moonbit-agent-guide（MoonbitAgent 指南）

Agent Workflow

For fast, reliable task execution, follow this order:

1. Clarify goal and constraints

- Confirm expected behavior, non-goals, and compatibility constraints (target backend, public API stability, performance limits).

1. Locate module/package boundaries

- Find moon.mod.json (module root) and relevant moon.pkg files (package boundaries and imports).

1. Discover APIs before coding

- Prefer moon ide doc queries to discover existing functions/types/methods before adding new code. - Use moon ide outline, moon ide peek-def, and moon ide find-references for semantic navigation.

1. Reliable refactoring

- Use moon ide rename for semantic refactoring. If multiple symbols share a name, add --loc filename:line:col. - If you want maintain backwards compatibility, use #alias(old_api, deprecated).

1. Edit minimally and package-locally

- Keep changes inside the correct package, use ///| top-level delimiters, and split code into cohesive files.

1. Validate in a tight loop

- Run moon check after edits, adding --warn-list +unnecessary_annotation to enable warning 73 for redundant annotations and over-qualified constructors (--warn-list +73 is equivalent). - Run targeted tests with moon test [dirname|filename] --filter 'glob' and use moon test --update for snapshot changes.

1. Finalize before handoff

- Run moon fmt. - Run moon info to verify whether public APIs changed (pkg.generated.mbti diff). - Report changed files, validation commands, and any remaining risks.

Fast Task Playbooks

Use the smallest playbook that matches the request.

Bug Fix (No API Change Intended)

1. Reproduce or identify the failing behavior.
2. Locate symbols with moon ide outline, moon ide peek-def, moon ide find-references.
3. Implement minimal fix in the current package.
4. Validate with:

- moon check - moon test [dirname|filename] --filter 'glob' (or closest targeted test scope) - moon fmt - moon info (confirm pkg.generated.mbti unchanged)

Refactor (Behavior Preserving)

1. Confirm behavior/API invariants first.
2. Prefer semantic rename/navigation tools:

- moon ide rename - moon ide find-references - moon ide peek-def - If multiple symbols share a name, use moon ide rename <symbol> <new_name> --loc filename:line:col.

1. Keep edits package-local and file-organization-focused.
2. Validate with:

- moon check - moon test [dirname|filename] - moon fmt - moon info (API should remain unchanged unless requested)

New Feature or Public API

1. Discover existing idioms with moon ide doc before introducing new names.
2. Add implementation in cohesive files with ///| delimiters.
3. Add/extend black-box tests and docstring examples for public APIs.
4. Validate with:

- moon check - moon test [dirname|filename] (use --update for snapshots when needed) - moon fmt - moon info (review and keep intended pkg.generated.mbti changes)

MoonBit Project Layouts

MoonBit uses the .mbt extension for source code files and interface files with the .mbti extension. At the top-level of a MoonBit project there is a moon.mod.json file specifying the metadata of the project. The project may contain multiple packages, each with its own moon.pkg. Subdirectories may also contain moon.mod.json files indicating that a different set of dependencies can be used for that subdir.

Example layout

my_module
├── moon.mod.json             # Module metadata, source field (optional) specifies the source directory of the module
├── moon.pkg             # Package metadata (each directory is a package like Golang)
├── README.mbt.md             # Markdown with tested code blocks (`test "..." { ... }`)
├── README.md -> README.mbt.md
├── cmd                       # Command line directory
│   └── main
│       ├── main.mbt
│       └── moon.pkg     # executable package with `options("is-main": true)`
├── liba/                     # Library packages
│   └── moon.pkg         # Referenced by other packages as `@username/my_module/liba`
│   └── libb/                 # Library packages
│       └── moon.pkg     # Referenced by other packages as `@username/my_module/liba/libb`
├── user_pkg.mbt              # Root packages, referenced by other packages as `@username/my_module`
├── user_pkg_wbtest.mbt       # White-box tests (only needed for testing internal private members, similar to Golang's package mypackage)
└── user_pkg_test.mbt         # Black-box tests
└── ...                       # More package files, symbols visible to current package (like Golang)

