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Merge pull request #453 from sshine/master

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[standard-ml/en-en] Format fixing, variable renaming, some more about exceptions
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Adam Bard 2013-12-16 12:47:36 -08:00
commit b6e73eeb3a
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@ -13,21 +13,21 @@ to update variables can feel severely inhibiting.
```ocaml
(* Comments in Standard ML begin with (* and end with *). Comments can be
nested which means that all (* tags must end with a *) tag. This comment
contains two nested comments. *)
nested which means that all (* tags must end with a *) tag. This comment,
for example, contains two nested comments. *)
(* A Standard ML program consists of declarations, e.g. value declarations: *)
val rent = 1200
val phone_no = 5551337
val pi = 3.14159
val negative_number = ~15 (* Yeah, unary minus is a so-called 'tilde' *)
val negative_number = ~15 (* Yeah, unary minus uses the 'tilde' symbol *)
(* And just as importantly, functions: *)
fun is_large(x : int) = if x > 37 then true else false
(* Floating-point numbers are called "reals". *)
val tau = 2.0 * pi (* You can multiply reals *)
val twice_rent = 2 * rent (* You can multiply ints *)
val tau = 2.0 * pi (* You can multiply two reals *)
val twice_rent = 2 * rent (* You can multiply two ints *)
(* val meh = 1.25 * 10 *) (* But you can't multiply an int and a real *)
(* +, - and * are overloaded so they work for both int and real. *)
@ -42,16 +42,16 @@ val negative_rent = ~(rent) (* Would also have worked if rent were a "real" *)
(* There are also booleans and boolean operators *)
val got_milk = true
val got_bread = false
val has_breakfast = got_milk andalso got_bread (* Yes, it's called andalso *)
val has_something = got_milk orelse got_bread (* Yes, it's called orelse *)
val has_breakfast = got_milk andalso got_bread (* 'andalso' is the operator *)
val has_something = got_milk orelse got_bread (* 'orelse' is the operator *)
val is_sad = not(has_something) (* not is a function *)
(* Many values can be compared using equality operators: = and <> *)
val pays_same_rent = (rent = 1300) (* false *)
val is_wrong_phone_no = (phone_no <> 5551337) (* false *)
(* The operator <> is what most other languages call != *)
(* The operator <> is what most other languages call !=. *)
(* 'andalso' and 'orelse' are called && and || in many other languages. *)
(* Actually, most of the parentheses above are unnecessary. Here are some
different ways to say some of the things mentioned above: *)
@ -61,7 +61,7 @@ val pays_same_rent = rent = 1300 (* Looks confusing, but works *)
val is_wrong_phone_no = phone_no <> 5551337
val negative_rent = ~rent (* ~ rent (notice the space) would also work *)
(* Parens are mostly necessary when grouping things: *)
(* Parentheses are mostly necessary when grouping things: *)
val some_answer = is_large (5 + 5) (* Without parens, this would break! *)
(* val some_answer = is_large 5 + 5 *) (* Read as: (is_large 5) + 5. Bad! *)
@ -84,32 +84,37 @@ val bar = [ #"H", #"e", #"l", #"l", #"o" ] (* SML also has lists! *)
are functions available in that library that take strings as argument. *)
val bob = String.implode bar (* gives "Hello" *)
val bob_char_count = String.size bob (* gives 5 *)
val _ = print (bob ^ "\n") (* For good measure, add a linebreak *)
val _ = print (bob ^ "\n") (* For good measure, add a linebreak *)
(* You can have lists of any kind *)
val numbers = [1, 3, 3, 7, 229, 230, 248] (* : int list *)
val names = [ "Fred", "Jane", "Alice" ] (* : string list *)
(* Even lists of lists of things *)
val groups = [ [ "Alice", "Bob" ],
[ "Huey", "Dewey", "Louie" ],
[ "Bonnie", "Clyde" ] ] (* : string list list *)
val number_count = List.length numbers (* gives 7 *)
(* You can put single values in front of lists of the same kind
using the :: ("cons") operator *)
(* You can put single values in front of lists of the same kind using
the :: operator, called "the cons operator" (known from Lisp). *)
val more_numbers = 13 :: numbers (* gives [13, 1, 3, 3, 7, ...] *)
val more_groups = ["Batman","Superman"] :: groups
(* Lists of the same kind can be appended using the @ ("append") operator *)
val guest_list = [ "Mom", "Dad" ] @ [ "Aunt", "Uncle" ]
(* This could have been done with the "cons" operator *)
(* This could have been done with the "cons" operator. It is tricky because the
