Moonbit Language

MoonBit language

Verification contract

Everything in this skill was verified by compiling and running real code against moonc v0.10.4+2cc641edf (MoonBit release 0.10.4, build 2026-07-15) on Linux x86_64, targets wasm-gc, wasm, js, native, on 2026-07-18. MoonBit is pre-1.0 and changes fast; there is no language version separate from the compiler — language behavior is keyed to the moonc version.

Before relying on this skill, run moon version --all. If the local moonc differs from the pin above:

1. Treat every claim here as a hypothesis, not a fact.
2. For any load-bearing construct, run a minimal experiment first: put the snippet in a scratch file and run moon check /tmp/probe.mbt or a test { } block via moon test /tmp/probe.mbt.md (both work standalone, no project needed) — or pipe it through node scripts/verify_snippet.ts, which does exactly that.
3. Check the release notes at https://www.moonbitlang.com/updates/ for changes between 0.10.4 and the local version.
4. Say explicitly in your answer which version you verified against and where versions differed.

Non-negotiable working rules

• The compiler is the oracle. Never assert that MoonBit syntax exists, or that code is correct, from memory alone — MoonBit changed too fast for pretraining to be trusted. If you cannot run moon check, say so and mark the answer unverified.
• Write current syntax, not remembered syntax. The highest-risk drift areas, all verified 2026-07-18: entry point is fn main { } (no parens); fn init { } (bare init { } is gone); type parameters are prefix (fn[T] id, not fn id[T]); trait/impl bodies never use impl Type { } blocks; error handling is suberror + -> T raise E + try/catch/noraise (the !Error, f!(), try? forms are deprecated); loop is deprecated; derive(Show) for debugging is deprecated in favor of Debug.
• Distinguish status. Label knowledge as: verified (ran here), documented (official docs, not run), proposal (moonbit-evolution — NEVER present as shipped; that repo's own status fields lag), or unknown. The docs themselves lag in places (e.g. trait pages still show pre-0.10.0 fn-less signatures).
• Cross-language habits are the main failure mode. When input code looks like Rust, TypeScript, or Go, check references/cross-language-and-stale-syntax.md before "fixing" it into another language's idiom. Watch the silent traps: "${x}" is literal text (interpolation is \{x}), defer is block-scoped, and unnecessary let mut is a hard error.

• If the toolchain contradicts this skill, complete the reporting protocol before the final response rather than merely mentioning the mismatch. Reproduce it with a fresh minimal snippet, prepare a privacy-scrubbed public issue draft, show the user its exact title and body, and provide the issue-template link. Stop there: never submit the issue or invoke GitHub from this skill workflow, even if the user gave blanket or draft-specific permission. A later request for the host agent to send it is a separate outbound action governed by that client's own approval controls. Never edit the installed skill copy or write the draft into the user's repository. Exact protocol: references/reporting-errors.md

Reference routing

Load only what the task needs:

• Program structure, functions, methods, lambdas, pipelines, cascades → references/declarations-and-functions.mbt.md
• Structs, enums, newtypes, aliases, derives → references/types-structs-enums.mbt.md
• match, patterns (array/string/map/range), is, guard → references/pattern-matching.mbt.md
• Traits, generics, impls, operators, trait objects → references/traits-and-generics.mbt.md
• pub / pub(all) / pub(open) / priv, cross-package access → references/visibility.mbt.md
• Error types, raise, try/catch, Option/Result → references/errors-and-error-handling.mbt.md
• Numbers, strings, bytes, arrays, maps, views → references/data-types.mbt.md
• Loops, iteration, closures, guard flow, Iter → references/control-flow-and-iteration.mbt.md
• test blocks, inspect snapshots, doc tests, .mbt.md → references/tests-and-checked-docs.mbt.md
• async semantics, extern FFI declarations → references/async-and-ffi.md
• Attributes (#cfg, #deprecated, #alias, ...) → references/attributes.mbt.md
• Rust/TS/Go habits and stale MoonBit forms → references/cross-language-and-stale-syntax.md

Files ending in .mbt.md are executable documentation: every mbt check block in them is compiled and run by this repository's CI against the pinned toolchain, so their examples are guaranteed-current at the pin. Blocks marked mbt nocheck show rejected or deprecated forms.

• Toolchain contradicts this skill → report upstream (with consent): references/reporting-errors.md

Boundary

This skill owns language semantics (what code means, which programs compile). It does not cover moon commands, moon.mod/moon.pkg configuration, dependency management, targets, linking, or publishing — that is the moonbit-toolchain skill. For a task that spans both (new package with code, FFI plus build wiring, fixing a repo where both code and config are broken), load both skills. Using moon check/moon test to verify language claims as described above does not require the toolchain skill.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: hyfdev
• Source: hyfdev/moonbit-agent-skills
• License: MIT

Install and usage instructions live in the source repository linked above.