Phase 3.1. list<T> for scalar T
| Field | Value |
|---|---|
| MEP | MEP-54 |
| Status | LANDED |
| Started | 2026-05-27 (GMT+7) |
| Landed | 2026-05-27 (GMT+7) |
| Tracking PR | #22490 |
| Commit | 8bb154ea8f |
Gate
12 fixtures cover scalar list values: list_lit, list_index, list_len, list_append, list_set, list_for_each, list_strings, list_floats, list_bools, list_nested_loop, list_sum_loop, list_empty. All 29 fixtures (Phase 1+2+3.1) pass byte-equal under go test ./transpiler3/go/build/... -run TestPhase1Hello.
Lowering decisions
Mochi list<int|float|string|bool> lowers to a native Go slice []T. aotir.ListLit emits []T{...}; the lowerer threads the element type through ElemType so an empty []int{} is distinguishable from []string{}. IndexExpr emits xs[int(i)] with a narrow conversion (literal int indices stay bare to keep the printed source readable). LenExpr emits int64(len(xs)) to preserve the int pin established in Phase 2; without the wrap the len result would be Go's machine-width int, inviting a 32/64-bit divergence on cross-compilation. AppendExpr emits append(xs, v) directly (Go's variadic append is the canonical idiom).
ForEachStmt lowers for x in xs { ... } to for _, x := range xs { ... }. The bare _ ignores the index when the Mochi loop does not need it; Phase 7.4 later refines this to drop the binding entirely when unused (for range xs). ListSetStmt emits xs[int(i)] = v for in-place mutation. LetStmt is taught to consult VarType so a list<int> binding gets the right []int64 annotation.
The Go slice is value-typed (Go semantics) but list values in Mochi are mutable through aliasing once shared, matching Go's behaviour on a shared underlying array. The Mochi semantics page calls out that xs = ys is an alias; the Go lowerer relies on Go's reference-on-the-underlying-array behaviour to deliver that.
Files changed
| File | Purpose |
|---|---|
transpiler3/go/lower/types.go | lowerListType(elem) -> []Elem (scalar elem types only) |
transpiler3/go/lower/expr.go | ListLit -> []T{...}; IndexExpr -> xs[int(i)]; LenExpr -> int64(len(xs)); AppendExpr -> append(xs, v) |
transpiler3/go/lower/stmt.go | ForEachStmt -> for _, x := range xs {...}; ListSetStmt -> xs[int(i)] = v; LetStmt routes through letTypeText |
tests/transpiler3/go/fixtures/list_*/ | 12 list fixtures |
Test set
TestPhase1Hello/list_lit,list_index,list_len,list_append,list_set,list_for_each,list_strings,list_floats,list_bools,list_nested_loop,list_sum_loop,list_empty.
Closeout notes
Wrapping len in int64(...) is non-negotiable: Phase 7.5's ListSumExpr later relies on the wrapper to keep the accumulator type consistent. The narrow int(i) on IndexExpr is intentionally wider than necessary (Go's int is 64-bit on the supported tuples) because uniformly wrapping makes the printer trivial; gofmt won't shorten xs[int(0)] to xs[0] but a literal-only optimisation kept the printed source clean for the list_lit golden file.