• Module: characterized by a moon.mod.json file in the project root directory. A MoonBit *module* is like a Go module; it is a collection of packages in subdirectories, usually corresponding to a repository or project. Module boundaries matter for dependency management and import paths.
• Package: characterized by a moon.pkg file in each directory. All subcommands of moon will still be executed in the directory of the module (where moon.mod.json is located), not the current package. A MoonBit *package* is the actual compilation unit (like a Go package). All source files in the same package are concatenated into one unit and thereby share all definitions throughout that package. The name in the moon.mod.json file combined with the relative path to the package source directory defines the package name, not the file name. Imports refer to module + package paths, NEVER to file names.
• Files: A .mbt file is just a chunk of source code inside a package. File names do NOT create modules, packages, or namespaces. You may freely split/merge/move declarations between files in the same package. Any declaration in a package can reference any other declaration in that package, regardless of file.

Coding/layout rules you MUST follow:

1. Prefer many small, cohesive files over one large file.

- Group related types and functions into focused files (e.g. http_client.mbt, router.mbt). - If a file is getting large or unfocused, create a new file and move related declarations into it.

1. You MAY freely move declarations between files inside the same package.

- Each block is separated by ///|. Moving a function/struct/trait between files does not change semantics, as long as its name and pub-ness stay the same. The order of each block is irrelevant too. - It is safe to refactor by splitting or merging files inside a package.

1. File names are purely organizational.

- Do NOT assume file names define modules, and do NOT use file names in type paths. - Choose file names to describe a feature or responsibility, not to mirror type names rigidly.

1. When adding new code:

- Prefer adding it to an existing file that matches the feature. - If no good file exists, create a new file under the same package with a descriptive name. - Avoid creating giant "impl", “misc”, or “util” files.

1. Tests:

- Place tests in dedicated test files (e.g. *_test.mbt) within the appropriate package. For a package (besides *_test.mbtfiles), *.mbt.md files are also blackbox test files in addition to Markdown files. The code blocks (separated by triple backticks) mbt check are treated as test cases and serve both purposes: documentation and tests. You may have README.mbt.md files with mbt check code examples. You can also symlink README.mbt.md to README.md to make it integrate better with GitHub. - It is fine — and encouraged — to have multiple small test files.

1. Interface files (pkg.generated.mbti) pkg.generated.mbti files are compiler-generated summaries of each package's public API surface. They provide a formal, concise overview of all exported types, functions, and traits without implementation details. They are generated using moon info and useful for code review. When you have a commit that does not change public APIs, pkg.generated.mbti files will remain unchanged, so it is recommended to put pkg.generated.mbti in version control when you are done. For IDE navigation and symbol lookup commands, see the dedicated moon ide section below.

Common Pitfalls to Avoid

• Don't use uppercase for variables/functions - compilation error
• Don't forget mut for mutable record fields - immutable by default (note that Arrays typically do NOT need mut unless completely reassigning to the variable - simple push operations, for example, do not need mut)
• Don't ignore error handling - errors must be explicitly handled
• Don't use return unnecessarily - the last expression is the return value
• Don't create methods without Type:: prefix - methods need explicit type prefix
• Don't forget to handle array bounds - use get() for safe access
• Don't forget @package prefix when calling functions from other packages
• Don't use ++ or -- (not supported) - use i = i + 1 or i += 1
• Don't add explicit try for error-raising functions - errors propagate automatically (unlike Swift)
• Legacy syntax: Legacy code may use function_name!(...) or function_name(...)? - these are deprecated, use normal calls.
• Prefer range for loops over C-style - for i in 0..<(n-1) {...} and for j in 0..=6 {...} are more idiomatic in MoonBit
• Async - MoonBit has no await keyword; do not add it. Async functions and tests are characterized by those which call other async functions. To identify a function or test as async, simply add the async prefix (e.g. [pub] async fn..., async test...).

