Callbacks using mutable references (ML)

	val cbs: (int -> unit) list ref = ref []
	(*
	x = ref 4 means x has a reference to a value of type int with initial value 4
	x := 5 means update the value x refers to with value 5
	val y = (!x) means store the value x refers to in y
	*)
	fun onKeyEvent f = cbs := f :: (!cbs)
	fun onEvent i =
	let fun loop fs =
	case fs of
	[] => ()
	|f::fs' => (f i; loop fs')
	in loop (!cbs) end
	(* some client code *)
	val timesPressed = ref 0
	val _ = onKeyEvent (fn _ => timesPressed := (!timesPressed) + 1
	fun printIfPressed i =
	onKeyEvent (fn j =>
	if i=j
	then print ("you pressed " ^ Int.ToString i)
	else ()

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