🥮 mooncakes.io

    #Existing schemas: generics, AnyPointer and constants

    This example uses the unmodified Cap'n Proto 1.5.0 test.capnp and schema.capnp. The original files, imports, embedded data, MIT notices and SHA-256 manifest are in the upstream fixtures. No generic declarations are replaced with handwritten concrete schemas.

    test.capnp exercises the standard data model; schema.capnp is the real protocol spoken by the official compiler and its language plugins. The telemetry example separately demonstrates an application that exchanges measurements and computed results.

    #Run the two peers

    Requirements: MoonBit 0.10.16+b1a0e846e, Python 3.12, pycapnp==2.2.1, and Node.js 24 for the JS target. Run from the repository root:

    moon update
python -m pip install -r examples/telemetry/requirements.txt
python examples/schema_compat/peer.py prepare work/schema-compat
moon run --target js examples/schema_compat -- work/schema-compat
python examples/schema_compat/peer.py verify work/schema-compat

    The last command prints:

    Verified MoonBit output: generics, AnyPointer, constants, schema evolution and union/group transitions

    Use --target native to run the same MoonBit peer as a native executable. Generated bindings are committed in tests/generated/upstream and tests/generated/compiler, so this exchange does not require a local compiler.

    #What is checked

    Existing declarationInput and independently checked output
    TestUseGenerics.basicTestGenerics(TestAllTypes, TestAnyPointer) retains concrete field types; reversed bindings and a list of nested generic structs round-trip
    TestUseGenerics.inner2Ancestor Bar binding, local Baz=Text, bound inner scope and unbound inner scope keep their distinct meanings
    unspecified, bindEnumListUnbound parameter carries Text through AnyPointer; a bound enum list preserves known ordinals 0/7 and unknown ordinal 40000 in both directions
    Generic defaults and aliasesUnwritten fields expose 123, "text", 321, [12,34] and [12,34,56] from the original schema
    TestAnyPointerStruct, Int32 list, UTF-8 Text, arbitrary Data and null are interpreted and copied without dropping unknown content
    TestAnyOthersAnyStruct and AnyList use their constrained views; a null capability remains null
    genericConstant, anyPointerConstants, embeddedStructGenerated constant functions produce real messages that Python/C++ decodes and checks
    embeddedData, embeddedTextGenerated values match Python/C++ constants and original embedded files byte for byte, including the Text file's final newline
    CodeGeneratorRequestBoth peers check Node, Brand.scopeId, each bound Type.typeId, and AnyPointer-backed Value against the original schema declarations
    TestOldVersion, TestNewVersionAn old writer is read with new recursive defaults (987, "baz") in both languages; MoonBit forwards a new message through the older typed view, and Python verifies the unknown root and child extension fields survive
    TestOldUnionVersion, TestNewUnionVersionExisting a/b alternatives retain their meanings when a becomes a nested union group; Python reads the MoonBit nested alternative through both old and new views
    TestUnionInUnion, TestInterleavedGroupsNested union reinitialization resets its tag and value; switching or clearing one interleaved group preserves the other group's data, pointers and tag
    TestUnion viewed through TestOldUnionVersionA legal C++-built tag 4 is unknown to the smaller view: both implementations reject its known members, MoonBit exposes the raw tag and forwards the complete payload without loss

    Python deliberately uses a small initial segment, forcing multi-segment input. MoonBit verifies both the ordinary frame and the packed frame. The reply uses generated builders for new values and schema-free deep copy for open payloads. Assertions check decoded values; merely exiting the generator is insufficient. For an unknown enum ordinal, the Python peer uses the original generic's unbound view to populate a UInt16 list with the C++ builder, then interprets it through the original bound enum-list schema. It does not patch message bytes or add an enum declaration to the source schema.

    The unknown-union case deliberately uses two existing layout-test declarations whose discriminants share an offset. It tests the unknown-tag guard and opaque forwarding, not a protocol-evolution relationship between those two types. Python checks the original schema offsets and constructs the payload with the C++ builder; no wire bytes are patched. Actual old/new-version compatibility is tested separately with upstream's explicitly paired version declarations.

    #Regenerate from the original schema files

    Install the official capnp 1.5.0 compiler and a C compiler for MoonBit's native plugin, then run:

    moon build --target native --release cmd/capnpc-moonbit
capnp compile --no-standard-import -Ifixtures/upstream/capnproto-1.5.0 --src-prefix=fixtures/upstream/capnproto-1.5.0 -o./_build/native/release/build/cmd/capnpc-moonbit/capnpc-moonbit.exe:tests/generated/upstream fixtures/upstream/capnproto-1.5.0/capnp/test.capnp
capnp compile --no-standard-import -Ifixtures/upstream/capnproto-1.5.0 --src-prefix=fixtures/upstream/capnproto-1.5.0 -o./_build/native/release/build/cmd/capnpc-moonbit/capnpc-moonbit.exe:tests/generated/compiler fixtures/upstream/capnproto-1.5.0/capnp/schema.capnp
moon fmt
git diff --exit-code -- tests/generated/upstream tests/generated/compiler

    The include root also contains stream.capnp, an implicit dependency of the upstream streaming-method declaration. Removing that declaration to avoid its dependency would change the protocol and invalidate this check.

    #Scope

    The versioned serialization target defines the completion criterion. These selected cases provide reproducible evidence, not a proof that every possible schema is covered. Interface declarations can coexist with data declarations; this example does not invoke RPC methods or transfer live capability tables. Unbound and recursively growing generic boundaries use explicit dynamic views rather than an infinite set of static specializations. The schema and wire data remain unchanged.

    Source Files

    Powered by MoonBit

    Site sourceReport issueModulesSkillsCLIBuild queueStatistics

    © 2026 mooncakes.io