1	# lazy: branches that do not run both ways. Bool.pick, Bool.and and Bool.or
2	# are ordinary defs, so every argument is evaluated before the call: a
3	# search written as `Bool.pick(T, hit, here, go(rest))` walks the whole list
4	# whatever `hit` says (measured native, 100 iterations over 100k cells: the
5	# hit at the head costs 0.10 s, the same as the hit at the end, while the
6	# forms here cost 0.00 s). A thunk `Unit -> T` is a value, so the branch
7	# that is not taken is never applied.
8	import Base
9	
10	# `a` when c, else what rest gives: the early exit of a search
11	def stop(-A: Type, cc: Bool, aa: A, rest: Unit -> A) -> A:
12	  match cc:
13	    case True{}:
14	      aa
15	    case False{}:
16	      rest(Unit{})
17	
18	# True when a, else what rest gives: `or` that stops at the first True
19	def or_else(aa: Bool, rest: Unit -> Bool) -> Bool:
20	  match aa:
21	    case True{}:
22	      True{}
23	    case False{}:
24	      rest(Unit{})
25	
26	# what rest gives when a, else False: `and` that stops at the first False
27	def and_then(aa: Bool, rest: Unit -> Bool) -> Bool:
28	  match aa:
29	    case True{}:
30	      rest(Unit{})
31	    case False{}:
32	      False{}
33	
34	# what yes gives when c, else what no gives: a pick whose two branches each
35	# recurse, only the one taken run
36	def either(-A: Type, cc: Bool, yes: Unit -> A, no: Unit -> A) -> A:
37	  match cc:
38	    case True{}:
39	      yes(Unit{})
40	    case False{}:
41	      no(Unit{})