moon Essentials

Essential Commands

• moon new my_project - Create new project
• moon run cmd/main - Run main package
• moon run - < hello.mbt - Run code from stdin (useful for quick experiments)
• moon run -c "code snippet" - Run code from command line argument (good for one-liners) Example: cat hello.mbt | moon run - This allows you to quickly test small snippets of MoonBit code without creating a full project. It can also be used with heredoc syntax for multi-line snippets: moon run - <<'EOF' fn main {println("Hello, MoonBit!")} EOF moon run -c 'fn main {println("Hello, MoonBit!")}'
• moon build - Build project (moon run and moon build both support --target)
• moon check - Type check without building, use it REGULARLY, it is fast (moon check also supports --target)
• moon info - Type check and generate mbti files. Run it to see if any public interfaces changed. (moon info also supports --target.)
• moon check --target all - Type check for all backends moon check --output-json can be used with jq to filter the output, e.g, moon check --output-json 2>&1 | jq -R 'fromjson? | select(.message | contains("unused"))' or, for richer post-processing, pipe into a small MoonBit program via moon run -c. Use --target native (the default wasm-gc does not support async fn main or @stdio.stdin), a quoted heredoc (<<'EOF') so the shell does not expand $/backticks in the source, and a de-indented closing EOF: ``

moon check --output-json 2>&1 | moon run --target native -c "$(cat <<'EOF' import {"moonbitlang/async", "moonbitlang/async/stdio", "moonbitlang/core/json",}

async fn main {let seen = {} while @stdio.stdin.read_until("\n") is Some(line) {try @json.parse(line.trim()) catch {_ => ()} noraise {{"level": "warning", "path": String(p),..} => if!seen.contains(p) {seen[p] = () println(p)} _ => ()}}} EOF)"

Get the diagnostics with "unused" in the message, which can be used to find unused code.
- `moon add package` - Add dependency
- `moon remove package` - Remove dependency
- `moon fmt` - Format code - should be run periodically - note that the files may be rewritten
Note you can also use `moon -C dir check` to run commands in a specific directory.
### Test Commands

- `moon test` - Run all tests
(`moon test` also supports `--target`)
- `moon test --update` - Update snapshots
- `moon test -v` - Verbose output with test names
- `moon test [dirname|filename]` - Test specific directory or file
- `moon coverage analyze` - Analyze coverage
- `moon test [dirname|filename] --filter 'glob'` - Run tests matching filter

moon test float/float_test.mbt --filter "Float::*" moon test float -F "Float::*" // shortcut syntax

## `README.mbt.md` Generation Guide

- Output `README.mbt.md` in the package directory.
`*.mbt.md` file and docstring contents treats `mbt check` specially.
`mbt check` block will be included directly as code and also run by `moon check` and `moon test`.  If you don't want the code snippets to be checked, explicit `mbt nocheck` is preferred.
If you are only referencing types from the package, you should use `mbt nocheck` which will only be syntax highlighted.
Symlink `README.mbt.md` to `README.md` to adapt to systems that expect `README.md`.

## Testing Guide

Use snapshot tests as it is easy to update when behavior changes.

- **Snapshot Tests**: `inspect(value, content="...")`. If unknown, write `inspect(value)` and run `moon test --update` (or `moon test -u`).
- Use regular `inspect()` for simple values (uses `Show` trait)
- Use `@json.inspect()` for complex nested structures (uses `ToJson` trait, produces more readable output)
- It is encouraged to `inspect` or `@json.inspect` the whole return value of a function if
  the whole return value is not huge, this makes the test simple. You need `impl (Show|ToJson) for YourType` or `derive (Show, ToJson)`.
- **Update workflow**: After changing code that affects output, run `moon test --update` to regenerate snapshots, then review the diffs in your test files (the `content=` parameter will be updated automatically).
- **Validation order**: Follow the canonical sequence in `Agent Workflow` and `Fast Task Playbooks`.

- Black-box by default: Call only public APIs via `@package.fn`. Use white-box tests only when private members matter.
- Grouping: Combine related checks in one `test "..." { ... }` block for speed and clarity.
- Panics: Name tests with prefix `test "panic ..." {...}`; if the call returns a value, wrap it with `ignore(...)` to silence warnings.
- Errors: Use `try? f()` to get `Result[...]` and `inspect` it when a function may raise.

