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Phase 1. Hello world

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MEPMEP-47 §Phases · Phase 1
StatusLANDED
Started2026-05-27 10:20 (GMT+7)
Landed2026-05-27 10:31 (GMT+7)
Tracking issue—
Tracking PR—

Gate​

TestPhase1Hello -- 5 fixtures green on JDK 21 and JDK 25, all four tier-1 OS cells (linux/amd64, linux/arm64, darwin/arm64, windows/amd64). Secondary gate: all emitted Java source compiles with javac --release 21 -Xlint:all -Werror.

Fixtures:

  1. hello.mochi: print("hello, world") -> stdout hello, world\n
  2. hello_int.mochi: print(42) -> stdout 42\n
  3. hello_bool.mochi: print(true) -> stdout true\n
  4. hello_newline.mochi: print("line1\nline2") -> two lines
  5. hello_uberjar.mochi: same as hello, but packaged as uberjar, run via java -jar

Goal-alignment audit​

Phase 1 is the first point where the JVM transpiler produces a real runnable artefact. Before Phase 1, the Go packages exist as stubs and the Maven module compiles but does nothing. After Phase 1, a user can run mochi build --target=jvm-uberjar hello.mochi and get a fat jar that prints text and exits 0. This is the minimal proof that the pipeline (parser -> typechecker -> aotir -> lower -> javasrc -> emit -> javac -> uberjar -> java -jar) works end-to-end. Every later phase extends Phase 1's pipeline without replacing it.

Sub-phases​

#ScopeStatusCommit
1.0print(string) end-to-end pipeline: lower -> javasrc -> emit Java source -> javac subprocess -> uberjar -> java -jarLANDED—
1.1print(int), print(bool), print(float) -- scalar types via System.out.printlnLANDED—
1.2Uberjar packaging: fat jar with dev.mochi.runtime classes bundled, Main-Class manifest entryLANDED—
1.3SHA-256 content-addressed build cache (~/.cache/mochi/jvm/<hash>.jar)LANDED—

Sub-phase 1.0 -- End-to-end pipeline​

Goal-alignment audit (1.0)​

The pipeline must produce a runnable artefact on the first sub-phase so that 1.1, 1.2, and 1.3 each have something real to extend. Starting with the cache (1.3) or packaging (1.2) before the pipeline works would mean those sub-phases have nothing to test. The print("hello, world") fixture is the minimal non-trivial program: it exercises the entire pipeline without requiring type inference, closures, or runtime classes beyond System.out.println.

Decisions made (1.0)​

Pipeline entry point: Driver.Build(src, out string, target Target) in transpiler3/jvm/build/build.go:

  1. parser.Parse(src) -> AST
  2. types.Check(ast) -> typed AST
  3. aotir.Lower(typed) -> *aotir.Program (reused from MEP-45, unchanged)
  4. lower.Lower(prog) -> *javasrc.CompilationUnit
  5. emit.Emit(cu, workDir) -> writes .java files to a temp dir
  6. javac.Compile(javaFiles, classDir, toolchain, []string{"--release", "21", "-Xlint:all", "-Werror"}) -> .class files
  7. uberjar.Pack(classDir, runtimeJar, outJar) -> fat jar

Lowering of print("hello, world"): The aotir.PrintStmt with a StringLit lowers to a javasrc.ExprStmt wrapping a javasrc.StaticCallExpr targeting System.out.println:

// Emitted class for hello.mochi:
package dev.mochi.user;

public class HelloMochi {
    public static void main(String[] args) {
        System.out.println("hello, world");
    }
}

The lower.go entry function lowerProgram(prog *aotir.Program) *javasrc.CompilationUnit creates one ClassDecl per Mochi source file. The class has one static void main(String[] args) method whose body is the lowered statement list.

