(* Programming Languages (F28PL) lecture transcript - 5th October at 15:15 *)

(* Creating value x which refers to 5 *)
val x = ref 5;
x := 6; (* returns a unit *)
(x := 6, x := 7); (* returns a tuple of units *)
val y = 6; (* y is immutable *)
val s = [y]; (* returns [6] *)
val y = 7;
s; (* still returns [6] *)

val s = [x]; (* s = [ref 7] *)
x := 4;
s; (* returns [ref 4] *)

val y = ref 4;
val s = [x,x];
val t = [x,y];
(s, t);
y := 100;

(* Dereferencing x *)
!x;
val z = 5;
!z; (* Will give a type error *)

val s = [ref 5, ref 5, ref 5];
hd (tl s) := 7;
s;

(* The two operatations *)
op :=;
op !;

fun firstof l = (map hd) l;

(* Another way to write the function *)
fun firstof [] = []
  | firstof (h::t) = (hd h)::(firstof t);

(* Also another way to write it *)
fun firstof [] = []
  | firstof ((hh::tt)::t) = hh::(firstof t);

val countdown = [["one"], ["two", "one"], ["three", "two", "one"]];
firstof countdown;

map;
hd;
hd []; (* raises an error *)

(* Reverse function *)
fun allrev l = (map rev) l;
allrev countdown;

fun lastof l = (firstof o allrev) l;
lastof countdown;