### Docstring tests

Public APIs are encouraged to have docstring tests.

///| /// Get the largest element of a non-empty Array. /// /// # Example /// ``mbt check /// test { /// inspect(sum_array([1, 2, 3, 4, 5, 6]), content="21") /// } /// ` /// /// # Panics /// Panics if the xs` is empty. pub fn sum_array(xs : Array[Int]) -> Int { xs.fold(init=0, (a, b) => a + b) }

The MoonBit code in a docstring will be type checked and tested automatically (using `moon test --update`). In docstrings, `mbt check` should only contain `test` or `async test`.

## Spec-driven Development

- The spec can be written in a readonly `spec.mbt` file (name is conventional, not mandatory) with stub code marked as declarations:

///| declare pub type Yaml

///| declare pub fn Yaml::to_string(y : Yaml) -> String raise

///| declare pub impl Eq for Yaml

///| declare pub fn parse_yaml(s : String) -> Yaml raise

- Add `spec_easy_test.mbt`, `spec_difficult_test.mbt`, etc. to test the spec functions; everything will be type-checked(`moon check`).
- The AI or users can implement the `declare` functions in different files thanks to our package organization.
- Run `moon test` to check everything is correct.
- `declare` is supported for functions, methods, and types.
- The `pub type Yaml` line is an intentionally opaque placeholder; the implementer chooses its representation.
- Note the spec file can also contain normal code, not just declarations.

## `moon ide [doc|peek-def|outline|find-references|hover|rename|analyze]` for code navigation and refactoring

For project-local symbols and navigation, use:

- `moon ide doc <query>` to discover available APIs, functions, types, and methods in MoonBit. Always prefer `moon ide doc` over other approaches when exploring what APIs are available, it is **more powerful and accurate** than `grep_search` or any regex-based searching tools.
- `moon ide outline.` to scan a package,
- `moon ide find-references <symbol>` to locate usages, and
- `moon ide peek-def` for inline definition context and to locate toplevel symbols.
- `moon ide hover sym --loc filename:line:col` to get type information at a specific location.
- `moon ide rename <symbol> <new_name> [--loc filename:line:col]` to rename a symbol project-wide. Prefer `--loc` when symbol names are ambiguous.
- `moon ide analyze [path]` to inspect public API usage of a package or module when planning safe refactors. These tools save tokens and are more precise than grepping (`grep` displays results in both definitions and call sites including comments too).

### `moon ide doc` for API Discovery

`moon ide doc` uses a specialized query syntax designed for symbol lookup:

- **Empty query**: `moon ide doc ''`
  - In a module: shows all available packages in current module, including dependencies and moonbitlang/core
  - In a package: shows all symbols in current package
  - Outside package: shows all available packages
- **Function/value lookup**: `moon ide doc "[@pkg.]value_or_function_name"`
- **Type lookup**: `moon ide doc "[@pkg.]Type_name"` (builtin type does not need package prefix)
- **Method/field lookup**: `moon ide doc "[@pkg.]Type_name::method_or_field_name"`
- **Package exploration**: `moon ide doc "@pkg"`
  - Show package `pkg` and list all its exported symbols
  - Example: `moon ide doc "@json"` - explore entire `@json` package
  - Example: `moon ide doc "@encoding/utf8"` - explore nested package
- **Globbing**: Use `*` wildcard for partial matches, e.g. `moon ide doc "String::*rev*"` to find all String methods with "rev" in their name

#### `moon ide doc` Examples

search for String methods in standard library:

$ moon ide doc "String"

type String

pub fn String::add(String, String) -> String # ... more methods omitted ...

$ moon ide doc "@buffer" # list all symbols in package buffer: moonbitlang/core/buffer

fn from_array(ArrayView[Byte]) -> Buffer

... omitted ...

$ moon ide doc "@buffer.new" # list the specific function in a package: package "moonbitlang/core/buffer"

pub fn new(size_hint? : Int) -> Buffer Creates ... omitted ...

