17 KiB
% Cryptol version 2 Syntax % %
Layout
Groups of declarations are organized based on indentation.
Declarations with the same indentation belong to the same group.
Lines of text that are indented more than the beginning of a
declaration belong to that declaration, while lines of text that are
indented less terminate a group of declarations. Groups of
declarations appear at the top level of a Cryptol file, and inside
where blocks in expressions. For example, consider the following
declaration group
f x = x + y + z
where
y = x * x
z = x + y
g y = y
This group has two declaration, one for f and one for g. All the
lines between f and g that are indented more then f belong to
f.
This example also illustrates how groups of declarations may be nested
within each other. For example, the where expression in the
definition of f starts another group of declarations, containing y
and z. This group ends just before g, because g is indented
less than y and z.
Comments
Cryptol supports block comments, which start with /* and end with
*/, and line comments, which start with // and terminate at the
end of the line. Block comments may be nested arbitrarily.
Examples:
/* This is a block comment */
// This is a line comment
/* This is a /* Nested */ block comment */
Identifiers
Cryptol identifiers consist of one or more characters. The first
character must be either an English letter or underscore (_). The
following characters may be an English letter, a decimal digit,
underscore (_), or a prime ('). Some identifiers have special
meaning in the language, so they may not be used in programmer-defined
names (see Keywords).
Examples:
name name1 name' longer_name
Name Name2 Name'' longerName
Keywords and Built-in Operators
The following identifiers have special meanings in Cryptol, and may not be used for programmer defined names:
Arith Inf export import min property width
Bit SignedCmp extern inf module then
Cmp True fin lg2 newtype type
False else if max pragma where
The following table contains Cryptol's operators and their associativity with lowest precedence operators first, and highest precedence last.
Operator Associativity
==> right
\/ right
/\ right
-> (types) right
!= == not associative
> < <= >= <$ >$ <=$ >=$ not associative
|| right
^ left
&& right
# right
>> << >>> <<< >>$ left
+ - left
* / % /$ %$ left
^^ right
! !! @ @@ left
(unary) - ~ right
Table: Operator precedences.
Numeric Literals
Numeric literals may be written in binary, octal, decimal, or
hexadecimal notation. The base of a literal is determined by its
prefix: 0b for binary, 0o for octal, no special prefix for
decimal, and 0x for hexadecimal.
Examples:
254 // Decimal literal
0254 // Decimal literal
0b11111110 // Binary literal
0o376 // Octal literal
0xFE // Hexadecimal literal
0xfe // Hexadecimal literal
Numeric literals represent finite bit sequences (i.e., they have type
[n]). Using binary, octal, and hexadecimal notation results in bit
sequences of a fixed length, depending on the number of digits in the
literal. Decimal literals are overloaded, and so the length of the
sequence is inferred from context in which the literal is used.
Examples:
0b1010 // : [4], 1 * number of digits
0o1234 // : [12], 3 * number of digits
0x1234 // : [16], 4 * number of digits
10 // : {n}. (fin n, n >= 4) => [n]
// (need at least 4 bits)
0 // : {n}. (fin n) => [n]
Bits
The type Bit has two inhabitants: True and False. These values
may be combined using various logical operators, or constructed as
results of comparisons.
Operator Associativity Description
==> right Short-cut implication
\/ right Short-cut or
/\ right Short-cut and
!= == none Not equals, equals
> < <= >= <$ >$ <=$ >=$ none Comparisons
|| right Logical or
^ left Exclusive-or
&& right Logical and
~ right Logical negation
Table: Bit operations.
