][16]
\end{Verbatim}

As we have emphasized before, Cryptol is strongly typed, meaning that
every entity (whether a Cryptol primitive or a user-defined function)
must have a well-defined type. Clearly, the types we provided for
\texttt{tail} above are quite different from each other. In particular, the
first example uses numbers as the element type, while the second has
tuples. So, how can \texttt{tail} be assigned a type that will make it
work on all these inputs?

If you are familiar C++ templates or Java generics, you might think
that Cryptol has some sort of an overloading mechanism that allows one
to define functions that can work on multiple types. While templates
and generics do provide a mental model, the correspondence is not very
strong. In particular, we never write multiple definitions for the
same function in Cryptol, i.e., there is no ad-hoc
overloading. However, what Cryptol has is a much stronger notion:
polymorphism, as would be advocated by languages such as Haskell or
ML~\cite{ML,Has98}.\indPolymorphism\indOverloading

Here is the type of {\tt tail} in Cryptol:
\begin{Verbatim}
  Cryptol> :t tail
  tail : {n, a} [1 + n]a -> [n]a
\end{Verbatim}
This is quite a different type from what we have seen so far. In
particular, it is a polymorphic type, one that can work over multiple
concrete instantiations of it. Here's how we read this type in
Cryptol:
\begin{quote} \texttt{tail} is a polymorphic function, parameterized over
  \texttt{n} and \texttt{a}. The input is a sequence that contains
  \texttt{1 + n} elements. The elements can be of an arbitrary type
  \texttt{a}; there is no restriction on their structure. The result
  is a sequence that contains \texttt{n} elements, where the elements
  themselves have the same type as those of the input.
\end{quote}
In the case for \texttt{tail}, the parameter \texttt{n} is a size-parameter
(since it describes the size of a sequence), while \texttt{a} is a
shape-parameter, since it describes the shape of elements. The
important thing to remember is that each use of \texttt{tail} must
instantiate the parameters \texttt{n} and \texttt{a} appropriately. Let's
see how the instantiations work for our running examples:

\todo[inline]{Talk about the scope of the type system's expressiveness and its
  limitations. Perhaps call it a ``size-polymorphic type system'' here
  more explicitly so that readers who are into dependent types are not
  offended or disappointed.}

\begin{center}
%\begin{adjustbox}{width={\textwidth},keepaspectratio}
\begin{tabular}[h]{c||c|c|l}
{\tt [n+1]a -> [n]a}                 & {\tt n}   & {\tt a}          & Notes \\ \hline\hline
{\tt [5][8] -> [4][8]}               &    4      & {\tt [8]}        & {\tt 1+n = 5} $\Rightarrow$ {\tt n = 4}  \\\hline
{\tt [10][32] -> [9][32]}            &    9      & {\tt [32]}       & {\tt 1+n = 10} $ \Rightarrow$ {\tt n = 9} \\\hline
{\tt [3](Bit, [8]) -> [2](Bit, [8])} &    2      & {\tt (Bit, [8])} & The type {\tt a} is now a tuple	           \\\hline
{\tt [inf][16] -> [inf][16]}         & {\tt inf} & {\tt [16]}       & {\tt 1+n = inf} $\Rightarrow$ {\tt n = inf}
\end{tabular}
%\end{adjustbox}
\end{center}

In the last instantiation, Cryptol knows that $\infty - 1 = \infty$,
allowing us to apply {\tt tail} on both finite and infinite
sequences. The crucial point is that an instantiation must be found
that satisfies the required match.  It is informative to see what
happens if we apply {\tt tail} to an argument where an appropriate
instantiation can not be found:
\begin{Verbatim}
  Cryptol> tail True
  [error] at <interactive>:1:6--1:10:
    Type mismatch:
      Expected type: [1 + ?n`859]?a`860
      Inferred type: Bit
    where
    ?n`859 is type argument 'n' of 'tail' at <interactive>:1:1--1:5
    ?a`860 is type argument 'a' of 'tail' at <interactive>:1:1--1:5
\end{Verbatim}
Cryptol is telling us that it cannot match the types \texttt{Bit} and
the sequence \texttt{[1+n]a}, causing a type error statically at
compile time. (The funny notation of \texttt{?n`859} and
\texttt{?a`860} are due to how type instantiations proceed under the
hood. While they look funny at first, you soon get used to the
notation.)

We should emphasize that Cryptol polymorphism\indPolymorphism
uniformly applies to user-defined functions as well, as we shall see
in \autoref{sec:funcs}.

\begin{Exercise}\label{ex:poly:split}\indGroup
  Use the \texttt{:t} command to find the type of \texttt{split}. For
  each use case below, find out what the instantiations of its type
  variables are, and justify why the instantiation works. Can you find
  an instantiation in all these cases?
\begin{Verbatim}
  split`{3} [1..9]
  split`{3} [1..12]
  split`{3} [1..10] : [3][2][8]
  split`{3} [1..10]
\end{Verbatim}
Is there any way to make the last example work by giving a type signature?
\end{Exercise}
\begin{Answer}\ansref{ex:poly:split}\indGroup
Here is the type of \texttt{split}:
\begin{Verbatim}
  Cryptol> :t split
  split : {parts, each, a} (fin each) =>
            [parts * each]a -> [parts][each]a
\end{Verbatim}
At every use case of \texttt{split} we must instantiate the parameters
\texttt{parts}, \texttt{each}, and \texttt{a}, so that the resulting
instantiation will match the use case. In the first example, we can
simply take: \texttt{parts = 3}, \texttt{each = 3}, and \texttt{a = [4]}. In
the second, we can take \texttt{parts=3}, \texttt{each=4}, and
\texttt{a=[4]}. The third expression does not type-check. Cryptol
tells us:
\begin{Verbatim}
  Cryptol> split`{3} [1..10] : [3][2][8]
  [error] at <interactive>:1:11--1:18:
    Type mismatch:
      Expected type: 6
      Inferred type: 10
\end{Verbatim}
In this case, we are telling Cryptol that \texttt{parts = 3},
\texttt{each = 2}, and \texttt{a = [8]} by providing the explicit type
signature.\indSignature Using this information, Cryptol must ensure
that the argument matches the instantiation of the the polymorphic
type. The argument to \texttt{split} must have type \texttt{[parts *
    each]a}; instantiating the type variables, this is \texttt{[3 *
    2][8]}, or \texttt{[6][8]}. However, the given argument has length
\texttt{10}, which is not equal to \texttt{6}. It is not hard to see
that there is no instantiation to make this work, since \texttt{10} is
not divisible by \texttt{3}.
\end{Answer}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Predicates}
\label{sec:predicates}

\todo[inline]{Should we introduce a better predicates example?}

In the previous section we have seen how polymorphism is a powerful
tool in structuring programs. Cryptol takes the idea of polymorphism
on sizes one step further by allowing predicates on
sizes.\indPredicates To illustrate the notion, consider the type of
the Cryptol primitive {\tt take}\indTake:
\begin{Verbatim}
  Cryptol> :t take
  take : {front, back, a} (fin front) => [front + back]a -> [front]a
\end{Verbatim}

The type of {\tt take} says that it is parameterized over {\tt front}
and {\tt back}, {\tt front} must be a finite value,\indFin it takes a
sequence {\tt front + back} long, and returns a sequence {\tt front} long.

