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Phase 7.12. List HOFs

FieldValue
MEPMEP-54
StatusLANDED
Started2026-05-29 (GMT+7)
Landed2026-05-29 (GMT+7)
Tracking PR#22612
Commit24b8ac0274

Gate​

8 fixtures: hof_map, hof_filter, hof_reduce, hof_map_str, hof_filter_str, hof_reduce_float, hof_combined, hof_map_to_bool. 261 transpiler3/go fixtures green (was 253).

Lowering decisions​

The three Phase 6.1 higher-order-list nodes dispatch in lowerExpr. Each is emitted as an IIFE that walks the input slice and either projects, filters, or folds elements through the function value. The function value is already a Go func of the right signature (the FunLit / VarRef path in lowerExpr produces a callable closure from Phase 6.1), so the call site is a plain fn(x) or fn(acc, x).

  • ListMapExpr(xs, fn) -> func() []B { out := make([]B, 0, len(xs)); for _, x := range xs { out = append(out, fn(x)) }; return out }(). The result element type comes from ListMapExpr.ElemType (B in map(xs: list<A>, fn: fun(A): B): list<B>).
  • ListFilterExpr(xs, pred) -> same shape but if pred(x) { out = append(out, x) }. Element type comes from ListFilterExpr.ElemType (preserved input element type).
  • ListFoldlExpr(xs, fn, init) -> func() T { acc := init; for _, x := range xs { acc = fn(acc, x) }; return acc }(). Accumulator type from ListFoldlExpr.AccType.

The IIFE pattern keeps each HOF as an expression so it composes inside larger expressions (the hof_combined fixture chains filter -> map -> reduce). Using the named element type from the aotir node, rather than inferring from the function value's return type, avoids a Phase 7.12-time type-checker reimplementation.

Files changed​

FilePurpose
transpiler3/go/lower/expr.goListMapExpr, ListFilterExpr, ListFoldlExpr IIFE lowering
tests/transpiler3/go/fixtures/hof_*/8 fixtures

Test set​

  • 8 TestPhase1Hello/hof_* subtests covering int doubling, even-filter, sum-reduce, string upper-mapping, string-substring filter, float reduce, the canonical filter+map+reduce pipeline, and map-to-bool element-type changes.

Closeout notes​

Phase 7.12 wraps up the Phase 7 query / HOF surface. The next sub-phase plan is Phase 8 (Datalog) and Phase 9.2 (streams); Phase 9.1 (channels) shipped on the staging branch ahead of Phase 8 because channels turned out to be a small, self-contained patch while Datalog's compile-time semi-naive engine is a larger build.