Range for on-demand evaluation

lazy-range.red

Red [
	Author: "Toomas Vooglaid"
	Date: 27-11-2017
]
lazy-range: function [
	"Returns function that generates next value in (possibly infinite) range"
	val 		[scalar!]		"Initial value"
	/step 	stp 	[scalar! block!]	"By which to increment each next value"
	/limit 	lim 	[integer!]		"How many new values to generate"
	/stop 	stopval	[scalar! block!]	"On which value to stop generation of new values; if block, then also after how may matches"
	/circle	crcl	[scalar!]		"To generate (possibly infinite) new values in limited range"
][
	cx: context compose/deep [
		limit: 		lim
		stop: 		(either all [stop block? stopval][stopval/1][stopval])
		step: 		(either step [([stp])][if percent? val [1%][1]])
		val: 		(val)
		count: 		0
		val2: 		none
		cval: 		none
		inc: 		if block? step [get step/2] ;either block? step [get step/2][:+]
		circle: 	crcl
		stop-count: 	(if all [stop block? stopval][stopval/2])
		count-stop: 	0
		count-circle: 	0
		bstep: 		if block? step [either '- = step/2 [0 - step/3][step/3]]
		b2stp:		0
		next-val: does [
			if any [limit circle][count: count + 1]
			either any [
				all [limit limit < count]
				all [
					stop 
					either circle [
						if all [stop-count stop = val2][count-stop: count-stop + 1]
						and~ stop = val2 either stop-count [stop-count = count-stop][true] 
					][
						pick reduce [stop > val stop < val] either any [
							all [number? step negative? step]
							all [block? step '- = step/2]
						][true][false]					
					]
				]
			][none][
				either circle [
					cval: either block? step [ 
						either number? circle [
							val2: val
							b2stp: count - 1 * bstep % circle
							val2/(step/1): val2/(step/1) + b2stp
							val2
						][cause-error 'user 'message ["Only integer! type supported for circle in case of block step!"]]
					][
						val + case [
							or~ number? circle time? circle [count - 1 * step % circle]
							true [cause-error 'user 'message ["Only numeric and time types supported for circle!"]]
						]
					]
				][
					also val val: either block? step [
						val2: val
						val2/(step/1): val2/(step/1) inc step/3
						val2
					][
						val + step
					]
				]
			]
		]
	]
	:cx/next-val
]