The impact of this predicate shows up when we try to take more than
what is available:
\begin{Verbatim}
  Cryptol> take`{10} [1..5]
  [error] at <interactive>:1:11--1:17:
    Adding 10 will always exceed 5
\end{Verbatim}
Cryptol is telling us that it is unable to satisfy this instantiation
(since {\tt front} is 10 and the sequence has 5
elements).\indTake\indPredicates

In general, type predicates exclusively describe \textit{arithmetic
  constraints on type variables}.  Cryptol does not have a
general-purpose dependent type system, but a \emph{size-polymorphic
  type system}.  Often type variables' values are of finite size,
indicated with the constraint {\tt fin a}\indFin; otherwise no
constraint is mentioned, and the variables' values are unbounded,
possibly taking the value \texttt{inf}\indInf.  Arithmetic
relations are arbitrary relations over all type variables, such as
{\tt 2*a+b >= c}.  We shall see more examples as we work through
programs later on.

\begin{Exercise}\label{ex:preds:1}
  Write a predicate that allows a word of size {\tt 128}, {\tt 192},
  or {\tt 256}, but nothing else.
\end{Exercise}
\begin{Answer}\ansref{ex:preds:1}\indPredicates
  Here is one way of writing this predicate, following the fact that
  $128 = 2 * 64$, $192 = 3 * 64$, and $256 = 4 * 64$:
\begin{verbatim}
  {k} (2 <= k, k <= 4) => [k*64]
\end{verbatim}
\todo[inline]{FIXME: The following example type signature is rejected by Cryptol, as subtraction is undefined when it underflows.}
Here is another way, more direct but somewhat less satisfying:
\begin{verbatim}
  {k} ((k - 128) * (k - 192) * (k - 256) == 0) => [k]
\end{verbatim}
Note that Cryptol's type constraints do not include {\em or} predicates,
hence we cannot just list the possibilities in a list.
\end{Answer}

\note{Type inference in the presence of arithmetic predicates is an
  undecidable problem~\cite{erkok-matthews-cryptolEqChecking-09}. This
  implies that there is no algorithm to decide whether a given type is
  inhabited, amongst other things.  In practical terms, we might end
  up writing programs with arbitrarily complicated predicates (e.g.,
  this ``type contains all solutions to Fermat's last equation'' or
  ``this type contains all primes between two large numbers'') without
  Cryptol being able to simplify them, or notice that there is no
  instantiation that will ever work.\indUndecidable Here is a simple
  example of such a type:}
\begin{Verbatim}
  {k} (2 >= k, k >= 5) => [k]
\end{Verbatim}
While a moment of pondering suffices to conclude that there is no such
value in this particular case, there is no algorithm to decide this
problem in general.

That being said, Cryptol's type inference and type checking algorithms
are well-tuned to the use cases witnessed in the types necessary for
cryptographic algorithms.  Moreover, Cryptol uses a powerful SMT
solver capable of reasoning about complex arithmetic theories within
these algorithms.

\todo[inline]{Precisely write up type system and type inference algorithm in
  the Cryptol language guide or appendix.  Reference which decision
  procedures are actually used during type checking/inference.}

%%\subsection{Defaulting: A note on constants}
%% What is the type of {\tt 42}? We can ask Cryptol to find out:
%%\begin{Verbatim}
%%  Cryptol> :t 42
%%  42 : [6]
%%\end{Verbatim}
%% However, {\tt 42} can in fact be of any size that is at least 6 bits
%% wide. So, why does Cryptol think it is just 6 bits? The reason is
%% because of defaulting.\indDefaulting The defaulting rule says that
%% Cryptol will pick the minimum size that will satisfy the
%% constraints, in the following two scenarios:
%%\begin{itemize}
%%\item At the top-level, i.e., when you type a number at the command
%%  line,
%%\item In a nested expression, where the type variable does not
%%  contribute to the overall type of the definition.
%%\end{itemize}
%% For the time being, let us simply focus on the first case: When we
%% type a number in the command line. Cryptol does this so that simple
%% arithmetic expressions at the top-level can produce the {\em
%% expected} values, instead of being overly polymorphic.  You can
%% turn off this behavior by issuing the command {\tt :set
%% -d}:\indSettingDefaulting
%%\begin{Verbatim}
%%  Cryptol> :set -d
%%  Cryptol> :t 42
%%  42 : {a} (a >= 6) => [a]
%%\end{Verbatim}
%% Now, Cryptol is telling us {\tt 42} actually is a word that is at
%% least {\tt 6} bits wide. As a consequence, we now have more
%% polymorphic results:
%%\begin{Verbatim}
%%  Cryptol> 4+1
%%  <polymorphic value> : {a} (a >= 3) => [a]
%%\end{Verbatim}
%% When defaulting is off, even simple expressions like the one above
%% will produce polymorphic results, which would require the user to
%% be explicit about the types:
%%\begin{Verbatim}
%%  Cryptol> 4+1 : [8]
%%  5
%%\end{Verbatim}
%% Some consider this style to be superior to the implicit defaulting,
%% as it forces the user to be explicit.  However, the defaulting
%% behavior is enabled by default, since otherwise interactions with
%% the interpreter would be very painful. This is true especially in
%% the case of nested expressions in Cryptol programs, as we shall see
%% in \autoref{sec:pitfall:defaulting}.

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Why typed?}
\label{sec:why-typed}

There is a spectrum of type systems employed by programming languages,
all the way from completely untyped to fancier dependently typed
languages.  There is no simple answer to the question, what type system
is the best? It depends on the application domain. We have found that
Cryptol's size-polymorphic type system is a good fit for programming
problems that arise in the domain of cryptography. The bit-precise
type system makes sure that we never pass an argument that is 32 bits
wide in a buffer that can only fit 16. The motto is: {\em Well typed
  programs do not go wrong}.

In practical terms, this means that the type system catches most of
the common mistakes that programmers tend to make. Size-polymorphism
is a good fit for Cryptol, as it keeps track of the most important
invariant in our application domain: making sure the sizes of data can
be very precisely specified and the programs can be statically
guaranteed to respect these sizes.

Opponents of type systems typically argue that the type system gets in
the way.\footnote{Another complaint is that ``strong types are for
  weak minds.'' We do not disagree here: Cryptol programmers want to
  use the type system so we do not have to think as hard about writing
  correct code as we would without strong types.}  It is true that
the type system will reject some programs that makes perfect
sense. But what is more important is that the type system will reject
programs that will indeed go wrong at run-time. And the price you pay
to make sure your program type-checks is negligible, and the savings
due to type checking can be enormous.

The crucial question is not whether we want type systems, but rather
what type system is the best for a given particular application
domain. We have found that the size-polymorphic type system of Cryptol
provides the right balance for the domain of cryptography and
bit-precise programming problems it has been designed
for~\cite{lewis2003}.

%=====================================================================
% \section{Defining functions}
% \label{sec:funcs}
\sectionWithAnswers{Defining functions}{sec:funcs}

So far, we used Cryptol as a calculator: we typed in expressions and
it gave us answers. This is great for experimenting, and exploring
Cryptol itself. The next fundamental Cryptol idiom is the notion of a
function. You have already used built-in functions {\tt +}, {\tt
  take}, etc.{\indPlus\indLg}Of course, users can define their own
functions as well. The recommended method is to define them in a file
and load it, as in the next exercises.

