Phase 3. Lists
| Field | Value |
|---|---|
| MEP | MEP-56 §Phases |
| Status | LANDED |
| Started | 2026-05-29 12:01 (GMT+7) |
| Landed | 2026-05-29 12:01 (GMT+7) |
| Tracking issue | — |
| Tracking PR | #22510 |
| Commit | 4738082cde |
Gate
TestPhase3Lists in transpiler3/ruby/build/phase03_test.go: six inline subtests, list_lit_index, list_len, for_each, list_set, list_append, and str_len. Each subtest compiles a Mochi source via Driver.Build to a .rb, executes the script under the resolved Ruby toolchain with -I mochi-runtime/lib, and diffs stdout against the recorded expectation.
Lowering decisions
Mochi lists lower directly to Ruby Array. aotir.ListLit is rendered by lowerListLit as a comma-joined bracket literal [elem1, elem2, ...], packaged as a rtree.RawExpr because the result is one whole expression token (lower.go lines 1088 to 1098). aotir.IndexExpr becomes recv[idx] via lowerIndexExpr, also as a RawExpr, since Ruby's [] is already a method call and rtree.MethodCall with a bracket-form method would over-complicate the renderer.
aotir.LenExpr lowers to recv.length, picked over .size to give a uniform name with aotir.StrLenExpr (which also lowers to .length); both return Integer and behave identically on Array and String. aotir.ForEachStmt produces xs.each do |x| ... end via the same RawStmt style as the for-range loop, with rubyIdent mangling on the loop variable.
aotir.ListSetStmt is a statement: xs[1] = 99 becomes RawStmt{Text: "xs[1] = 99"}. The C lowerer surfaces this as a separate statement type rather than an AssignStmt with an indexed LHS, which the Ruby lowerer accepts as-is. aotir.AppendExpr is the canonical Mochi append(xs, v) and lowers to xs + [v], a functional append that returns a new array without mutating xs. The comment at lower.go:663 notes this is intentional: Mochi's append semantics are pure, matching the vm3 oracle.
The str_len subtest belongs to Phase 3 because it shares the len(...) surface with lists; lowerExpr's StrLenExpr arm lowers to the same .length call.
Files changed
| File | Purpose |
|---|---|
transpiler3/ruby/lower/lower.go | lowerListLit, lowerIndexExpr, LenExpr/StrLenExpr arms, lowerForEachStmt, ListSetStmt arm, AppendExpr arm |
transpiler3/ruby/build/phase03_test.go | TestPhase3Lists with 6 subtests |
Test set
TestPhase3Lists/list_lit_index,list_len,for_each,list_set,list_append,str_len.
Closeout notes
Phase 3 landed on CRuby 3.4 with all six subtests green. The functional append (xs + [v]) bumped past one early attempt that used <<, which mutates in place and would have broken any fixture that holds a reference to the pre-append list. Key implementation insight: Mochi's var xs = ...; xs = append(xs, v) pattern lowers to the same Ruby xs = xs + [v], so the user-facing var-rebinding is preserved without a separate ABI for in-place vs functional append. len(s) on strings reuses .length, giving a single name for both arrays and strings (matches Ruby community style).