moonbit-practice

name: moonbit-practice description: MoonBit code generation best practices. Use when writing MoonBit code to avoid common AI mistakes with syntax, tests, and benchmarks.

MoonBit Practice Guide

Best practices for AI when generating MoonBit code. If you don't understand something here, use moonbit-docs skill to search the official documentation.

Guidelines

Code Navigation: Prefer moon ide over Read/Grep

In MoonBit projects, prefer moon ide commands over Read tool or grep.

# ❌ Avoid: Reading files directly
Read src/parser.mbt

# ✅ Recommended: Find definitions from symbols
moon ide peek-def Parser::parse
moon ide goto-definition -tags 'pub fn' -query 'parse'

# ❌ Avoid: Searching with grep
grep -r "fn parse" .

# ✅ Recommended: Semantic search
moon ide find-references parse
moon ide outline src/parser.mbt

Why:

• moon ide provides semantic search (distinguishes definitions from call sites)
• grep picks up comments and strings
• moon doc quickly reveals APIs

Other Rules

• Use moon doc '<Type>' to explore APIs before implementing
• Check reference/configuration.md before editing moon.pkg.json / moon.mod.json
• Check reference/agents.md when updating CLAUDE.md

Common Pitfalls

• Don't use uppercase for variables/functions - compilation error
• mut is only for reassignment, not field mutation - Array push doesn't need it
• return is unnecessary - last expression is the return value
• Methods require Type:: prefix
• ++ -- not supported - use i = i + 1 or i += 1
• No try needed for error propagation - automatic (unlike Swift)
• No await keyword - just declare with async fn
• Prefer range for over C-style - for i in 0..<n {...}
• Legacy syntax: function_name!(...) and function_name(...)? are deprecated

Common Syntax Mistakes by AI

Type Parameter Position

///| NG: fn identity[T] is old syntax
fn identity[T](val: T) -> T { val }

///| OK: Type parameter comes right after fn
fn[T] identity(val: T) -> T { val }

raise Syntax

///|
/// NG: -> T!Error was removed
fn parse(s: String) -> Int!Error { ... }

///|
/// OK: Use raise keyword
fn parse(s: String) -> Int raise Error { ... }

Int raise is shorthand for Int raise Error. async fn implicitly raises by default; use noraise to enforce no errors.

Macro Calls

///|
/// NG: ! suffix was removed
assert_true!(true)

///|
/// OK
assert_true(true)

Multi-line Text

let text =
  #|line 1
  #|line 2

Comments and Block Separators

///| is a block separator. /// comments attach to the following ///| block.

///|
/// This function is foo
fn foo() -> Unit { ... }

///|
/// This function is bar
fn bar() -> Unit { ... }

Avoid consecutive ///| at the file beginning as they create separate blocks.

Snapshot Tests

moon test -u auto-updates content="" in inspect(val).

test "snapshot" {
  inspect([1, 2, 3], content="")  // auto-filled by moon test -u
}

After running:

test "snapshot" {
  inspect([1, 2, 3], content="[1, 2, 3]")
}

Doc Tests

Available in .mbt.md files or ///| inline comments.

Example (inline comment):

///|
/// Increment an integer by 1
/// ```mbt test
/// inspect(incr(41), content="42")
/// ```
pub fn incr(x : Int) -> Int {
  x + 1
}

Pre-release Checklist

Run before releasing:

moon fmt   # Format code
moon info  # Generate type definition files

pkg.generated.mbti is auto-generated by moon info. Don't edit it directly.

Exploring Built-in Type Methods

moon doc StringView   # StringView methods
moon doc Array        # Array methods
moon doc Map          # Map methods

Quick Reference

moon ide Tools

More accurate than grep for code navigation. See reference/ide.md for details.

# Show symbol definition
moon ide peek-def Parser::read_u32_leb128

# Package outline
moon ide outline .

# Find references
moon ide find-references TranslationUnit

# Jump to type definition (with location)
moon ide peek-def Parser -loc src/parse.mbt:46:4

Functional for loop

Prefer functional for loops whenever possible. More readable and easier to reason about.

// Functional for loop with state
for i = 0, sum = 0; i <= 10; {
  continue i + 1, sum + i  // Update state
} else {
  sum  // Value at loop exit
}

// Range for (recommended)
for i in 0..<n { ... }
for i, v in array { ... }  // index and value

Error Handling

MoonBit uses checked errors. See reference/ffi.md for details.

///| Declare error type
suberror ParseError {
  InvalidEof
  InvalidChar(Char)
}

///| Declare with raise, auto-propagates
fn parse(s: String) -> Int raise ParseError {
  if s.is_empty() { raise ParseError::InvalidEof }
  ...
}

///| Convert to Result
let result : Result[Int, ParseError] = try? parse(s)

///| Handle with try-catch
parse(s) catch {
  ParseError::InvalidEof => -1
  _ => 0
}

Assets

assets/ci.yaml is a GitHub Actions workflow for CI