(* Programming Languages (F28PL) lecture transcript - 20th September at 11:15 *)

(* A function which returns true of the character is a number *)
Char.isDigit;
Char.isDigit #"5";
Char.isDigit #"0";
Char.isDigit #"9";
Char.isDigit #"a"; (* returns false *)

(* A terribly vulgar function, Char.isDigit function returns a boolean, no reason for equality test *)
if Char.isDigit #"2" = true then true else false;

(* Import the functions from Char namespace using open *)
open Char;

(* Turn a char into an integer (ASCII representation) *)
ord #"5";
ord #"0";
ord #"9";

(* Since we opened the Char namespace, the > operator is replaced *)
5 > 6; (* Will give a type error *)
op >;  (* has type char * char -> bool *)

open Int;
(* We can now use the > operator which takes in int * int *)
op >;

(* isDigit which we imported from the Char namespace is still accessible *)
isDigit;

(* To see what is in a structure without opening it *)
signature S = INTEGER;
signature S = OS;
signature S = POSIX;
signature S = LIST;

val a = 5;
(* 'a' is bound to 5 forever in this scope *)
val a = 6;
(* We now overwritten 'a' with another variable *)

val b = a;
val a = 7;

(* What's in b? *)
b;

(* A function which returns its input *)
fun identity x = x;
(* Equivalent formulation *)
val identity = fn x => x;

(* Runs a variable to a function (square) twice *)
(fun f => fn x => f(f(x))) (fn x => x*x);
val poweroffour = it;
poweroffour 2;
poweroffour 3;

(fn f => fn x => f(f(x)));