\note{Reviewing the contents of \autoref{sec:technicalities} might
  help at this point. Especially the commands that will let you load
  files ({\tt :l} and {\tt :r}) in Cryptol.\indCmdLoad\indCmdReload}

\begin{Exercise}\label{ex:fn:0}
  Type the following definition into a file and save it. Then load it
  in Cryptol and experiment.
\begin{code}
  increment : [8] -> [8]
  increment x = x+1
\end{code}
In particular, try the following invocations:
\begin{Verbatim}
  increment 3
  increment 255
  increment 912
\end{Verbatim}
Do you expect the last call to type-check?
\end{Exercise}
\begin{Answer}\ansref{ex:fn:0}
Here are some example uses of {\tt increment}:
\begin{Verbatim}
  Cryptol> increment 3
  4
  Cryptol> increment 255
  0
  Cryptol> increment 912
    [error] at <interactive>:1:1--1:14:
      Unsolvable constraint: 8 >= 10
\end{Verbatim}
Note how type inference rejects application when applied to an
argument of the wrong size: 912 is too big to fit into 8 bits.
\end{Answer}

\note{We do not have to parenthesize the argument to {\tt
    increment}, as in {\tt increment(3)}. Function application is
  simply juxtaposition in Cryptol. However, you can write the
  parentheses if you want to, and you must use parentheses if you want
  to pass a negative argument, e.g. \texttt{increment(-2)} (recall
  \exerciseref{ex:arith:5}).}\indFunApp

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Definitions in the interpreter with \texttt{let}}
\label{sec:interpreter-let}
As an alternative to creating and loading files, the Cryptol
interpreter also supports simple definitions with \texttt{let}. Unlike
file-based definitions, \texttt{let}-definitions are not persistent:
They exist only until the same name is redefined or the interpreter
exits with \texttt{:q}.
\begin{code}
  Cryptol> let r = {a = True, b = False}
  Cryptol> r.a
  True
\end{code}
Simple functions can be defined in the interpreter with \texttt{let},
with some restrictions: They must be entered on a single line, and
they cannot have a separate type signature. If we want a function to
have a more specific type than the inferred one, we must add type
annotations to variables or other parts of the body of the definition.
\begin{Exercise}\label{ex:fn:0b}
  Enter the following definition into the Cryptol interpreter and check
  its type with \texttt{:t}.
\begin{code}
  let increment x = x+1
\end{code}
Modify the \texttt{let}-definition of \texttt{increment} so that it
will have type \texttt{[8] -> [8]}.
\end{Exercise}
\begin{Answer}\ansref{ex:fn:0b}
Any of the following definitions will have the desired type:
\begin{Verbatim}
  Cryptol> let increment (x : [8]) = x + 1
  Cryptol> let increment x = (x : [8]) + 1
  Cryptol> let increment x = x + (1 : [8])
  Cryptol> let increment x = x + 1 : [8]
  Cryptol> :t increment
  increment : [8] -> [8]
\end{Verbatim}
\end{Answer}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Local names: {\ttfamily{\textbf where}} clauses}
\label{sec:local-names:-ttfam}

You can create local bindings in a {\tt where} clause\indWhere, to
increase readability and give names to common subexpressions.

\begin{Exercise}\label{ex:fn:1}
Define and experiment with the following function:
\begin{code}
  twoPlusXY : ([8], [8]) -> [8]
  twoPlusXY (x, y) = 2 + xy
     where xy = x * y
\end{code}
What is the signature of the function telling us?
\end{Exercise}
\begin{Answer}\ansref{ex:fn:1}
  The signature indicates that {\tt twoPlusXY} is a function that
  takes two 8-bit words as a tuple, and returns an 8-bit word.
\end{Answer}
\note{When calling {\tt twoPlusXY}, you do need to parenthesize the
  arguments.  But this is because you are passing it a
  tuple!\indFunApp The parentheses there are not for the application
  but rather in construction of the argument tuple.  Cryptol does not
  automatically convert between tuples and curried application like in
  some other programming languages (e.g., one cannot pass a pair of
  type \texttt{(a, b)} to a function with type \texttt{a -> b -> c}).}

\begin{Exercise}\label{ex:fn:1.1}
  Comment out the type signature of {\tt twoPlusXY} defined above, and
  let Cryptol infer its type. What is the inferred type? Why?
\end{Exercise}
\begin{Answer}\ansref{ex:fn:1.1}
Here is the type Cryptol infers:
\begin{verbatim}
   Cryptol> :t twoPlusXY
   twoPlusXY : {a} (a >= 2, fin a) => ([a],[a]) -> [a]
\end{verbatim}
That is, our function will actually work over arbitrary (finite) sized
words, as long as they are at least 2 bits wide. The 2-bit requirement
comes from the constant 2, which requires at least 2 bits to
represent.
\end{Answer}

\todo[inline]{The Cmp class has not been introduced yet. At least, add a forward reference to the ``Type classes'' section.}
\begin{Exercise}\label{ex:fn:2}
Define a function with the following signature:
\begin{Verbatim}
   minMax4 : {a} (Cmp a) => [4]a -> (a, a)
\end{Verbatim}
such that the first element of the result is the minimum and the
second element is the maximum of the given four elements. What happens
when you try:
\begin{Verbatim}
   minMax4 [1 .. 4]
   minMax4 [1 .. 5]
\end{Verbatim}
\end{Exercise}
\noindent Why do we need the {\tt \Verb+(Cmp a)+} constraint?\indFin
\begin{Answer}\ansref{ex:fn:2}
Here is one way of defining this function:\indMin\indMax
\begin{code}
  minMax4 : {a} (Cmp a) => [4]a -> (a, a)
  minMax4 [a, b, c, d] = (e, f)
      where   e = min a (min b (min c d))
              f = max a (max b (max c d))
\end{code}
Note that ill-typed arguments will be caught at compile time! So, the
second invocation with the 5 element sequence will fail to type-check.
The {\tt Cmp a} constraint arises from the types of {\tt min} and {\tt
  max} primitives:\indMin\indMax
\begin{Verbatim}
  min, max : {a} (Cmp a) => a -> a -> a
\end{Verbatim}
\end{Answer}

\begin{Exercise}\label{ex:fn:3}
  Define a function {\tt butLast} that returns all the elements of a
  given non-empty sequence, except for the last.  How can you make
  sure that {\tt butLast} can never be called with an empty sequence?
  \lhint{You might find the Cryptol primitive functions {\tt reverse}
    and {\tt width} useful.}\indReverse\indTail\indWidth
\end{Exercise}
\begin{Answer}\ansref{ex:fn:3}
  Using {\tt reverse} and {\tt tail}, {\tt butLast} is easy to
  define:\indReverse\indTail\indWidth
\begin{code}
  butLast : {n, t} (fin n) => [n+1]t -> [n]t
  butLast xs = reverse (tail (reverse xs))
\end{code}
Here is another way to define {\tt butLast}:
\begin{code}
  butLast' : {count, x} (fin count) => [count+1]x -> [count]x
  butLast' xs = take`{count} xs
\end{code}
The type signature sets {\tt count} to the desired width of the
output, which is one shorter than the width of the input:\indSignature
\begin{Verbatim}
  Cryptol> butLast []

  [error] at <interactive>:1:1--1:11:
    Unsolved constraint:
      0 >= 1
        arising from
        matching types
        at <interactive>:1:1--1:11
\end{Verbatim}
At first the error message might be confusing. What Cryptol is telling
us that it deduced {\tt count+1} must be {\tt 1}, which makes {\tt
  count} have value {\tt 0}.  But the {\tt count+1} we gave it was 0,
which is not greater than or equal to 1.

Finally, note that {\tt butLast} requires a finite sequence as input,
for obvious reasons, and hence the {\tt fin n} constraint.
\end{Answer}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{\texorpdfstring{\lamexs}{Lambda-expressions}}\label{sec:lamex}

One particular use case of a {\tt where} clause\indWhere is to
introduce a helper function. If the function is simple enough, though,
it may not be worth giving it a name. A \lamex fits the bill in these
cases, where you can introduce an unnamed function as an
expression. The syntax differs from ordinary definitions in two minor
details: instead of the name we use the backslash or ``whack''
character, `{\tt \Verb|\|}', and the equals sign is replaced by an
arrow `{\tt ->}'.  (Since these functions do not have explicit names,
they are sometimes referred to as ``anonymous functions'' as well. We
prefer the term \lamex, following the usual functional programming
terminology~\cite{Has98}.)

\todo[inline]{See
  \href{https://www.galois.com/cryptol/ticket/89}{ticket \#89}.  This
  is also about our currently-weak REPL.}

Below is an example of a \lamex, allowing us to write functions inline:
\begin{Verbatim}
  Cryptol> f 8 where f x = x+1
  9
  Cryptol> (\x -> x+1) 8
  9
\end{Verbatim}
\lamexs are especially handy when you need to write small functions at
the command line while interacting with the interpreter.

