Higher order functions Map and Filter (ML)

	(* ('a -> 'b) * 'a list -> 'b list *)
	fun map(f,xs) =
	case xs of
	[] => []
	| x::xs' => (f x)::map(f,xs')
	val x1 = map((fn x => x + 1), [4,8,12,16])
	val x2 = map(hd, [ [1,2], [3,4], [5,6,7] ])
	(* ('a -> bool) * 'a list -> 'a list *)
	fun filter(f,xs) =
	case xs of
	[] => []
	| x::xs' => if f x
	then x::(filter (f, xs'))
	else filter(f,xs')
	fun is_even v =
	(v mod 2 = 0)
	fun filter_even xs = filter(is_even, xs)
	val x3 = filter_even([1,2,3,4,5,6])
	(* output of program
	- use "mapfilter.sml";
	[opening mapfilter.sml]
	val map = fn : ('a -> 'b) * 'a list -> 'b list
	val x1 = [5,9,13,17] : int list
	val x2 = [1,3,5] : int list
	val filter = fn : ('a -> bool) * 'a list -> 'a list
	val is_even = fn : int -> bool
	val filter_even = fn : int list -> int list
	val x3 = [2,4,6] : int list
	val it = () : unit
	*)

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