Skip to main content

Phase 1. Hello world

FieldValue
MEPMEP-54 (Mochi-to-Go transpiler under transpiler3/go/)
StatusLANDED
Started2026-05-26 (GMT+7)
Landed2026-05-26 (GMT+7)
Tracking issuenone (combined with Phase 0)
Tracking PR#22485
Commitb836c348fb

Gate​

TestPhase1Hello in transpiler3/go/build/phase01_test.go walks every fixture directory under tests/transpiler3/go/fixtures, runs the end-to-end pipeline, and diffs the binary's stdout against the fixture's expect.txt. Each sub-test gets its own CacheDir so a stray file in one fixture's work directory cannot bleed into the next. The runner skips on Windows because the driver hard-codes a POSIX go invocation (the cross-tuple sweep moves under Phase 16).

Five fixtures cover Phase 1: hello, hello_int, hello_bool, hello_float, hello_newline. All pass byte-equal under go test ./transpiler3/go/build/....

Lowering decisions​

Phase 1 wires the smallest end-to-end surface. parser.Parse(src) produces a Mochi AST, types.Check(prog, types.NewEnv(nil)) validates it (errors short-circuit before lowering), clower.Lower(prog) returns the shared aotir IR (the same IR consumed by MEP-45/46/47/48/49/50/51/52/53/55/56), glower.Lower(ir) returns a gotree.File, and gemit.Emit writes main.go. writeGoMod(workDir, modulePath) writes a go.mod pinning Go 1.26.0 + toolchain 1.26.3 so the build is reproducible; goBuild(GoBin, workDir, absOut, nil) shells out to go build and writes the binary to the caller's out.

The Go lowerer handles the single Mochi node that hello-world needs: aotir.PrintStmt for print("hello, mochi!"). Print lowers to fmt.Println(args...) with the fmt import added on demand via addImport. The println call uses Go's variadic forwarding so multi-arg prints (print("x", 1)) work without per-arg conversions in Phase 1; later phases lift more value shapes into the format args.

The work directory is created under CacheDir (defaulting to os.TempDir()) with MkdirTemp so concurrent fixture runs cannot collide. KeepWorkDir is exposed so failing tests can keep the emitted main.go for inspection; the default deletes the directory on Build return. WorkDirPath is always set on success so callers can inspect the path even when KeepWorkDir is true.

Files changed​

FilePurpose
transpiler3/go/lower/lower.goLower(prog) recognises aotir.PrintStmt and emits fmt.Println(args...)
transpiler3/go/lower/expr.goLowers string / int / bool / float literals for print args
transpiler3/go/build/driver.goDriver.Build wires parser -> types.Check -> clower.Lower -> glower.Lower -> gemit.Emit -> goBuild
transpiler3/go/build/gobuild.gogoBuild(goBin, workDir, out, env) shelling out to go build
transpiler3/go/build/gomod.gowriteGoMod(dir, modulePath) pinning Go 1.26.0 + toolchain 1.26.3
transpiler3/go/build/phase01_test.goTestPhase1Hello fixture walker
tests/transpiler3/go/fixtures/hello{,_int,_bool,_float,_newline}/Five hello-world fixtures with expect.txt

Test set​

  • TestPhase1Hello/hello
  • TestPhase1Hello/hello_int
  • TestPhase1Hello/hello_bool
  • TestPhase1Hello/hello_float
  • TestPhase1Hello/hello_newline

Closeout notes​

Secondary gates verified during Phase 1 closeout: go vet ./transpiler3/go/... clean, gofmt reaches a fixed point on every emitted main.go, go build emits the binary in under 2 seconds on the macOS arm64 reference machine. Pinning the toolchain in go.mod avoided the "developer has Go 1.27 installed locally, CI runs 1.26" drift that bit MEP-46. The shared aotir layer paid off immediately: the Go lowerer only had to translate aotir.PrintStmt, not re-derive call-site type information from the parser AST.