$ moon ide doc "String::*rev*" # globbing package "moonbitlang/core/string"

pub fn String::rev(String) -> String Returns ... omitted ... # ... more

pub fn String::rev_find(String, StringView) -> Int? Returns ... omitted ...

**Best practice**: Treat this section as command reference; execution order is defined in `Agent Workflow`.

### `moon ide rename sym new_name [--loc filename:line:col]` example

When the user asks: "Can you rename the function `compute_sum` to `calculate_sum`?"

$ moon ide rename compute_sum calculate_sum --loc math_utils.mbt:2

* Begin Patch * Update File: cmd/main/main.mbt @@ ///| fn main {

• println(@math_utils.compute_sum(1, 2))

+ println(@math_utils.calculate_sum(1, 2)) } * Update File: math_utils.mbt @@ ///| -pub fn compute_sum(a: Int, b: Int) -> Int { +pub fn calculate_sum(a: Int, b: Int) -> Int { a + b } * Update File: math_utils_test.mbt @@ ///| test {

• inspect(@math_utils.compute_sum(1, 2))

+ inspect(@math_utils.calculate_sum(1, 2)) } *** End Patch

### `moon ide hover sym --loc filename:line:col` example

When the user asks: "What is the signature and docstring of `filter`? at line 14 of hover.mbt"

$ moon ide hover filter --loc hover.mbt:14 test { let a: Array[Int] = [1] inspect(a.filter((x) => {x > 1})) ^^^^^^

            fn[T] Array::filter(self : Array[T], f : (T) -> Bool raise?) -> Array[T] raise?

Creates a new array containing all elements from the input array that satisfy ... omitted ... }

### `moon ide peek-def sym [--loc filename:line:col]` example

When the user asks: "Can you check if `Parser::read_u32_leb128` is implemented correctly?" you can run `moon ide peek-def Parser::read_u32_leb128` to get the definition context (this is better than `grep` since it searches the whole project by semantics):

L45:|///| L46:|fn Parser::read_u32_leb128(self : Parser) -> UInt raise ParseError { L47:| ... ...:| }

Now if you want to see the definition of the `Parser` struct, you can run:

$ moon ide peek-def Parser --loc src/parse.mbt:46:4 Definition found at file src/parse.mbt | ///| 2 | priv struct Parser { | ^^^^^^ | bytes : Bytes | mut pos : Int | } |

For the `--loc` argument, the line number must be precise; the column can be approximate since the positional argument `Parser` helps locate the position.

If the "sym" is a toplevel symbol, the location can be omitted:

$ moon ide peek-def String::rev Found 1 symbols matching 'String::rev':

pub fn String::rev in package moonbitlang/core/builtin at /Users/usrname/.moon/lib/core/builtin/string_methods.mbt:1039-1044 1039 | ///| | /// Returns a new string with the characters in reverse order. It respects | /// Unicode characters and surrogate pairs but not grapheme clusters. | pub fn String::rev(self : String) -> String { | self[:].rev() | }

### `moon ide outline [dir|file]` and `moon ide find-references <sym>` for Package Symbols

Use `moon ide outline` to scan a package or file for top-level symbols and locate usages without grepping.

- `moon ide outline dir` outlines the current package directory (per-file headers)
- `moon ide outline parser.mbt` outlines a single file This is useful when you need a quick inventory of a package, or to find the right file before `peek-def`.
- `moon ide find-references TranslationUnit` finds all references to a symbol in the current module

$ moon ide outline . spec.mbt: L003 | pub(all) enum CStandard { ... L013 | pub(all) struct Position { ...

$ moon ide find-references TranslationUnit

## Package Management

### Adding Dependencies

moon add moonbitlang/x # Add latest version moon add moonbitlang/x@0.4.6 # Add specific version

### Updating Dependencies

moon update # Update package index

### Typical Module configurations (`moon.mod.json`)

{ "name": "username/hello", // Required format for published modules "version": "0.1.0", "source": ".", // Source directory(optional, default: ".") "repository": "", // Git repository URL "keywords": [], // Search keywords "description": "...", // Module description "deps": { // Dependencies from mooncakes.io, usingmoon add to add dependencies "moonbitlang/x": "0.4.6" } }

### Typical Package configuration (`moon.pkg`)

moon.pkg for simplicity

import { "username/hello/liba", "moonbitlang/x/encoding" @libb } import {...} for "test" import {...} for "wbtest" options("is-main" : true) // other options

Packages are per directory and packages without a `moon.pkg` file are not recognized.

