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. Keep them for work that does not
8	# recurse: a thunk around a search's own recursive step allocates a closure
9	# and leaves the loop on every step, where carrying the test as a Bool into
10	# the next call keeps it a loop (the `thunk` rule, U015).
11	import Base
12	
13	# `a` when c, else what rest gives: the early exit of a search
14	def stop(-A: Type, cc: Bool, aa: A, rest: Unit -> A) -> A:
15	  match cc:
16	    case True{}:
17	      aa
18	    case False{}:
19	      rest(Unit{})
20	
21	# True when a, else what rest gives: `or` that stops at the first True
22	def or_else(aa: Bool, rest: Unit -> Bool) -> Bool:
23	  match aa:
24	    case True{}:
25	      True{}
26	    case False{}:
27	      rest(Unit{})
28	
29	# what rest gives when a, else False: `and` that stops at the first False
30	def and_then(aa: Bool, rest: Unit -> Bool) -> Bool:
31	  match aa:
32	    case True{}:
33	      rest(Unit{})
34	    case False{}:
35	      False{}
36	
37	# what yes gives when c, else what no gives: a pick whose two branches each
38	# recurse, only the one taken run
39	def either(-A: Type, cc: Bool, yes: Unit -> A, no: Unit -> A) -> A:
40	  match cc:
41	    case True{}:
42	      yes(Unit{})
43	    case False{}:
44	      no(Unit{})