If Then Else with Multiway
If then else has been extended to support multi-way
conditionals. Examples:
x = if y % 2 == 0 then 22 else 33
x = if y % 2 == 0 then 1
| y % 3 == 0 then 2
| y % 5 == 0 then 3
else 7
Tuples and Records
Tuples and records are used for packaging multiples values together. Tuples are enclosed in parenthesis, while records are enclosed in braces. The components of both tuples and records are separated by commas. The components of tuples are expressions, while the components of records are a label and a value separated by an equal sign. Examples:
(1,2,3) // A tuple with 3 component
() // A tuple with no components
{ x = 1, y = 2 } // A record with two fields, `x` and `y`
{} // A record with no fields
The components of tuples are identified by position, while the components of records are identified by their label, and so the ordering of record components is not important. Examples:
(1,2) == (1,2) // True
(1,2) == (2,1) // False
{ x = 1, y = 2 } == { x = 1, y = 2 } // True
{ x = 1, y = 2 } == { y = 2, x = 1 } // True
The components of a record or a tuple may be accessed in two ways: via pattern matching or by using explicit component selectors. Explicit component selectors are written as follows:
(15, 20).0 == 15
(15, 20).1 == 20
{ x = 15, y = 20 }.x == 15
Explicit record selectors may be used only if the program contains sufficient type information to determine the shape of the tuple or record. For example:
type T = { sign : Bit, number : [15] }
// Valid definition:
// the type of the record is known.
isPositive : T -> Bit
isPositive x = x.sign
// Invalid definition:
// insufficient type information.
badDef x = x.f
The components of a tuple or a record may also be accessed using pattern matching. Patterns for tuples and records mirror the syntax for constructing values: tuple patterns use parentheses, while record patterns use braces. Examples:
getFst (x,_) = x
distance2 { x = xPos, y = yPos } = xPos ^^ 2 + yPos ^^ 2
f p = x + y where
(x, y) = p
Sequences
A sequence is a fixed-length collection of elements of the same type.
The type of a finite sequence of length n, with elements of type a
is [n] a. Often, a finite sequence of bits, [n] Bit, is called a
word. We may abbreviate the type [n] Bit as [n]. An infinite
sequence with elements of type a has type [inf] a, and [inf] is
an infinite stream of bits.
[e1,e2,e3] // A sequence with three elements
[t .. ] // Sequence enumerations
[t1, t2 .. ] // Step by t2 - t1
[t1 .. t3 ]
[t1, t2 .. t3 ]
[e1 ... ] // Infinite sequence starting at e1
[e1, e2 ... ] // Infinite sequence stepping by e2-e1
[ e | p11 <- e11, p12 <- e12 // Sequence comprehensions
| p21 <- e21, p22 <- e22 ]
Note: the bounds in finite unbounded (those with ..) sequences are type expressions, while the bounds in bounded-finite and infinite sequences are value expressions.
Operator Description
# Sequence concatenation
>> << Shift (right,left)
>>> <<< Rotate (right,left)
>>$ Arithmetic right shift (on bitvectors only)
@ ! Access elements (front,back)
@@ !! Access sub-sequence (front,back)
update updateEnd Update the value of a sequence at a location (front,back)
updates updatesEnd Update multiple values of a sequence (front,back)
Table: Sequence operations.
There are also lifted point-wise operations.
[p1, p2, p3, p4] // Sequence pattern
p1 # p2 // Split sequence pattern
Functions
\p1 p2 -> e // Lambda expression
f p1 p2 = e // Function definition
Local Declarations
e where ds
Note that by default, any local declarations without type signatures are monomorphized. If you need a local declaration to be polymorphic, use an explicit type signature.
Explicit Type Instantiation
If f is a polymorphic value with type:
f : { tyParam }
f = zero
you can evaluate f, passing it a type parameter:
f `{ tyParam = 13 }
Demoting Numeric Types to Values
The value corresponding to a numeric type may be accessed using the following notation:
`{t}
Here t should be a type expression with numeric kind. The resulting
expression is a finite word, which is sufficiently large to accommodate
the value of the type:
`{t} : {w >= width t}. [w]
Explicit Type Annotations
Explicit type annotations may be added on expressions, patterns, and in argument definitions.
e : t
p : t
f (x : t) = ...
Type Signatures
f,g : {a,b} (fin a) => [a] b
Type Synonym Declarations
type T a b = [a] b
Modules
A module is used to group some related definitions.
module M where
type T = [8]
f : [8]
f = 10
Hierarchical Module Names
Module may have either simple or hierarchical names.
Hierarchical names are constructed by gluing together ordinary
identifiers using the symbol ::.
module Hash::SHA256 where
sha256 = ...
The structure in the name may be used to group together related
modules. Also, the Cryptol implementation uses the structure of the
name to locate the file containing the definition of the module.