\todo[inline]{Is the following note exclusively from Cryptol v1?  Should there
  by a ticket?  Removing for now.}

% \note{The former example shows that in addition to the
%     indentation-based layout, Cryptol supports the more traditional
%     curly-braces and semicolon-based block structure. At the command
%     line is the primary (only?) place this option should be used.}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Using {\tt zero} in functions}
\label{sec:using-tt-zero}

The constant {\tt zero}\indZero comes in very handy in Cryptol
whenever we need a polymorphic shape that consists of all {\tt False}
bits. The following two exercises utilize {\tt zero} to define the
functions {\tt all}\indAll and {\tt any}\indAny which, later in this
book, you will find are very helpful functions for producing boolean
values from a sequence.

\begin{Exercise}\label{ex:zero:1}
Write a function {\tt all} with the following signature:\indAll
\begin{code}
  all : {n, a} (fin n) => (a -> Bit) -> [n]a -> Bit
\end{code}
such that {\tt all f xs} returns {\tt True} if all the elements in the
sequence {\tt xs} yield {\tt True} for the function {\tt f}.
\lhint{Use a complemented {\tt zero}.} You should see:
\begin{Verbatim}
  Cryptol> all eqTen [10, 10, 10, 10] where eqTen x = x == 10
  True
  Cryptol> all eqTen [10, 10, 10, 5] where eqTen x = x == 10
  False
\end{Verbatim}
(The {\tt where} clause introduces a local definition that is in scope
in the current expression. We have seen the details in
\autoref{sec:funcs}.\indWhere) What is the value of {\tt all f []}
for an arbitrary function {\tt f}? Is this reasonable?
\end{Exercise}
\begin{Answer}\ansref{ex:zero:1}
\begin{code}
  all f xs = [ f x | x <- xs ] == ~zero
\end{code}
Note how we apply {\tt f} to each element in the sequence and check
that the result consists of all {\tt True}s, by using a complemented
{\tt zero}.\indZero\indAll If we pass {\tt any} an empty sequence,
then we will always get {\tt False}:
\begin{Verbatim}
  Cryptol> any eqTen [] where eqTen x = x == 10
  False
\end{Verbatim}
This is intuitively the correct behavior as well. The predicate is
satisfied by {\em none} of the elements in the sequence, but {\tt any}
requires at least one.
\end{Answer}

\begin{Exercise}\label{ex:zero:2}
Write a function {\tt any} with the following signature:\indAny
\begin{code}
  any : {n, a} (fin n) => (a -> Bit) -> [n]a -> Bit
\end{code}
such that {\tt any f xs} returns {\tt True} if any the elements in the
sequence {\tt xs} yield {\tt True} for the function {\tt f}. What is
the value of {\tt any f []}? Is this reasonable?
\end{Exercise}
\begin{Answer}\ansref{ex:zero:2}
\begin{code}
  any f xs = [ f x | x <- xs ] != zero
\end{code}
This time all we need to make sure is that the result is not {\tt
  zero}, i.e., at least one of elements yielded {\tt
  True}.\indZero\indTrue\indAny If we pass {\tt all} an empty
sequence, then we will always get {\tt False}:
\begin{Verbatim}
  Cryptol> any eqTen [] where eqTen x = x == 10
  False
\end{Verbatim}
Again, this is the correct response since there are no elements in an
empty sequence that can satisfy the predicate.
\end{Answer}

%=====================================================================
% \section{Recursion and recurrences}
% \label{sec:recandrec}
\sectionWithAnswers{Recursion and recurrences}{sec:recandrec}

\todo[inline]{This section represents a big opportunity to emphasize sequence
  comprehensions, motivate comprehensions, and help readers think that
  way.}

\todo[inline]{This section is weak on recurrances.  What is the \emph{real}
  impact of a functional vs. recurrance style to the use of Cryptol?
  Does such a style choice only impact type checking or inference, or
  does it have implications on execution and reasoning (wrt
  performance, overhead, soundness, and completeness)?}

Cryptol allows both recursive function and value definitions. A
recursive function is one that calls itself in its definition. Cryptol
also allows the more general form of mutual recursion, where multiple
functions can call each other in a cyclic
fashion.\indRecurrence\indRecursion

\todo[inline]{Are there any standard limitations witnessed in other systems
  that are inherent in Cryptol as well that should be mentioned?
  E.g., how does the typing infrastructure handle missing fixpoints?
  Chains of type synonyms?  Funny interactions with the module system?
  Remind the reader about forward-referencing types.  }

\begin{Exercise}\label{ex:recfun:1}
  Define two functions {\tt isOdd} and {\tt isEven} that each take
  a finite arbitrary sized word and returns a {\tt Bit}. The functions
  should be mutually recursive. What extra predicates do you have to
  put on the size?\indPredicates
\end{Exercise}
\begin{Answer}\ansref{ex:recfun:1}
\begin{code}
  isOdd, isEven : {n} (fin n, n >= 1) => [n] -> Bit
  isOdd  x = if x == 0 then False else isEven (x - 1)
  isEven x = if x == 0 then True  else isOdd  (x - 1)
\end{code}
The extra predicate we need to add is {\tt n >=
  1}.\indFin\indPredicates\indEq\indMinus This constraint comes from
the subtraction with 1, which requires at least 1 bit to represent.
\end{Answer}

\begin{Exercise}\label{ex:recfun:1:5}
  While defining {\tt isOdd} and {\tt isEven} mutually recursively
  demonstrates the concept of recursion, it is not the best way of
  coding these two functions in Cryptol. Can you implement them using
  a constant time operation? \lhint{What is the least significant bit
    of an even number? How about an odd one?}
\end{Exercise}
\begin{Answer}\ansref{ex:recfun:1:5}
  A number is even if its least least significant bit is {\tt False},
  and odd otherwise. Hence, we can define these functions as:
\begin{code}
  isOdd', isEven' : {n} (fin n, n >= 1) => [n] -> Bit
  isOdd'  x = x ! zero
  isEven' x = ~(x ! zero)
\end{code}
Note the use of {\tt zero} which permits Cryptol to choose the width
of the \texttt{0} constant appropriately.
\end{Answer}

\paragraph*{Recurrences} While Cryptol does support recursion, the
explicit recursive function style is typically discouraged: Arbitrary
recursion is hard to compile down to hardware.\indRecurrence A much
better notion is that of {\em recurrences}. A recurrence is a way of
cyclically defining a value, typically a stream.\indStream It turns
out that most recursive functions can be written in a recurrence style
as well, something that might first come as a surprise. In particular,
most recursive definitions arise from recurrence equations in
cryptographic and data flow style programs, and Cryptol's
comprehensions can succinctly represent these computations.\indComp

\todo[inline]{Justify/explain the following exercise's purpose and location.
  Show the solution using recursion, either before or after, and show
  how to convert from the recursive solution to the recurrance-based
  one.  Show the limitations of the recursive version.}

\begin{Exercise}\label{ex:recfun:2}
  In this exercise, we will define a function to compute the maximum
  element in a given sequence of numbers.  Define the following
  function and load it into Cryptol:\indRIndex\indRecurrence\indMax
\begin{code}
  maxSeq xs = ys ! 0
      where ys = [0] # [ max x y | x <- xs
                                 | y <- ys
                       ]
\end{code}
What is the type of {\tt maxSeq}? Try out the following calls:
\begin{Verbatim}
  maxSeq []
  maxSeq [1 .. 10]
  maxSeq ([10, 9 .. 1] # [1 .. 10])
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:recfun:2}
The type of {\tt maxSeq} is:
\begin{Verbatim}
  maxSeq : {a, b} (fin a, fin b) => [a][b] -> [b]
\end{Verbatim}
It takes a sequence of words and returns a word of the same size.  The
suggested expressions produce {\tt 0}, {\tt 10}, and {\tt 10},
respectively.
\end{Answer}

