Moonbit language

MoonBit language

Verification contract

Executable claims in this skill are checked against moonc v0.10.4+2cc641edf (MoonBit release 0.10.4, build 2026-07-15) on Linux x86_64. The repository runs every mbt check fence on wasm-gc, wasm, js, and native and runs diagnostic fixtures against the same pin. Facts that depend on an external runtime or were not executed are explicitly labeled Documented, not executed with an official source. Blocks marked mbt nocheck are explanatory rejected or deprecated forms; they are not executed.

MoonBit is pre-1.0 and changes quickly. Before relying on a load-bearing claim, run moon version --all. If local moonc differs from the pin:

1. Treat the claim as a hypothesis.
2. Reproduce the smallest relevant example with moon check, moon test, or node scripts/verify_snippet.ts.
3. Check the official release notes between 0.10.4 and the local version.
4. State which version was actually tested and any remaining difference.

Work by task

• Understand code: identify unfamiliar syntax and the local compiler version; search this file for the exact token or diagnostic; load the routed reference; explain behavior only after matching the code to a checked example or an explicitly documented fact.
• Write code: load the smallest relevant reference, use its current forms, then run moon check and the narrowest useful moon test. Do not invent syntax from another language or an old MoonBit tutorial.
• Fix code: preserve the program's intent, reproduce the diagnostic, search both the diagnostic text and suspicious token in the references, apply the current migration, and rerun with warnings denied when deprecation is involved. Do not silence a warning before proving the replacement.
• Judge a feature: distinguish verified, documented, proposal, and unknown. Official docs can lag or suppress warnings; the compiler at the stated pin decides executable behavior.

If a token, warning, or construct is not named below, search all references/ for the exact spelling and the compiler's diagnostic before concluding that the skill does not cover it.

Feature index

Each row is both a capability map and a search route. Load only the rows relevant to the task.

Need or search terms	Reference
Program structure; fn main; fn init; naming conventions; Keywords and Reserved Keywords; // comments; /// Doc Comments and doc tests; labelled arguments; optional arguments and forwarding; autofill / #callsite; declare fn; function aliases / #alias; deprecated fnalias; letrec; lambdas; partial application; pipelines |> and <|; methods; cascades; TODO placeholder ...	references/declarations-and-functions.mbt.md
Structs; struct update; enums; mutable enum fields; extenum; custom constructors; tuple structs; aliases; opaque types; derives; Eq / Compare / Default; JSON Enum styles; Deriving Option; Container, Case, and Field arguments; local type definitions are deprecated	references/types-structs-enums.mbt.md
match; guards; array, string, map, bitstring, and Json patterns; regex literal and =~; or-pattern defaults with with; lexscan; deprecated lexmatch	references/pattern-matching.mbt.md
Traits; generics; impls; explicit extend and pub extend; implicit_impl_as_method; supertrait dot-call migration; Builtin traits; Deriving builtin traits; operators; trait objects	references/traits-and-generics.mbt.md
pub; pub(all); pub(open); priv; alias visibility; pub impl; source-level using; cross-package access and re-export	references/visibility.mbt.md
Error types; raise; try / catch / noraise; Option; Result; deprecated !Error, bang calls, and try?	references/errors-and-error-handling.mbt.md
Unit; Bool; Boolean negation; !value; deprecated not(value); Ref; Option; Result; Json; Numbers and overloaded literals; strings, bytes, escapes, interpolation, and Choosing a Byte Container; arrays, maps, views; conditional spread; ambiguous {}; immutable Type(array) / deprecated from_array; negative view indices; removed collection aliases and IterResult; Json::empty_object, String predicates, Int16 / UInt16 lnot	references/data-types.mbt.md
if; while; for; list comprehensions; Iter and [| ... |]; deprecated implicit array-to-Iter and loop; labels; closures; defer	references/control-flow-and-iteration.mbt.md
test; snapshots; inspect; blackbox and whitebox tests; doc tests; .mbt.md	references/tests-and-checked-docs.mbt.md
async; task groups; cancellation and cleanup; JavaScript Promise bridge; native/JS/Wasm1 runtime boundaries; extern "js", extern "wasm", extern "C"; stable FFI Types; constant enum ABI values; exported functions; FFI ownership and callbacks	references/async-and-ffi.md
Attributes including #cfg, #deprecated, #alias, #as_free_fn, #callsite, #doc(hidden), #internal, #label_migration, #module, #must_implement_one, #visibility, #warnings; deprecated @ warning switch and --deny-warn; derives	references/attributes.mbt.md
Rust, TypeScript, Go, and stale MoonBit forms; silent interpolation, defer, integer division, and indexing traps	references/cross-language-and-stale-syntax.md
Skill guidance contradicts the compiler; minimal reproduction and privacy-scrubbed issue draft	references/reporting-errors.md

Current syntax and migrations

• Entry points are fn main { } and fn init { }; type parameters precede names (fn[T] id); top-level function parameters and results need annotations.
• Trait implementations do not automatically create dot-call methods. Attach intended methods with extend Type with Trait::{method}; use pub extend when downstream packages need the dot call. Rename identifiers called extend, and use qualified Trait::method(value) for deprecated supertrait or ambiguous constrained dot calls.
• Current checked-error syntax is suberror, -> T raise E, and try / catch / noraise. The !Error, f!(), and try? forms are deprecated.
• loop and expected-type conversion from an array literal to Iter are deprecated. Use while true / for and explicit [| ... |] respectively.
• Local type definitions still parse but emit deprecated_syntax; move the type to top level. Do not copy the official local-type example without checking warnings.
• derive(Show) for debugging is deprecated; use Debug. Unnecessary let mut is an error. String interpolation is \{value}, not ${value}.

Reporting contradictions

If the pinned compiler contradicts this skill, reproduce it with a fresh minimal example and follow references/reporting-errors.md before the final response. Prepare a privacy-scrubbed issue title and body and show the exact draft plus issue-template link. The skill workflow never submits the issue, invokes GitHub, edits the installed skill, or writes the draft into the user's repository.

Boundary

This skill owns language semantics: what MoonBit code means and which programs compile. It does not own moon commands, moon.mod / moon.pkg, dependencies, targets, linking, publishing, or project setup; use moonbit-toolchain for those. Load both skills when a task changes code and project operation together. Running moon check or moon test solely to verify a language claim does not require the toolchain skill.