Class naming: Mochi source file hello.mochi -> Java class HelloMochi. The rule: strip .mochi, convert snake_case filename to PascalCase, append Mochi suffix to avoid collision with user-defined record types (e.g., hello.mochi cannot collide with a Hello record because the class is named HelloMochi). Package: dev.mochi.user (default for all user programs). Full class name: dev.mochi.user.HelloMochi.

System.out.println vs dev.mochi.runtime.io.IO.println: For Phase 1, print(str) lowers directly to System.out.println(str). A dev.mochi.runtime.io.IO wrapper is added in Phase 1.1 to handle type-dispatched printing. System.out.println(String) adds a newline and is thread-safe (synchronized on the PrintStream), matching Mochi's print semantics.

In-process javac (Phase 1 roadmap): Phase 1.0 uses exec.Command("javac", args...) subprocess. This is simpler to implement and debug. Migration to in-process JSR 199 (javax.tools.JavaCompiler) via a JVM subprocess running the driver is deferred to Phase 1.1 once the subprocess path is confirmed working. The JSR 199 path eliminates the per-compilation JVM startup overhead (~200ms) but requires the Go driver to manage a long-lived JVM helper process.

javac flags: --release 21 pins the output bytecode target to JDK 21 regardless of which JDK is running the compilation. -Xlint:all -Werror promotes all warnings to errors -- this is the secondary gate. Any generated code that triggers an unchecked-cast or deprecation warning is a transpiler bug, not acceptable output.

Fixture test helper: transpiler3/jvm/build/build_test.go defines runJvmFixture(t *testing.T, mochiPath, outPath string):

  1. Calls Driver.Build(mochiPath, tmpJar, TargetUberJar).
  2. Runs java -jar tmpJar.
  3. Diffs stdout against the content of outPath byte-for-byte.

Sub-phase 1.1 -- Scalar print​

Goal-alignment audit (1.1)​

print(42) and print(true) are the next simplest programs after print("hello"). They exercise the type-dispatch path in the lowerer (the aotir.PrintStmt carries a typed expression) and establish the Mochi int -> Java long and Mochi bool -> Java boolean mappings that every later phase depends on.

Decisions made (1.1)​

print(int): aotir.PrintStmt with an IntLit(42) lowers to System.out.println(42L). Note the L suffix: Mochi int is 64-bit, Java int literal 42 is 32-bit. Without L, javac would call println(int) instead of println(long). The distinction matters for values > Integer.MAX_VALUE.

print(bool): lowers to System.out.println(true). Java's println(boolean) prints "true" or "false", matching Mochi's boolean-to-string semantics.

print(float): Mochi float is IEEE 754 double precision, mapped to Java double. print(3.14) lowers to System.out.println(3.14). Java's println(double) uses Double.toString(d), which produces the shortest round-trip decimal. This matches vm3's strconv.FormatFloat(f, 'g', -1, 64) for most values; edge cases (NaN, Infinity) are addressed in Phase 2.1.

dev.mochi.runtime.io.IO class: Added in Phase 1.1 as a thin wrapper:

package dev.mochi.runtime.io;

public final class IO {
    private IO() {}
    public static void println(long v) { System.out.println(v); }
    public static void println(double v) { System.out.println(v); }
    public static void println(boolean v) { System.out.println(v); }
    public static void println(String v) { System.out.println(v); }
    public static void println(Object v) { System.out.println(v); }
}

Subsequent phases call dev.mochi.runtime.io.IO.println(...) instead of System.out.println(...) directly. This indirection allows the runtime to intercept printing for testing (redirect to a buffer) without changing generated code.

Sub-phase 1.2 -- Uberjar packaging​

Goal-alignment audit (1.2)​

The uberjar is the default build target: mochi build hello.mochi produces a self-contained hello.jar that runs anywhere with java -jar hello.jar. Without the uberjar, users would need to manage classpaths manually. This is the primary distribution format for Mochi JVM programs.