### Package Importing (used in moon.pkg)

- **Import format**: `"module_name/package_path"`
- **Usage**: `@alias.function()` to call imported functions
- **Default alias**: Last part of path (e.g., `liba` for `username/hello/liba`)
- **Package reference**: Use `@packagename` in test files to reference the tested package

**Package Alias Rules**:

- Import `"username/hello/liba"` → use `@liba.function()` (default alias is the last path segment)
- Import with custom alias `import {"moonbitlang/x/encoding" @enc}` → use `@enc.function()` (Note that this is unnecessary when the last path segment is identical to the alias name.)
- In `_test.mbt` or `_wbtest.mbt` files, the package being tested is auto-imported

Example:

///| /// In main.mbt after importing "username/hello/liba" in moon.pkg fn main { println(@liba.hello()) // Calls hello() from liba package }

### Using the Standard Library (moonbitlang/core)

**MoonBit standard library (moonbitlang/core) packages were automatically imported**. MoonBit is transitioning to explicit imports—you will see a warning to add imports like `moonbitlang/core/strconv` to `moon.pkg` if you use them. The module is always available without adding to dependencies.

### Creating Packages

To add a new package `fib` under `.`:

1. Create directory: `./fib/`
2. Add `./fib/moon.pkg`
3. Add `.mbt` files with your code
4. Import in dependent packages: `import {"username/hello/fib",}`

For more advanced topics like `conditional compilation`, `link configuration`, `warning control`, and `pre-build commands`, see `references/advanced-moonbit-build.md`.

# MoonBit Language Tour

## Core facts

- **Expression‑oriented**: `if`, `match`, loops return values; the last expression is the return value.
- **References by default**: Arrays/Maps/structs mutate via reference; use `Ref[T]` for primitive mutability.
- **Blocks**: Separate top‑level items with `///|`. Generate code block‑by‑block. If a blank line is desired within a block (enclosed by curly braces), add a comment line after the blank line (with or without comment text).
- **Visibility**: `fn` is private by default; `pub` exposes read/construct as allowed; `pub(all)` allows external construction.
- **Naming convention**: lower_snake for values/functions; UpperCamel for types/enums; enum variants start UpperCamel.
- **Packages**: No `import` in code files; call via `@alias.fn`. Configure imports in `moon.pkg`.
- **Placeholders**: `...` is a valid placeholder in MoonBit code for incomplete implementations.
- **Global values**: immutable by default and generally require type annotations.
- **Garbage collection**: MoonBit has a GC, there is no lifetime annotation, there's no ownership system. Unlike Rust, like F#, `let mut` is only needed when you want to reassign a variable, not for mutating fields of a struct or elements of an array/map.

## MoonBit Error Handling (Checked Errors)

MoonBit uses checked error-throwing functions, not unchecked exceptions. All errors are a subtype of `Error` and you can declare your own error types using `suberror`. Use `raise` in signatures to declare error types and let errors propagate by default. `try {} catch {}` to handle errors explicitly. Use `try!` to abort if it does raise. Occasionally, use `try?` to convert to `Result[...]` in tests for inspection.

///| /// Declare error types with 'suberror' suberror ValueError { ValueError(String) }

///| /// Tuple struct to hold position info struct Position(Int, Int) derive(ToJson, Debug, Eq)

///| /// ParseError is subtype of Error pub(all) suberror ParseError { InvalidChar(pos~ : Position, Char) // pos is labeled InvalidEof(pos~ : Position) InvalidNumber(pos~ : Position, String) InvalidIdentEscape(pos~ : Position) } derive(Eq, ToJson, Debug)

///| /// Functions declare what they can throw fn parse_int(s : String, position~ : Position) -> Int raise ParseError { // 'raise' throws an error if s is "" { raise ParseError::InvalidEof(pos=position) } ... // parsing logic }