\paragraph*{Patterns of recurrence} The definition pattern for {\tt ys}
in the definition of {\tt maxSeq} above is very common in Cryptol, and
it is well worth understanding it clearly.\indRecurrence The basic
idea is to create a sequence of running results, for each prefix of
the input.

\begin{Exercise}\label{ex:recfun:3}
  Define a variant of {\tt maxSeq} (let us call it {\tt maxSeq'})
  which returns the sequence {\tt ys}, instead of its last element.
\end{Exercise}
\begin{Answer}\ansref{ex:recfun:3}
  We can simply drop the selection of the last element ({\tt ! 0}),
  and write {\tt maxSeq'} as follows:\indRIndex
\begin{code}
  maxSeq' : {a, b} (fin a,fin b) => [a][b] -> [1 + a][b]
  maxSeq' xs = ys
      where ys = [0] # [ max x  y | x <- xs
                                  | y <- ys
                       ]
\end{code}
\end{Answer}

\paragraph*{Running results} It is very instructive to look at the
results returned by {\tt maxSeq'} that you have just defined in
\exerciseref{ex:recfun:3}:
\begin{Verbatim}
  Cryptol> maxSeq' []
  [0]
  Cryptol> maxSeq' [1 .. 10]
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  Cryptol> maxSeq' [10, 9 .. 1]
  [0, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10]
  Cryptol> maxSeq' [1, 3, 2, 4, 3, 5, 4, 7, 6, 8, 5]
  [0, 1, 3, 3, 4, 4, 5, 5, 7, 7, 8, 8]
\end{Verbatim}
We clearly see the running results as they accumulate in {\tt ys}. For
the empty sequence, it only has {\tt [0]} in it. For the monotonically
increasing sequence {\tt \Verb+[1 .. 10]+}, the maximum value keeps
changing at each point, as each new element of {\tt xs} is larger than
the previous running result. When we try the sequence that always goes
down ({\tt [10, 9 .. 1]}), we find that the running maximum never
changes after the first. The mixed input in the last call clearly
demonstrates how the execution proceeds, the running maximum changing
depending on the next element of {\tt xs} and the running maximum so
far. In the {\tt maxSeq} function of \exerciseref{ex:recfun:2}, we
simply project out the last element of this sequence, obtaining the
maximum of all elements in the given sequence.

\paragraph*{Folds}\label{par:fold}\indFold The pattern of recurrence
employed in {\tt maxSeq} is an instance of what is known as a {\em
  fold}~\cite{Bir98}.  Expressed in Cryptol terms, it looks as
follows:
\begin{Verbatim}
  ys = [i] # [ f (x, y) | x <- xs
                        | y <- ys
             ]
\end{Verbatim}
where {\tt xs} is some input sequence, {\tt i} is the result
corresponding to the empty sequence, and {\tt f} is a transformer to
compute the next element, using the previous result and the next
input.  This pattern can be viewed as generating a sequence of running
values, accumulating them in {\tt ys}. To illustrate, if {\tt xs} is a
sequence containing the elements {\tt [$x_1$, $x_2$, $x_3$ .. $x_n$]},
then successive elements of {\tt ys} will be:
\begin{Verbatim}[commandchars=\\\{\}, codes={\catcode`$=3\catcode`^=7\catcode`_=8}]
  $y_0$ = i
  $y_1$ = f($x_1$, i)
  $y_2$ = f($x_2$, $y_1$)
  $y_3$ = f($x_3$, $y_2$)
  ...
  $y_n$ = f($x_n$, $y_{n-1}$)
\end{Verbatim}
Note how each new element of {\tt ys} is computed by using the
previous element and the next element of the input. The value {\tt i}
provides the seed. Consequently, {\tt ys} will have one more element
than {\tt xs} does.

\paragraph*{While loops} An important use case of the above pattern is
when we are interested in the final value of the accumulating values,
as in the definition of {\tt maxSeq}.\indRIndex\indWhileLoop When used
in this fashion, the execution is reminiscent of a simple while loop
that you might be familiar with from other languages, such as C:
\begin{Verbatim}[commandchars=\\\{\}, codes={\catcode`$=3\catcode`^=7\catcode`_=8}]
  -- C-like Pseudo-code!
  y = i;                 // running value, start with i
  idx = 0;               // walk through the xs "array" using idx
  while(idx < length (xs)) \{
    y = f (xs[idx], y);  // compute next elt using the previous
    ++idx;
  \}
  return y;
\end{Verbatim}
\note{If the while-loop analogy does not help you, feel free to ignore
  it. It is not essential. The moral of the story is this: if you feel
  like you need to write a while-loop in Cryptol to compute a value
  dependent upon the values in a datatype, you probably want to use a
  fold-like recurrence instead.}

\begin{Exercise}\label{ex:recfun:4}
  Define a function that sums up a given sequence of elements. The
  type of the function should be:
\begin{Verbatim}
  sumAll : {n, a} (fin n, fin a) => [n][a] -> [a]
\end{Verbatim}
\lhint{Use the folding pattern to create a sequence containing the
  partial running sums.  What is the last element of this sequence?}
Try it out on the following examples:
\begin{Verbatim}
  sumAll []
  sumAll [1]
  sumAll [1, 2]
  sumAll [1, 2, 3]
  sumAll [1, 2, 3, 4]
  sumAll [1, 2, 3, 4, 5]
  sumAll [1 .. 100]
\end{Verbatim}
Be mindful that if you do not specify the width of the result that you
may get unexpected answers.

\end{Exercise}
\begin{Answer}\ansref{ex:recfun:4}
  Here is one answer.  Note that in this solution the width of the
  answer is specified in terms of the width of the elements, so is
  likely to overflow. You can prevent the overflow by explicitly
  specifying the width of the output.
\begin{code}
  sumAll : {n, a} (fin n, fin a) => [n][a] -> [a]
  sumAll xs = ys ! 0
    where ys = [0] # [ x+y | x <- xs
                           | y <- ys
                     ]
\end{code}
\todo[inline]{Insert the recursive solution and a transformation to the
  recurrance-based solution.}
\todo[inline]{Show how rich the type of \texttt{sumAll} could be with a full
  dependent type system vs. what we have at our disposal.}
In this code, the sequence {\tt ys} contains the partial running sums.
This is precisely the same pattern we have seen in
Example~\ref{ex:recfun:3}.  The output for the example calls are:
\begin{Verbatim}
  CrashCourse> sumAll []
  0
  CrashCourse> sumAll [1]
  1
  CrashCourse> sumAll [1, 2]
  3
  CrashCourse> sumAll [1, 2, 3]
  2
  CrashCourse> sumAll [1, 2, 3, 4]
  2
  CrashCourse> sumAll [1, 2, 3, 4, 5]
  7
  CrashCourse> sumAll [1 .. 100]
  58
\end{Verbatim}
If we do not explicitly tell Cryptol how wide the result is, then it
will pick the width of the input elements, which will cause overflow
and be subject to modular arithmetic as usual. Experiment with
different signatures for {\tt sumAll}, to avoid the overflow
automatically, to get the answers:
\begin{Verbatim}
  0
  1
  3
  6
  10
  15
  5050
\end{Verbatim}
\indModular\indOverflow \end{Answer}

\begin{Exercise}\label{ex:recfun:4:1}
Define a function {\tt elem} with the following signature:\indElem
\begin{code}
  elem : {n, t} (fin n, Cmp t) => (t, [n]t) -> Bit
\end{code}
such that {\tt elem (x, xs)} returns {\tt True} if {\tt x} appears in
{\tt xs}, and {\tt False} otherwise.
\end{Exercise}
\begin{Answer}\ansref{ex:recfun:4:1}
Using a fold, it is easy to write {\tt elem}:
\begin{code}
  elem (x, xs) = matches ! 0
      where matches = [False] # [ m || (x == e) | e <- xs
                                                | m <- matches
                                ]
\end{code}
Note how we or ({\tt ||})\indOr the previous result {\tt m} with the
current match, accumulating the result as we walk over the
sequence. Starting with {\tt False} ensures that if none of the
matches succeed we will end up returning {\tt False}.  We have:
\begin{Verbatim}
  Cryptol> elem (2, [1..10])
  True
  Cryptol> elem (0, [1..10])
  False
  Cryptol> elem (10, [])
  False
\end{Verbatim}
\end{Answer}

\paragraph*{Generalized folds} The use of fold we have seen above is
the simplest use case for recurrences in Cryptol.\indFold\indComp It
is very common to see Cryptol programs employing some variant of this
idea for most of their computations.

