; Iter 15g — fifth consumer demo. First demo to import four stdlib
; modules (std_list, std_either, std_pair, std_either_list); first to
; thread a List<Either<Int, Int>> through three combinators that
; together exercise every monomorphisation site introduced by 15g.
;
; The list is constructed via the monomorphic helpers `mkleft` and
; `mkright` rather than bare `(term-ctor std_either.Either Left ...)` /
; `(term-ctor std_either.Either Right ...)`. This sidesteps a codegen
; bug that 15g surfaced: when an inline list literal mixes
; `Either Left n` (pins e, leaves a as `$u`) and `Either Right n` (pins
; a, leaves e as `$u`), the synth-time unification of the outer
; `List<a>`'s parameter against the `$u`-bearing element types fails
; on the param side because `unify_for_subst` only accepts arg-side
; `$u` wildcards. The helpers fully pin both type vars at the call
; site, so each list element arrives with concrete `Either<Int, Int>`
; and the bug is dodged. Logged as known-debt under 15g; the fix is
; a one-line symmetric extension of the `$u` early-return.

(module std_either_list_demo

  (import std_list)
  (import std_either)
  (import std_pair)
  (import std_either_list)

  (fn mkleft
    (doc "Monomorphic Left-injector at Either<Int, Int>. Pins both type vars at the call site so the codegen synth doesn't leave a `$u` wildcard on the second var.")
    (type (fn-type (params (con Int)) (ret (con std_either.Either (con Int) (con Int)))))
    (params x)
    (body (term-ctor std_either.Either Left x)))

  (fn mkright
    (doc "Monomorphic Right-injector at Either<Int, Int>. Symmetric to mkleft.")
    (type (fn-type (params (con Int)) (ret (con std_either.Either (con Int) (con Int)))))
    (params x)
    (body (term-ctor std_either.Either Right x)))

  (fn main
    (doc "Drive lefts, rights, and partition_eithers on the same five-element List<Either<Int, Int>> (Left 1, Right 10, Left 2, Right 20, Right 30). Expected output (one per line): 2, 3, 2, 3.")
    (type (fn-type (params) (ret (con Unit)) (effects IO)))
    (params)
    (body
      (seq (do io/print_int (app std_list.length (app std_either_list.lefts (term-ctor std_list.List Cons (app mkleft 1) (term-ctor std_list.List Cons (app mkright 10) (term-ctor std_list.List Cons (app mkleft 2) (term-ctor std_list.List Cons (app mkright 20) (term-ctor std_list.List Cons (app mkright 30) (term-ctor std_list.List Nil)))))))))
      (seq (do io/print_int (app std_list.length (app std_either_list.rights (term-ctor std_list.List Cons (app mkleft 1) (term-ctor std_list.List Cons (app mkright 10) (term-ctor std_list.List Cons (app mkleft 2) (term-ctor std_list.List Cons (app mkright 20) (term-ctor std_list.List Cons (app mkright 30) (term-ctor std_list.List Nil)))))))))
      (seq (do io/print_int (app std_list.length (app std_pair.fst (app std_either_list.partition_eithers (term-ctor std_list.List Cons (app mkleft 1) (term-ctor std_list.List Cons (app mkright 10) (term-ctor std_list.List Cons (app mkleft 2) (term-ctor std_list.List Cons (app mkright 20) (term-ctor std_list.List Cons (app mkright 30) (term-ctor std_list.List Nil))))))))))
           (do io/print_int (app std_list.length (app std_pair.snd (app std_either_list.partition_eithers (term-ctor std_list.List Cons (app mkleft 1) (term-ctor std_list.List Cons (app mkright 10) (term-ctor std_list.List Cons (app mkleft 2) (term-ctor std_list.List Cons (app mkright 20) (term-ctor std_list.List Cons (app mkright 30) (term-ctor std_list.List Nil))))))))))))))))