fun curry f x y = f(x,y);
val $ = curry;

fun add x y = x + y;
add 1 2;

(*add 1.1 2.2;*)
fun first x y = x;
val One = first 1;

(*One 2;*)

(* equality poymorphism *)
fun same x y = (x = y);
same 1 2;
same 1 1;
same "gato" "gato";

(* True polymorphism *)
(* C or Java: Ad Hoc polymorphism *)
(* function composition with two arguments *)
(* fun (f oo g) x y = f  (g x y); *)

fun add x y = x + y;
fun mul x y = x * y;
fun double x = 2 * x;

(* second element *)
val second = hd o tl;
(* apply a function twice *)
fun twice f = f o f;
twice double 4;

fun square x = x * x;
val divd   = $ op/;
val inverse = $ op/ 1.0;

val f1 = inverse o real o square;
f1 2;

(* combinators *)
fun I x      = x;		(* identity *)
fun K x y    = x;		(* select *)
fun S x y z  = x z (y z);	(* composition *)

val square = S mul I;
square 5;



(* Higher order functions for lists *)
(* map: apply function f to each element in the list *)
fun map (f:'a->'b) (nil:'a list) :('b list) = []
 |  map f (x::xs) = (f x) :: map f xs;
(* filter: return a list where the elements pass test f *)
fun filter (f:'a->bool) (nil:'a list) :('a list)= []
 |  filter f (x::xs) = 
      if f x 
      then x :: filter f xs
      else      filter f xs;
(* reduce: combine all the elments in the list *)
(* u is the identity element for operation f *)
fun reduce (f:'a->'b->'b) (u:'b) (nil:'a list) :'b = u
 |  reduce f u (x::xs) = f x (reduce f u xs);

 (* create a list of integers from lo to hi*)
fun fromTo lo hi =
  if lo > hi 
  then []
  else lo :: fromTo (lo+1) hi;
(* create a list of integers from 0 to ...*)
val upto = fromTo 0;

(* more complex function *)
fun combine nil nil = []
 |  combine (x::xs) (y::ys) = (x, y) :: combine xs ys;
combine [1, 2, 3] [10, 20, 30];
(* flipadd takes one pascal list and generates the next *)
fun flipadd xs = map (op +) (combine ([0] @ xs) (xs @ [0]));
flipadd [1];
flipadd [1, 1];
flipadd [1, 2, 1];
flipadd [1, 3, 3, 1];
(* generates "n" lists
   start is the "seed" list
   function "next" generates a next list
   based on the previous one *)
fun genlist start next 0 = nil
 |  genlist start next n = start :: genlist (next start) next (n-1);
(* generates n pascal lists *)
val pascal = genlist [1] flipadd;
pascal 4;

(* prime numbers example *)
fun divides n i = (n mod i = 0);
fun factors n = filter (divides n) (fromTo 2 (n - 1));
factors 50;

(* test if something is null *)
fun null x = (x = nil);
(* test for primality *)
(* the list of all numbers that divide n is null *)
val prime = null o factors;
(* make a list of prime numbers *)
val primes = filter prime o upto;
(* use it *)
primes 20;

(* From Watt *)
fun sqr x:real = x * x;
val pi = 3.1415;

datatype shape =
  point
  | circle of real
  | box of (real * real);
  
fun area (point)    = 0.0
 |  area (circle r) = pi * sqr (r)
 |  area (box (w, h)) = w * h;

val myBox = box(3.0, 4.0);
area myBox;

(* From Michaelson *)
datatype booleanValue = true | false;

datatype traffic_light = red | red_amber | green | amber;

fun change red 		= red_amber |
    change red_amber	= green |
    change green	= amber |
    change amber	= red;

change red;

datatype inttree = empty | node of int * inttree * inttree;
fun tadd (v:int) empty = node(v, empty, empty)
 |  tadd (v:int) (node(nv:int, l:inttree, r:inttree)) =
      if v < nv
      then node (nv, tadd v l, r)
      else node (nv, l       , tadd v r);

(* use it *)
val root = empty;

val root = tadd 5 root;
val root = tadd 3 root;
val root = tadd 7 root;
val root = tadd 4 root;
val root = tadd 9 root;

(* from tree.ml *)
datatype 'a tree = Lf | Br of 'a * 'a tree * 'a tree;

val tree1 = Br( 1, Br(2, Lf, Lf), Br(3, Lf, Br(4, Lf, Lf)));

(* count non-leaf nodes *)
fun count (Lf) = 0
 |  count (Br(v, t1, t2)) = 1 + count t1 + count t2;
count tree1;

(* depth of the tree *)
fun max (x:int, y:int):int =
  if x > y 
  then x
  else y;
  
fun depth (Lf) = 0
 |  depth (Br(v, t1, t2)) = 1 + max( depth t1, depth t2 );
depth tree1;

(* list with an inorder traversal of the tree *)
fun getin (Lf) = []
 |  getin (Br(v, t1, t2)) = getin(t1) @ [v] @ getin(t2);
getin tree1;

    Source: geocities.com/htrefftz/mum/cs505/smlExamples

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