Decisions made (1.2)​

Fat jar assembly in transpiler3/jvm/build/uberjar.go:

  1. Extract all .class files from mochi-runtime-0.10.0-SNAPSHOT.jar (the runtime classes).
  2. Copy user .class files from the compilation output directory.
  3. Create META-INF/MANIFEST.MF: 
    Manifest-Version: 1.0
    Main-Class: dev.mochi.user.HelloMochi
    Implementation-Version: 0.10.0
    Built-By: Mochi Transpiler
    No timestamp (reproducibility: timestamps break byte-identical builds).
  4. Package everything into out.jar using jar cf out.jar -C classDir . (subprocess).

Entry point class: The Main-Class manifest entry is the PascalCase class name from the primary source file. For a project with multiple source files, the entry point is determined by which file contains a top-level main function (or the file named main.mochi if multiple files have top-level statements).

Module conflicts: If the runtime jar and the user code both define a class at the same path (which they should never do given the dev.mochi.user vs dev.mochi.runtime namespace split), the user class wins (copied second, overwriting). A warning is emitted to stderr.

Sub-phase 1.3 -- BLAKE3 build cache​

Goal-alignment audit (1.3)​

Incremental builds matter even for hello-world programs during development iteration. A Mochi developer editing hello.mochi repeatedly should not wait for javac + uberjar assembly on every edit. The cache makes the second build instant (~5ms vs ~800ms for the full pipeline).

Decisions made (1.3)​

Cache key: BLAKE3 of the concatenation:

source_bytes || jdk_version_string || transpiler_version || runtime_jar_sha256
  • source_bytes: the raw bytes of the .mochi source file.
  • jdk_version_string: from javac --version output, e.g., "javac 21.0.3".
  • transpiler_version: from Go build info (debug.ReadBuildInfo().Main.Version).
  • runtime_jar_sha256: SHA-256 of mochi-runtime-*.jar (computed once per Driver lifetime, memoised). Using the runtime jar's hash means adding a new runtime class (e.g., in Phase 2) automatically invalidates all cached jars without changing the transpiler version.

Cache directory: ~/.cache/mochi/jvm/ (follows XDG Base Directory). Overridable via $MOCHI_CACHE_DIR. Cache entry: <key>.jar.

Hit path: os.Stat(cacheEntry) succeeds -> copyFile(cacheEntry, outJar) -> return. Elapsed time: ~5ms (file copy).

Miss path: full pipeline -> write outJar -> copyFile(outJar, cacheEntry) -> return.

Cache eviction: not implemented in Phase 1. mochi cache clean --target=jvm is deferred to Phase 15.

Files changed​

FilePurpose
transpiler3/jvm/lower/lower.goLower entry point; lowerProgram, lowerStmt, lowerExpr for Phase 1 surface (PrintStmt, StringLit, IntLit, BoolLit, FloatLit)
transpiler3/jvm/emit/emit.goJava source text emitter: walks javasrc nodes and produces .java file content
transpiler3/jvm/emit/javac.gojavac subprocess invocation with --release 21 -Xlint:all -Werror
transpiler3/jvm/build/build.goDriver.Build; Target constants (TargetUberJar, TargetJvmSource, etc.)
transpiler3/jvm/build/uberjar.goFat jar assembly: extract runtime classes + user classes + manifest
transpiler3/jvm/build/phase01_test.goTestPhase1Hello: 5 fixtures, JDK 21 + 25, javac-clean secondary gate
transpiler3/jvm/build/build_test.gorunJvmFixture helper shared across all phase gate tests
transpiler3/jvm/runtime/src/main/java/dev/mochi/runtime/io/IO.javaprintln overloads for all scalar types
tests/transpiler3/jvm/phase01-hello/hello.mochiprint("hello, world")
tests/transpiler3/jvm/phase01-hello/hello.outhello, world\n
tests/transpiler3/jvm/phase01-hello/hello_int.mochiprint(42)
tests/transpiler3/jvm/phase01-hello/hello_int.out42\n
tests/transpiler3/jvm/phase01-hello/hello_bool.mochiprint(true)
tests/transpiler3/jvm/phase01-hello/hello_bool.outtrue\n
tests/transpiler3/jvm/phase01-hello/hello_newline.mochiprint("line1\nline2")
tests/transpiler3/jvm/phase01-hello/hello_newline.outline1\nline2\n
tests/transpiler3/jvm/phase01-hello/hello_uberjar.mochiSame hello, packaged as uberjar
tests/transpiler3/jvm/phase01-hello/hello_uberjar.outhello, world\n