\todo[inline]{Again, show the recursion-based version of \texttt{fibs}
  and its limitations.}

\begin{Exercise}\label{ex:recfun:5}
  Define the sequence of Fibonacci numbers {\tt fibs}, so that {\tt
    fibs @ n} is the $n^{\mbox{th}}$ Fibonacci
  number~\cite{wiki:fibonacci}.\glosFibonacci You can assume 32 bits
  is sufficient for representing these fast growing
  numbers. \lhint{Use a recurrence where the seed consists of two
    elements.}
\end{Exercise}
\begin{Answer}\ansref{ex:recfun:5}
\begin{code}
  fibs : [inf][32]
  fibs = [0, 1] # [ x+y | x <- fibs
                        | y <- drop`{1} fibs
                  ]
\end{code}
In this case we use the sequence {\tt [0, 1]} as the seed, and both
branches recursively refer to the defined value {\tt fibs}. In the
second branch, we {\tt drop} the first element to skip over the first
element of the sequence, effectively pairing the previous two elements
at each step.  The $n$th fibonacci number is obtained by the
expression {\tt fibs @ n}:\indDrop
\begin{Verbatim}
  Cryptol> fibs @ 3
  2
  Cryptol> fibs @ 4
  3
  Cryptol> take`{10} fibs
  [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
\end{Verbatim}
Note that {\tt fibs} is an infinite stream of 32 bit numbers, so it
will eventually be subject to wrap-around due to modular
arithmetic.\indModular
\end{Answer}

%=====================================================================
% \section{Stream equations}
% \label{sec:streameq}
\sectionWithAnswers{Stream equations}{sec:streameq}

Most cryptographic algorithms are described in terms of operations on
bit-streams.\indStreamEquation A common way of depicting operations on
bit-streams is using a {\it stream equation,} as shown in
Figure~\ref{fig:streamDiagram}:
\begin{figure}[htbp]
\centering
\includegraphics[width=3.5in]{crashCourse/streamDiagram.pdf}
\caption{Equation for producing a stream of {\tt as}}
\label{fig:streamDiagram}
\end{figure}

In this diagram the stream is seeded with four initial values ({\tt
  3F, E2, 65, CA}). The subsequent elements ({\tt new}) are appended
to the stream, and are computed by xor-ing the current stream element
with two additional elements extracted from further into the stream.
The output from the stream is a sequence of values, known as $a$s.

The Cryptol code corresponding to this stream equation is:
\begin{code}
  as  = [0x3F, 0xE2, 0x65, 0xCA] # new
    where
      new = [ a ^ b ^ c | a <- as
                        | b <- drop`{1} as
                        | c <- drop`{3} as ]
\end{code}

% \vfill
% \eject
\todo[inline]{Make sure pagination looks good, particularly for figures.}

\begin{Exercise}\label{ex:streamEq}
  Write the Cryptol code corresponding to the stream equation in
  Figure~\ref{fig:streamExercise}:
\end{Exercise}
\begin{figure}[htbp]
\centering
\includegraphics[width=4in]{crashCourse/streamExercise}
\caption{Equation for producing a stream of {\tt as} from an initial
  seed and an input stream.}
\label{fig:streamExercise}
\end{figure}

\todo[inline]{Update diagram so that the output is called \texttt{xs}
  to match the solution and we avoid getting an error from Cryptol
  about redeclaration of \texttt{as}.}

\begin{Answer}\ansref{ex:streamEq}
\begin{code}
  xs input = as where
     as = [0x89, 0xAB, 0xCD, 0xEF] # new
     new = [ a ^ b ^ c | a <- as
                       | b <- drop`{2} as
                       | c <- input ]
\end{code}
\end{Answer}

%=====================================================================
% \section{Type synonyms}
% \label{sec:tsyn}
\sectionWithAnswers{Type synonyms}{sec:tsyn}\indTypSynonym

\todo[inline]{Motivate type synonyms better; NQueens with nice
  synonyms is a good example.}

\todo[inline]{Should we insert a section on currying-style functions
  vs. tuples here?}

Types in Cryptol can become fairly complicated, especially in the
presence of records.  Even for simple types, meaningful names should
be used for readability and documentation.  Type synonyms allow users
to give names to arbitrary types.  In this sense, they are akin to
{\tt typedef} declarations in C~\cite{TheCProgrammingLanguage}.
However, Cryptol's type synonyms are significantly more powerful than
C's {\tt typedef}s, since they can be parameterized by other types,
much like in Haskell~\cite{Has98}.

\todo[inline]{Add a discussion of N-queens or AES or something more compelling
  to show off type synonyms.}

Here are some simple type synonym definitions:
\begin{code}
  type Word8       = [8]
  type CheckedWord = (Word8, Bit)
  type Point a     = {x : [a], y : [a]}
\end{code}

\todo[inline]{2.0: Rewrite this paragraph and much of this section.}

Type synonyms are either unparameterized (as in {\tt Word8} and {\tt
  CheckedWord}, or parameterized with other types (as in {\tt Point}).
Synonyms may depend upon other synonyms, as in the {\tt CheckedWord}
example.  Once the synonym is given, it acts as an additional name for
the underlying type, making it much easier to read and
maintain.

For instance, we can write the function that returns the x-coordinate
of a point as follows:
\begin{code}
  xCoord : {a} Point a -> [a]
  xCoord p = p.x
\end{code}

Type synonyms look like distinct types when they are printed in the
output of the \texttt{:t} and \texttt{:browse} commands. However,
declaring a type synonym does not actually introduce a new
\emph{type}; it merely introduces a new \emph{name} for an existing
type. When Cryptol's type checker compares types, it expands all type
synonyms first. So as far as Cryptol is concerned, \texttt{Word8} and
\texttt{[8]} are the \emph{same} type. Cryptol preserves type synonyms
in displayed types as a convenience to the user.

For example, consider the following declarations:
%% not "code" to avoid conflicting with previous Word8
\begin{Verbatim}
  type Word8     = [8]
  type Word8'    = [8]
  type B         = Word8
  type A         = B
  type WordPair  = (Word8, Word8')
  type WordPair' = (Word8', Word8)

  foo : Word8 -> Bit
  foo x = True

  bar : Word8' -> Bit
  bar x = foo x
\end{Verbatim}
Within this type context, six different type \emph{names} are
declared, but there are not six distinct \emph{types}. The first four
(\texttt{Word8}, \texttt{Word8'}, \texttt{B}, and \texttt{A}) are all
interchangeable abbreviations for \texttt{[8]}. The last two are
synonymous and interchangeable with the pair type \texttt{([8], [8])}.
Likewise, the function types of \texttt{foo} and \texttt{bar} are
identical; thus \texttt{bar} can call \texttt{foo}.

