First stdlib fn that imports three other stdlib modules (std_list,
std_either, std_pair) and returns a compound polymorphic ADT tree
(Pair<List<e>, List<a>>). Stresses monomorphisation across nested
parameterised ADTs.
What shipped:
- examples/std_either_list.{ailx,ail.json}: 3 combinators —
- lefts : forall e a. (List<Either<e, a>>) -> List<e>
- rights : forall e a. (List<Either<e, a>>) -> List<a>
- partition_eithers : forall e a. (List<Either<e, a>>)
-> Pair<List<e>, List<a>>
lefts/rights use depth-2 nested Ctor patterns (Cons (Left l) t)
— exercises 16a's desugar pass at a depth not reached by any
prior fixture.
- examples/std_either_list_demo.{ailx,ail.json}: drives all three
combinators on a five-element List<Either<Int, Int>>; expected
output one per line: 2, 3, 2, 3.
- crates/ail/tests/e2e.rs::std_either_list_demo: e2e count 34 → 35.
- docs/JOURNAL.md: Iter 15g entry.
Compiler bug surfaced (queued as 15g-aux, not fixed):
unify_for_subst in ailang-codegen/src/lib.rs accepts $u wildcards
only on the arg side. Mixing inline (Either Left n) and (Either
Right n) in a list literal lands $u on the param side via the
outer List<a>'s binding and errors. Reduced repro: `length [Left
1, Right 10]`. Demo works around with monomorphic mkleft/mkright
helpers; workaround documented inline. Fix is a one-line symmetric
extension of the early-return.
Tests: 94/94. Stdlib: 5 modules, 27 combinators.
