Using pattern matching instead of using head or tail

	(* in a previous post we saw how we could define a max function by testing
	- if a list is empty with null
	- extracting a value from an option with valOf
	BUT that is considered bad practise so instead we will switch to using pattern matching
	Because both list AND option are defined as algebraic datatypes
	fun max(numbers: int list) =
	if null numbers
	then NONE
	else
	let val max_tail = max(tl numbers)
	in
	if max_tail = NONE orelse hd numbers >= valOf max_tail
	then SOME (hd numbers)
	else max_tail
	end;
	can be rewritten to
	*)
	fun max(numbers: int list): int option =
	case numbers of
	[] => NONE
	| x::xs =>
	case max xs of
	NONE => SOME x
	|SOME y => SOME (Int.max(x, y))
	val test1 = max([3,6,4,2])
	val test2 = max([])
	(* output of program
	[opening patternmatching.sml]
	val max = fn : int list -> int option
	val test1 = SOME 6 : int option
	val test2 = NONE : int option
	val it = () : unit
	*)

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