\begin{Exercise}\label{ex:tsyn:1}
  Define a type synonym for 3-dimensional points and write a function
  to determine if the point lies on any of the 3 axes.
\end{Exercise}
\begin{Answer}\ansref{ex:tsyn:1}
  A point is on the $a^{\text{th}}$ axis if its non-$a^{\text{th}}$
  components are $0$. Hence we have:
\begin{code}
  type Point3D a = {x : [a], y : [a], z : [a]}

  onAnAxis : {a} (fin a) => Point3D a -> Bit
  onAnAxis p = onX || onY || onZ
    where onX = (p.y == 0) && (p.z == 0)
          onY = (p.x == 0) && (p.z == 0)
          onZ = (p.x == 0) && (p.y == 0)
\end{code}
\todo[inline]{Reflect upon this example.}
\end{Answer}

\paragraph*{Predefined type synonyms} The following type synonyms are
predefined in Cryptol:
\begin{Verbatim}
  type Bool = Bit
  type Char = [8]
  type String n = [n]Char
  type Word n = [n]
\end{Verbatim}
For instance, a {\tt String n} is simply a sequence of precisely $n$
8-bit words.\indTSWord\indTSString\indTSBool

\todo[inline]{Discussion of \texttt{String} as a type synonym is an
  important example, so more discussion is warranted.}

%=====================================================================
\section{Type classes}\indTypeClasses
\label{sec:type-classes}

\todo[inline]{This section needs a full rewrite.}

\todo[inline]{Add discussion of Haskell's type classes and other
  typeclass-like things.}

Type classes are a way of describing behaviors shared by multiple
types.  As an example, consider the type of the function {\tt ==}:
\begin{Verbatim}
    Cryptol> :t (==)
    (==) : {a} (Cmp a) => a -> a -> Bit
\end{Verbatim}

This type signature may be read as, ``equality is an operator that
takes two objects of any single type that can be compared and returns
a Bit.''

Cryptol defines a collection of basic type classes: \texttt{Logic},
\texttt{Zero}, \texttt{Cmp}, \texttt{SignedCmp}, \texttt{Arith}, and
\texttt{Literal}. These appear in the type signature of operators and
functions that require them. If a function you define calls, for
example, \texttt{+}, on two arguments both of type \texttt{a}, the
type constraints for \texttt{a} will include \texttt{(Arith a)}.

\begin{itemize}
\item
The \texttt{Logic} typeclass includes the binary logical operators
\texttt{\&\&}, \texttt{||}, and \verb+^+, as well as the unary
operator \verb+~+. Cryptol types made of bits (but not those
containing unbounded integers) are instances of class \texttt{Logic}.

\item
The \texttt{Zero} typeclass includes the special constant
\texttt{zero}. The shifting operators \texttt{<<} and \texttt{>>} are
also in class \texttt{Zero}, because they can shift in zero values.
All of the built-in types of Cryptol are instances of class
\texttt{Zero}.

\item
The \texttt{Cmp} typeclass includes the binary relation operators
\texttt{<}, \texttt{>}, \texttt{<=}, \texttt{>=}, \texttt{==}, and
\texttt{!=}, as well as the binary functions \texttt{min} and
\texttt{max}. Note that equality and other comparisons are bundled
into a single typeclass. Function types are not in class \texttt{Cmp}
and cannot be compared with \texttt{==}, but Cryptol does provide a
special pointwise comparison operator for functions, \texttt{(===) :
  \{a b\} (Cmp b) => (a -> b) -> (a -> b) -> a -> Bit}.

\item
The \texttt{SignedCmp} typeclass includes the binary relation
operators \texttt{<\$}, \texttt{>\$}, \texttt{<=\$}, and
\texttt{>=\$}. These are like \texttt{<}, \texttt{>}, \texttt{<=}, and
\texttt{>=}, except that they interpret bitvectors as \emph{signed}
2's complement numbers, whereas the \texttt{Cmp} operations use the
\emph{unsigned} ordering.

\item
The \texttt{Arith} typeclass includes the binary operators \texttt{+},
\texttt{-}, \texttt{*}, \texttt{/}, \verb+%+, and \verb+^^+, as well
as the unary operators \texttt{lg2} and \texttt{-}, and the function
\texttt{fromInteger}. Infinite sequence enumerations \texttt{[x ...]}
and \texttt{[x, y ...]} are also defined for class \texttt{Arith}.

\item
The \texttt{Literal} typeclass includes numeric literals.
\end{itemize}

\begin{Exercise}\label{ex:tvar:1}
  Without including an explicit type declaration, define a function
  that Cryptol infers has the following type:
\begin{Verbatim}
  cmpArith : {a,b} (Cmp a, Arith b) => a -> a -> b -> b
\end{Verbatim}
\end{Exercise}
\begin{Answer}\ansref{ex:tvar:1}
This code:
\begin{code}
  cmpArith x y z = if x == y then z else z+z
\end{code}
yields the inferred type:
\begin{Verbatim}
  cmpArith : {a, b} (Arith b, Cmp a) => a -> a -> b -> b
\end{Verbatim}
\end{Answer}

%=====================================================================
\section{Type vs.\ value variables}\indTypeVariables
\label{sec:type-vs.-value}

\todo[inline]{Rewrite this whole section to better clarify object- vs. type
  variables, their scope and limitations, where they can be left
  abstract and when and where they must be concretized, etc.}

Its powerful type system is one of the key features of Cryptol. We
have encountered many aspects of types already. You may have noticed,
in functions such as \texttt{split}, that when you call a function in
Cryptol, there are two kinds of parameters you can pass: \textit{value
  variables} and \textit{type variables}.

Consider the \texttt{split} function that we previously examined in
Exercise \autoref{ex:poly:split}. Recall that \texttt{split}'s
type is:
\begin{verbatim}
    split : {parts, each, a} (fin each) =>
              [parts * each]a -> [parts][each]a
\end{verbatim}
When applying \texttt{split}, one typically specifies a concrete
value for the formal parameter \texttt{parts}:
\begin{Verbatim}
    Cryptol> split`{parts=3} [1..12]
    [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
\end{Verbatim}
In this example, the term {\tt\Verb|`{parts=3}|} passes \texttt{3} to
the \texttt{parts} type variable argument, and the \texttt{[1..12]} is
passing a sequence as the first (and only) \textit{value argument}.

A \emph{value variable} is the kind of variable you are used to from
normal programming languages.  This kind of variable represents a
normal run-time value.

A \emph{type variable}, on the other hand, allows you to express
interesting (arithmetic) constraints on \emph{types}. These variables
express things like lengths of sequences or relationships between
lengths of sequences.  Type variable values are computed
statically---they never change at runtime.\footnote{In this way,
  they are similar (but more expressive than) templates in languages
  like C++ or Java. If you want to learn more about this area, look up
  the term ``type-level naturals''.}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Positional vs.\ named type
  arguments}\indTypePositionalArguments
\label{sec:positional-vs.-named}

Cryptol permits type variables to be passed either by name (as in {\tt
  \Verb|`{parts=3}|} above), or by position (leaving out the name).
For functions you define, the position is the order in which the type
variables are declared in your function's type signature. If you are
not sure what that is, you can always use the {\tt :t} command to find
out the position of type variables.

For example:
\begin{Verbatim}
    Cryptol> :t groupBy
    groupBy : {each, parts, elem}
              (fin each) => [parts * each]elem
                         -> [parts][each]elem
\end{Verbatim}
tells us that that {\tt parts} is in the second position of {\tt
  groupBy}'s type signature, so the positional-style call equivalent
to our example is:
\begin{Verbatim}
    Cryptol> groupBy`{_,3}[1..12]
\end{Verbatim}

Note the use of an underscore in order to pass \texttt{3} in the
second position.  Positional arguments are most often used when the
type argument is the first argument and when the name of the argument
does not add clarity.  The {\tt groupBy\Verb|`{_,3}|} is not as
self-explanatory as {\tt groupBy\Verb|`{parts=3}|}.  On the other hand, our
use of positional arguments to {\tt take} in previous chapters is very
clear, as in:
\begin{Verbatim}
    Cryptol> take`{3}[1..12]
    [1, 2, 3]
\end{Verbatim}

\begin{tip}
  Cryptol programs that use named arguments are more maintainable and
  robust during program evolution.  E.g., you can reorder parameters or
  refactor function definitions much more easily if invocations of
  those functions use named, rather than positional, arguments.
\end{tip}

\todo[inline]{What are the implications to type inference, especially
  resolution, in the presence of positional arguments.}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Type context vs.\ variable context}\indTypeContext
\label{sec:type-context-vs}

You have seen, in the discussion of type variables above, that Cryptol
has two kinds of variables---type variables and value variables. Type
variables normally show up in type signatures, and value variables
normally show up in function definitions. Sometimes you may want to
use a type variable in a context where value variables would normally
be used.  To do this, use the backtick character {\tt \Verb|`|}.

