moonbit-practice

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

```moonbit
///| 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.

Code Block	Behavior
```mbt check	Checked by LSP
```mbt test	Executed as test {...}
```moonbit	Display only (not executed)

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

Topic	Command	Details
Test	moon test	https://docs.moonbitlang.com/en/stable/language/tests
Update snapshots	moon test -u	Same as above
Filtered test	moon test --filter 'glob'	Run specific tests
Benchmark	moon bench	https://docs.moonbitlang.com/en/stable/language/benchmarks
Doc Test	moon check / moon test	https://docs.moonbitlang.com/en/stable/language/docs
Format	moon fmt	-
Generate types	moon info	-
Doc reference	moon doc <Type>	-

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