1	import Base
2	
3	# A statically shaped product. Pairing readers preserves one buffer owner.
4	type Twin<-Element: Data> is Data:
5	  Twin{first: Element,second: Element}
6	
7	# Static product composition also lifts an operation to any pair of elements.
8	# Nesting this template gives wider products without another arithmetic kernel.
9	def zip_map(~Element: Data,~Context: Data,~combine: Element -> Element -> Context -> Element,
10	  left: Twin<Element>,right: Twin<Element>,context: Context) -> Twin<Element>:
11	  Twin{left_first,left_second} = left
12	  Twin{right_first,right_second} = right
13	  +context = context
14	  Twin{combine(left_first,right_first,context),combine(left_second,right_second,context)}
15	
16	def finish_pair(~State: Type,~Element: Data,first: Element,result: State & Element) -> State & Twin<Element>:
17	  (state,second) = result
18	  (state,Twin{first,second})
19	
20	def read_second(
21	  ~State: Type,~Context: Data,~Element: Data,~Index: Data,
22	  ~read: State -> Context -> Index -> State & Element,
23	  result: State & Element,context: Context,index: Index
24	) -> State & Twin<Element>:
25	  (state,first) = result
26	  finish_pair(~State,~Element,first,read(state,context,index))
27	
28	def read_at(
29	  ~State: Type,~Context: Data,~Element: Data,~Index: Data,
30	  ~read: State -> Context -> Index -> State & Element,
31	  state: State,+context: Context,first: Index,second: Index
32	) -> State & Twin<Element>:
33	  read_second(~State,~Context,~Element,~Index,~read,read(state,context,first),context,second)
34	
35	def read_pair(
36	  ~State: Type,~Context: Data,~Element: Data,
37	  ~read: State -> Context -> Nat -> State & Element,~stride: Nat,
38	  state: State,context: Context,index: Nat
39	) -> State & Twin<Element>:
40	  +context = context
41	  +index = index
42	  read_at(~State,~Context,~Element,~Nat,~read,state,context,index,Nat.add(index,stride))