The definition of the built-in {\tt width} function is a good example
of the use of backtick:
\begin{Verbatim}
    width : {bits, n, a} (fin n, fin bits, bits >= width n) => [n]a -> [bits]
    width _ = `n
\end{Verbatim}

\begin{tip}
  Note there are some subtle things going on in the above definition
  of \texttt{width}.  First, arithmetic constraints on types are
  position-independent; properties of formal parameters early in a
  signature can depend upon those late in a signature.  Second, type
  constraints can refer to not only other functions, but recursively
  to the function that is being defined (either directly, or
  transitively).

  Type constraints can get pretty crazy in practice, especially deep
  in the signature of crypto code subsystems.  Our suggestion is that
  you should not chase the dragon's tail of feedback from the
  typechecker in attempting to massage your specification's types for
  verification.  Instead, think carefully about the meaning and
  purpose of the concepts in your specification, introduce appropriate
  type synonyms, and ensure that the specification is clear and
  precise.  Trust that the interpreter and the verifier will do the
  right thing.
\end{tip}

The bounds in a finite sequence literal (such as \texttt{[1 ..\ 10]}) in
Cryptol are type-level values because the length of a sequence is part
of its type.  Only type-level values can appear in a finite sequence
definition.  You cannot write \texttt{[a ..\ b]} where either \texttt{a} or
\texttt{b} are arguments to a function.  On the other hand, an infinite
sequence's type is fixed (\texttt{[inf]a}), so the initial value in an
infinite sequence can be a runtime variable or a type variable, but
type variables are escaped here using a {\tt \Verb|`|}.

\todo[inline]{Add an example of this here.  Rewrite if something great
  occurs to us.}

\todo[inline]{A proper discussion of type-context vs.~value-context is
  necessary here, and it should be formalized as part of the type
  system and type-inference algorithm, of course.}

This is probably obvious, but there is no way to get a value variable
to appear in a type context.  Types must be known at ``compile time,''
and (non-literal) values are not, so there is no way to use them in
that way.

\todo[inline]{It is more subtle than this because we do not have a proper
  semantic for when arithmetic terms in types (or expressions, for
  that matter!) are evaluated.}

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\subsection{Inline argument type declarations}\indTypeInline
\label{sec:inline-argument-type}

So far when we have defined a function, we have declared the type of
its arguments and its return value in a separate type declaration.
When you are initially writing code, you might not know exactly what a
function's full type is (including the constraints), but you may know
(and need to express) the types of the function's arguments. Cryptol's
syntax for this should look familiar:
\begin{code}
  addBytes (x:[8]) (y:[8]) = x + y
\end{code}

This defines a function that takes two bytes as input, and returns their sum.
Note that the use of parentheses \texttt{( )} is mandatory.

Here is a more interesting example:
\begin{code}
  myWidth (x:[w]a) = `w
\end{code}

\todo[inline]{Why is this more interesting?  What are the reflections the
  reader should have?}

%=====================================================================
\section{Type constraint synonyms}

Sometimes programs have certain combinations of type constraints that
appear over and over in many places. For convenience, Cryptol allows
the programmer to declare named sets of type constraints that can be
used in other function type signatures. Typically a named type
constraint will have one or more type parameters. The syntax is very
similar to a type synonym declaration:

\begin{code}
  type constraint myConstraint a b = (fin a, fin b, b >= width a)
\end{code}

Wherever a type constraint synonym is used, it is as if its definition
is expanded in place. So the following two signatures would be
equivalent:

\begin{code}
    width : {bits,len,elem} (fin len, fin bits, bits >= width len) =>
            [len] elem -> [bits]
    width : {bits,len,elem} (myConstraint len bits) => [len] elem -> [bits]
\end{code}

%=====================================================================
\section{Program structure with modules}

When a cryptographic specification gets very large it can make sense
to decompose its functions into modules.\indModuleSystem\indImport
Doing this well encourages
code reuse, so it's a generally good thing to do. Cryptol's module
system is simple and easy to use. Here's a quick overview:

A module's name should be the same as the filename the module is
defined in. For example, the \verb+utilities+ module should be
defined in a file called \verb+utilities.cry+. To specify that a file
defines a module, its first non-commented line should be:

\begin{verbatim}
  module utilities where
\end{verbatim}

After that the variables and functions you define will be contained
(in this example) in the {\it utilities} module.

In the code where you want to use a module, you \verb+import+ it like this:
\begin{verbatim}
  import utilities
\end{verbatim}

Cryptol will look for the file \verb+utilities.cry+ in the current directory. Once you've imported a module, all of its variables and functions are available to use in your code.

If you're writing a module that has both {\it private} and {\it public}
definitions, you can hide the ones that shouldn't be exported to modules
that include it by using the \verb+private+ keyword, like this:\indPrivate

\begin{verbatim}
  private internalDouble x = x + x
  exportedDouble = x * 2
\end{verbatim}

As you can tell, by default definitions are exported to including modules.

For a large project it can be convenient to place modules in a directory
structure.  In this case, the directory structure becomes part of the modules'
names.  For example, when placing \verb+SHA3.cry+ in the \verb+Hash+ directory and
accessing it from \verb+HMAC.cry+ you would need to name the modules
accordingly:

\begin{verbatim}

sha3 : {n} (fin n) => [n] -> [512]
sha3 = error "Stubbed, for demonstration only: sha3-512"

blocksize : {n} (fin n, n >= 10) => [n]
blocksize = 576
\end{verbatim}

\begin{verbatim}
module Hash::SHA3 where
import Hash::SHA3

hmac : {keySize, msgSize} (fin keySize, fin msgSize) => [keySize] -> [msgSize] -> [512]
hmac k m = sha3 (ko # sha3 (ki # m))
 where ko    = zipWith (^) kFull (join (repeat 0x5c))
       ki    = zipWith (^) kFull (join (repeat 0x36))
       kFull = if `keySize == blocksize
                 then take (k#zero)
                 else sha3 k
\end{verbatim}

Finally, if you're importing a module that defines something with
a name that you would like to define in your code, you can do a
{\it qualified} import of that module like this:

\begin{verbatim}
  import utilities as util
\end{verbatim}

Now, instead of all the definitions being available in your module,
they are qualified with the name you provided, in this case \verb+util+.
This means you will prefix those names with \verb+util::+ when you call them,
and the unqualified names are able to be defined in your own code.

\begin{verbatim}
  import utilities as util
  // let's say utililities.cry defines "all", and we want to use
  // it in our refined definition of all:
  all xs = util::all xs && (width xs) > 0
\end{verbatim}

%=====================================================================
\section{The road ahead}
\label{sec:road-ahead}

In this introductory chapter, we have seen essentially all of the
language elements in Cryptol. The concepts go deeper, of course, but
you now have enough knowledge to tackle large Cryptol programming
tasks. As with any new language, the more exercises you do, the more
you will feel comfortable with the concepts. In fact, we will take
that point of view in the remainder of this document to walk you
through a number of different examples (both small and large),
employing the concepts we have seen thus far.

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