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/language.md for detailed language feature examples (types, traits, pattern matching, etc.)

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 {...}
• nobreak not else for functional for-loop exit values (else is deprecated)
• 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

# Show type signature and docs at location (unlike peek-def, shows inferred type + doc comments)
moon ide hover my_func --loc src/lib.mbt:10:4

# Rename symbol across the project
moon ide rename old_name new_name

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
} nobreak {
  sum  // Value at loop exit (nobreak, not else)
}

// 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