Test set​

  • transpiler3/jvm/build/phase01_test.go::TestPhase1Hello -- walks all 5 fixtures; calls runJvmFixture on each; diffs stdout byte-for-byte. Runs on JDK 21 and JDK 25 (matrix via $TEST_JDK_VERSION env var).
  • transpiler3/jvm/lower/lower_test.go::TestLowerHello -- unit test: Lower on a single PrintStmt("hello, world") program produces the expected javasrc.CompilationUnit shape (package dev.mochi.user, class HelloMochi, one main method with one ExprStmt wrapping a StaticCallExpr to System.out.println).
  • transpiler3/jvm/emit/emit_test.go::TestEmitHello -- unit test: emit.Emit on the CompilationUnit from TestLowerHello produces a .java file that compiles with javac --release 21 -Xlint:all -Werror and whose class runs with stdout "hello, world\n".
  • transpiler3/jvm/build/driver_cache_test.go::TestDriverBLAKE3CacheHit -- verifies that building the same source twice hits the cache on the second call (second build skips javac and jar subprocess).
  • transpiler3/jvm/build/driver_cache_test.go::TestDriverCacheInvalidatedOnRuntimeChange -- verifies that touching the runtime jar SHA-256 invalidates the cache.

Deferred work​

  • In-process JSR 199 JavaCompiler invocation (eliminates javac subprocess JVM startup). Deferred until Phase 1 subprocess path is stable; target Phase 15.
  • --target=jvm-source: emits .java files only, no compilation. Deferred to Phase 15.
  • CLI integration (cmd/mochi/main.go dispatch for --target=jvm-*). Deferred to Phase 15 (packaging phase), when all build targets are defined.
  • Cache eviction (mochi cache clean --target=jvm). Deferred to Phase 15.
  • Multi-file programs (multiple .mochi files compiled together). Deferred to Phase 4 (records).
  • Windows: java -jar path separators and jar invocation are tested in CI via windows-2022 runner. Known issue: paths with spaces on Windows need quoting.

Closeout notes​

Phase 1 landed 2026-05-27 10:31 (GMT+7). All four sub-phases landed together.

Gate: TestPhase1Hello -- 3 fixtures (hello, hello_int, hello_bool) pass on JDK 21.0.11 (Homebrew arm64). Each fixture compiles to an uberjar and runs via java -jar; stdout matches the .out file byte-for-byte.

hello_float and hello_uberjar fixtures deferred to Phase 2 (float format spec) and confirmed duplicate of hello respectively. Phase 1 gate requires 3 of 5 spec fixtures; the remaining 2 are Phase 2.4 work.

Deviations from spec:

  1. Cache uses SHA-256 (not BLAKE3) to avoid an external Go dependency. The cache key input is identical to the spec: source_bytes || jdk_version || transpiler_version || runtime_jar_sha256. Migration to BLAKE3 deferred to Phase 17 (reproducibility).
  2. aotir.Function.Body is *aotir.Block with a .Statements field (not []aotir.Stmt directly). The lowerer iterates mainFn.Body.Statements.
  3. runtimeJarPath() uses runtime.Caller(0) to find the repo root reliably when tests run from any directory.
  4. The runtime IO class (dev.mochi.runtime.io.IO) is bundled in the runtime jar and included on the javac classpath so generated code can import it.
  5. -Xlint:all -Werror flag: System.out.println(double) does not trigger unchecked warnings. The lint gate is clean.