left-hand-side must be an element whereas the right-hand-side must be a list
of those elements. *)
val guest_list = "Mom" :: "Dad" :: [ "Aunt", "Uncle" ]
val guest_list = "Mom" :: ("Dad" :: ("Aunt" :: ("Uncle" :: [])))
(* If you have many lists of the same kind, you can concatenate them all *)
val everyone = List.concat groups (* [ "Alice", "Bob", "Huey", ... ] *)
(* A list can contain any (finite) amount of values *)
(* A list can contain any (finite) number of values *)
val lots = [ 5, 5, 5, 6, 4, 5, 6, 5, 4, 5, 7, 3 ] (* still just an int list *)
(* Lists can only contain one kind of thing... *)
@ -264,21 +269,23 @@ fun map f [] = []
(* 'a is called a type variable. *)
(* We can define functions as infix *)
fun plus (x, y) = x + y
infix plus
(* We can now call plus like "2 plus 5" *)
(* We can declare functions as infix *)
val plus = add_them (* plus is now equal to the same function as add_them *)
infix plus (* plus is now an infix operator *)
val seven = 2 plus 5 (* seven is now bound to 7 *)
(* Functions can also be made infix before they are defined *)
(* Functions can also be made infix before they are declared *)
infix minus
fun x minus y = x - y
fun x minus y = x - y (* It becomes a little hard to see what's the argument *)
val four = 8 minus 4 (* four is now bound to 4 *)
(* An infix function/operator can be made prefix with "op" *)
val n = op + (5, 5)
(* n is now 10 *)
(* An infix function/operator can be made prefix with 'op' *)
val n = op + (5, 5) (* n is now 10 *)
(* op is useful when combined with high order functions *)
val listSum = foldl op + 0 [1,2,3,4,5]
(* 'op' is useful when combined with high order functions because they expect
functions and not operators as arguments. Most operators are really just
infix functions. *)
val sum_of_numbers = foldl op+ 0 [1,2,3,4,5]
(* Datatypes are useful for creating both simple and complex structures *)
@ -291,6 +298,8 @@ fun say(col) =
if col = Blue then "You are blue!" else
raise Fail "Unknown color"
val _ = print (say(Red) ^ "\n")
(* Datatypes are very often used in combination with pattern matching *)
fun say Red = "You are red!"
| say Green = "You are green!"
@ -318,28 +327,40 @@ val myTree = Node (Leaf 9, 8, Node (Leaf 3, 5, Leaf 7))
fun count (Leaf n) = n
| count (Node (leftTree, n, rightTree)) = count leftTree + n + count rightTree
val myTreeCount = count myTree (* myTreeCount is now bound to 32 *)
(* Exceptions! *)
(* Exceptions can be raised using "raise" *)
fun raiseException msg = raise Fail msg
(* This raises exception `Fail "hello from exception"` *)
(* val _ = raiseException "hello from exception" *)
(* Exceptions can be raised/thrown using the reserved word 'raise' *)
fun calculate_interest(n) = if n < 0.0
then raise Domain
else n * 1.04
(* Exceptions can be caught using "handle" *)
val x = raiseException "hello" handle Fail msg => msg
(* x now has the value "hello" *)
val balance = calculate_interest ~180.0
handle Domain => ~180.0 (* x now has the value ~180.0 *)
(* We can pattern match in "handle" to make sure
(* Some exceptions carry extra information with them *)
(* Here are some examples of built-in exceptions *)
fun failing_function [] = raise Empty (* used for empty lists *)
| failing_function [x] = raise Fail "This list is too short!"
| failing_function [x,y] = raise Overflow (* used for arithmetic *)
| failing_function xs = raise Fail "This list is too long!"
(* We can pattern match in 'handle' to make sure
a specfic exception was raised, or grab the message *)
val y = raiseException "..." handle Fail _ => "Fail was raised"
| Domain => "Domain was raised"
(* y now has the value "Fail was raised" *)
val err_msg = failing_function [1,2] handle Fail _ => "Fail was raised"
| Domain => "Domain was raised"
| Empty => "Empty was raised"
| _ => "Unknown exception"
(* err_msg now has the value "Unknown exception" because Overflow isn't
listed as one of the patterns -- thus, the catch-all pattern _ is used. *)
(* We can define our own exceptions like this *)
exception MyException
exception MyExceptionWithMessage of string
exception SyntaxError of string * (int * int)
(* File I/O! *)
(* Write a nice poem to a file *)
@ -372,4 +393,4 @@ val test_poem = readPoem "roses.txt" (* gives [ "Roses are red,",
[SML/NJ](http://smlnj.org/).
* Follow the Coursera course [Programming Languages](https://www.coursera.org/course/proglang).
* Get the book *ML for the Working Programmer* by Larry C. Paulson.
* Use [StackOverflow's sml tag](http://stackoverflow.com/questions/tagged/sml).