For example, when searching for module Hash::SHA256, Cryptol
will look for a file named SHA256.cry in a directory called
Hash, contained in one of the directories specified by CRYPTOLPATH.
Module Imports
To use the definitions from one module in another module, we use
import declarations:
// Provide some definitions
module M where
f : [8]
f = 2
// Uses definitions from `M`
module N where
import M // import all definitions from `M`
g = f // `f` was imported from `M`
Import Lists
Sometimes, we may want to import only some of the definitions from a module. To do so, we use an import declaration with an import list.
module M where
f = 0x02
g = 0x03
h = 0x04
module N where
import M(f,g) // Imports only `f` and `g`, but not `h`
x = f + g
Using explicit import lists helps reduce name collisions. It also tends to make code easier to understand, because it makes it easy to see the source of definitions.
Hiding Imports
Sometimes a module may provide many definitions, and we want to use most of them but with a few exceptions (e.g., because those would result to a name clash). In such situations it is convenient to use a hiding import:
module M where
f = 0x02
g = 0x03
h = 0x04
module N where
import M hiding (h) // Import everything but `h`
x = f + g
Qualified Module Imports
Another way to avoid name collisions is by using a qualified import.
module M where
f : [8]
f = 2
module N where
import M as P
g = P::f
// f was imported from M
// but when used it needs to be prefixed by the qualified P
Qualified imports make it possible to work with definitions that happen to have the same name but are defined in different modules.
Qualified imports may be combined with import lists or hiding clauses:
import A as B (f) // introduces B::f
import X as Y hiding (f) // introduces everything but `f` from X
// using the prefix `X`
It is also possible the use the same qualifier prefix for imports from different modules. For example:
import A as B
import X as B
Such declarations will introduces all definitions from A and X
but to use them, you would have to qualify using the prefix B:::.
Private Blocks
In some cases, definitions in a module might use helper functions that are not intended to be used outside the module. It is good practice to place such declarations in private blocks:
module M where
f : [8]
f = 0x01 + helper1 + helper2
private
helper1 : [8]
helper1 = 2
helper2 : [8]
helper2 = 3
The keyword private introduce a new layout scope, and all declarations
in the block are considered to be private to the module. A single module
may contain multiple private blocks. For example, the following module
is equivalent to the previous one:
module M where
f : [8]
f = 0x01 + helper1 + helper2
private
helper1 : [8]
helper1 = 2
private
helper2 : [8]
helper2 = 3
Parameterized Modules
module M where
parameter
type n : # // `n` is a numeric type parameter
type constraint (fin n, n >= 1)
// Assumptions about the parameter
x : [n] // A value parameter
// This definition uses the parameters.
f : [n]
f = 1 + x
Named Module Instantiations
One way to use a parameterized module is trough a named instantiation:
// A parameterized module
module M where
parameter
type n : #
x : [n]
y : [n]
f : [n]
f = x + y
// A module instantiation
module N = M where
type n = 32
x = 11
y = helper
helper = 12
The second module, N, is computed by instantiating the parameterized
module M. Module N will provide the exact same definitions as M,
except that the parameters will be replaced by the values in the body
of N. In this example, N provides just a single definition, f.
Note that the only purpose of the body of N (the declarations
after the where keyword) is to define the parameters for M.
Parameterized Instantiations
It is possible for a module instantiations to be itself parameterized. This could be useful if we need to define some of a module's parameters but not others.
// A parameterized module
module M where
parameter
type n : #
x : [n]
y : [n]
f : [n]
f = x + y
// A parameterized instantiation
module N = M where
parameter
x : [32]
type n = 32
y = helper
helper = 12
In this case N has a single parameter x. The result of instantiating
N would result in instantiating M using the value of x and 12
for the value of y.
Importing Parameterized Modules
It is also possible to import a parameterized module without using a module instantiation:
module M where
parameter
x : [8]
y : [8]
f : [8]
f = x + y
module N where
import `M
g = f { x = 2, y = 3 }
A backtick at the start of the name of an imported module indicates that we are importing a parameterized module. In this case, Cryptol will import all definitions from the module as usual, however every definition will have some additional parameters corresponding to the parameters of a module. All value parameters are grouped in a record.
This is why in the example f is applied to a record of values,
even though its definition in M does not look like a function.