///| /// Just declare raise to not track specific error types fn div(x : Int, y : Int) -> Int raise { if y is 0 { fail("Division by zero") } x / y }

///| test "inspect raise function" { let result : Result[Int, Error] = try? div(1, 0) guard result is Err(Failure::Failure(msg)) && msg.contains("Division by zero") else { fail("Expected error") } }

// Three ways to handle errors:

///| /// Propagate automatically fn use_parse(s : String, position~ : Position) -> Int raise ParseError { let x = parse_int(s, position~) // label punning, equivalent to position=position // Error auto-propagates by default. // Unlike Swift, you do not need to mark try for functions that can raise // errors; the compiler infers it automatically. This keeps error handling // explicit but concise. x * 2 }

///| /// Use try! to abort if it raises, no raise in the signature fn use_parse2(position~ : Position) -> Int { let x = try! parse_int("123", position~) // label punning x * 2 }

///| /// Handle with try-catch fn handle_parse(s : String, position~ : Position) -> Int { parse_int(s, position~) catch { ParseError::InvalidEof(pos=_) => { println("Parse failed: InvalidEof") -1 // Default value } _ => 2 } }

Important: When calling a function that can raise errors, if you only want to propagate the error, you do not need any marker; the compiler infers it. Note that all `async` functions automatically can raise errors without explicitly stating this.

## Integer, Char and overloaded literals

MoonBit supports `Byte`, `Int16`, `Int`, `UInt16`, `UInt`, `Int64`, `UInt64`, etc. When the type is known, the literal can be overloaded:

///| test "integer and char literal overloading disambiguation via type in the current context" { let (int, uint, uint16, int64, byte) : (Int, UInt, UInt16, Int64, Byte) = ( 1, 1, 1, 1, 1, ) // The literal 1 is overloaded based on the expected type in the current context. // compile time error if the literal cannot be represented in the target type, // e.g. let a7 : Byte = 256 // ❌ won't compile, 256 exceeds Byte max value 255 assert_eq(int, uint16.to_int()) let (a1, a2, a3) : (Int, Char, UInt16) = ('b', 'b', 'b') // char literal overloading, a1 will be the unicode value of 'b', // compile time error when the literal cannot be represented in the target type // e.g, let a6 : UInt16 = '𐍈' // ❌ won't compile, '𐍈' is U+10348, which exceeds UInt16 max value 0xffff let a4 : Byte = b'b' // Byte literal }

## Bytes (Immutable)

///| test "bytes literals" { let b0 : Bytes = b"abcd" let b1 : Bytes = [0xff, 0x00, 0x01] // Array literal overloading guard b0 is [b'a', ..] && b0[1] is b'b' else { // Bytes can be pattern matched as BytesView and indexed fail("unexpected bytes content") } }

## Array (Resizable)

///| test "array literals overloading: disambiguation via type in the current context" { let (a0, a1, a2, a3) : ( Array[Int], FixedArray[Int], ReadOnlyArray[Int], ArrayView[Int], ) = ([1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3]) // The literal [1, 2, 3] is overloaded based on the expected type in the current context. // Defaults to Array[_] }

## String (Immutable UTF-16)

`s[i]` returns a code unit (UInt16), `s.get_char(i)` returns `Char?`. Since MoonBit supports char literal overloading, you can write code snippets like this:

///| test "string indexing and utf8 encode/decode" { let s = "hello world" let b0 : UInt16 = s[0] guard b0 is ('\n' | 'h' | 'b' | 'a'..='z') && s is [.. "hello", .. rest] else { fail("unexpected string content") } guard rest is " world" // otherwise will crash (guard without else)

// In check mode (expression with explicit type), ('\n' : UInt16) is valid.

