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authorGeoff Liu <cangming.liu@gmail.com>2015-06-06 11:34:13 -0600
committerGeoff Liu <cangming.liu@gmail.com>2015-06-06 11:34:13 -0600
commit958093f30eae89579ea23b88874122556378d55c (patch)
tree35b9e705aeadd9b1b5228e219570b9073cba8b10
parent9e44a5b9eec2017ae33ca213efd1c0b1c86e93cb (diff)
parent21c3187f11cbb9926959af23d76743718e96d77b (diff)
Merge pull request #1125 from Inaimathi/master
Add record notes to the Standard ML page
-rw-r--r--standard-ml.html.markdown42
1 files changed, 36 insertions, 6 deletions
diff --git a/standard-ml.html.markdown b/standard-ml.html.markdown
index 07896beb..143980e7 100644
--- a/standard-ml.html.markdown
+++ b/standard-ml.html.markdown
@@ -4,6 +4,7 @@ contributors:
- ["Simon Shine", "http://shine.eu.org/"]
- ["David Pedersen", "http://lonelyproton.com/"]
- ["James Baker", "http://www.jbaker.io/"]
+ - ["Leo Zovic", "http://langnostic.inaimathi.ca/"]
---
Standard ML is a functional programming language with type inference and some
@@ -136,9 +137,29 @@ val mixup = [ ("Alice", 39),
val good_bad_stuff =
(["ice cream", "hot dogs", "chocolate"],
- ["liver", "paying the rent" ]) (* string list * string list *)
+ ["liver", "paying the rent" ]) (* : string list * string list *)
+(* Records are tuples with named slots *)
+
+val rgb = { r=0.23, g=0.56, b=0.91 } (* : {b:real, g:real, r:real} *)
+
+(* You don't need to declare their slots ahead of time. Records with
+ different slot names are considered different types, even if their
+ slot value types match up. For instance... *)
+
+val Hsl = { H=310.3, s=0.51, l=0.23 } (* : {H:real, l:real, s:real} *)
+val Hsv = { H=310.3, s=0.51, v=0.23 } (* : {H:real, s:real, v:real} *)
+
+(* ...trying to evaluate `Hsv = Hsl` or `rgb = Hsl` would give a type
+ error. While they're all three-slot records composed only of `real`s,
+ they each have different names for at least some slots. *)
+
+(* You can use hash notation to get values out of tuples. *)
+
+val H = #H Hsv (* : real *)
+val s = #s Hsl (* : real *)
+
(* Functions! *)
fun add_them (a, b) = a + b (* A simple function that adds two numbers *)
val test_it = add_them (3, 4) (* gives 7 *)
@@ -225,17 +246,26 @@ fun fibonacci 0 = 0 (* Base case *)
| fibonacci 1 = 1 (* Base case *)
| fibonacci n = fibonacci (n - 1) + fibonacci (n - 2) (* Recursive case *)
-(* Pattern matching is also possible on composite types like tuples and lists.
- Writing "fun solve2 (a, b, c) = ..." is in fact a pattern match on the one
- three-tuple solve2 takes as argument. Similarly, but less intuitively, you
- can match on a list consisting of elements in it (from the beginning of the
- list only). *)
+(* Pattern matching is also possible on composite types like tuples, lists and
+ records. Writing "fun solve2 (a, b, c) = ..." is in fact a pattern match on
+ the one three-tuple solve2 takes as argument. Similarly, but less intuitively,
+ you can match on a list consisting of elements in it (from the beginning of
+ the list only). *)
fun first_elem (x::xs) = x
fun second_elem (x::y::xs) = y
fun evenly_positioned_elems (odd::even::xs) = even::evenly_positioned_elems xs
| evenly_positioned_elems [odd] = [] (* Base case: throw away *)
| evenly_positioned_elems [] = [] (* Base case *)
+(* When matching on records, you must use their slot names, and you must bind
+ every slot in a record. The order of the slots doesn't matter though. *)
+
+fun rgbToTup {r, g, b} = (r, g, b) (* fn : {b:'a, g:'b, r:'c} -> 'c * 'b * 'a *)
+fun mixRgbToTup {g, b, r} = (r, g, b) (* fn : {b:'a, g:'b, r:'c} -> 'c * 'b * 'a *)
+
+(* If called with {r=0.1, g=0.2, b=0.3}, either of the above functions
+ would return (0.1, 0.2, 0.3). But it would be a type error to call them
+ with {r=0.1, g=0.2, b=0.3, a=0.4} *)
(* Higher order functions: Functions can take other functions as arguments.
Functions are just other kinds of values, and functions don't need names