// Using get_char for Option handling let b1 : Char? = s.get_char(0) assert_true(b1 is Some('a'..='z'))

// ⚠️ Important: Variables won't work with direct indexing let eq_char : Char = '=' // s[0] == eq_char // ❌ Won't compile - eq_char is not a literal, lhs is UInt while rhs is Char // Use: s[0] == '=' or s.get_char(0) == Some(eq_char) let bytes = @utf8.encode("中文") // utf8 encode package is in stdlib assert_true(bytes is [0xe4, 0xb8, 0xad, 0xe6, 0x96, 0x87]) let s2 : String = @utf8.decode(bytes) // decode utf8 bytes back to String assert_true(s2 is "中文") for c in "中文" { let _ : Char = c // unicode safe iteration println("char: \{c}") // iterate over chars } }

### String Interpolation && StringBuilder

MoonBit uses `\{}` for string interpolation, for custom types, they need to implement trait `Show`.

///| test "string interpolation basics" { let name : String = "Moon" let config = { "cache": 123 } let version = 1.0 println("Hello \{name} v\{version}") // "Hello Moon v1.0" // ❌ Wrong - quotes inside interpolation not allowed: // println(" - Checking if 'cache' section exists: \{config["cache"]}")

// ✅ Correct - extract to variable first: let has_key = config["cache"] // " not allowed in interpolation println(" - Checking if 'cache' section exists: \{has_key}") let sb = StringBuilder::new() sb.write_char('[') sb.write_view([ for x in [1, 2, 3] => "\{x}" ].join(",")) sb.write_char(']') inspect(sb.to_string(), content="[1,2,3]") }

Expressions inside `\{}` can only be *basic expressions* (no quotes, newlines, or nested interpolations). String literals are not allowed as they make lexing too difficult.

### Multiple line strings

///| test "multi-line string literals" { let multi_line_string : String = #|Hello "world" #|World #| let multi_line_string_with_interp : String = $|Line 1 "" $|Line 2 \{1+2} $| // no escape in #|, // only escape '\{..} in $|` assert_eq(multi_line_string, "Hello \"world\"\nWorld\n") assert_eq(multi_line_string_with_interp, "Line 1 \"\"\nLine 2 3\n") }

## Map (Mutable, Insertion-Order Preserving)

///| test "map literals and common operations" { // Map literal syntax let map : Map[String, Int] = { "a": 1, "b": 2, "c": 3 } let empty : Map[String, Int] = {} // Empty map, preferred let also_empty : Map[String, Int] = Map::new() // From array of pairs let from_pairs : Map[String, Int] = Map::from_array([("x", 1), ("y", 2)])

// Set/update value map["new-key"] = 3 map["a"] = 10 // Updates existing key

// Get value - returns Option[T] guard map is { "new-key": 3, "missing"? : None, .. } else { fail("unexpected map contents") }

// Direct access (panics if key missing) let value : Int = map["a"] // value = 10

// Iteration preserves insertion order for k, v in map { println("\{k}: \{v}") // Prints: a: 10, b: 2, c: 3, new-key: 3 }

// Other common operations map.remove("b") guard map is { "a": 10, "c": 3, "new-key": 3, .. } && map.length() == 3 else { // "b" is gone, only 3 elements left fail("unexpected map contents after removal") } }

## View Types

**Key Concept**: View types (`StringView`, `BytesView`, `ArrayView[T]`) are zero-copy, non-owning read-only slices created with the `[:]` syntax. They don't allocate memory and are ideal for passing sub-sequences without copying data, for functions which take `String`, `Bytes`, `Array`, they also take `*View` (implicit conversion).

- `String` → `StringView` via `s[:]` or `s[start:end]` or `s[start:]` or `s[:end]`
- `Bytes` → `BytesView` via `b[:]` or `b[start:end]`, etc.
- `Array[T]`, `FixedArray[T]`, `ReadOnlyArray[T] →`ArrayView[T]`via`a[:]`or`a[start:end]`, etc.

**Important**: StringView slice is slightly different due to unicode safety: `s[a:b]` may raise an error at surrogate boundaries (UTF-16 encoding edge case). You have two options:

- Use `try! s[a:b]` if you're certain the boundaries are valid (crashes on invalid boundaries)
- Let the error propagate to the caller for proper handling

**When to use views**:

- Pattern matching with rest patterns (`[first,.. rest]`)
- Passing slices to functions without allocation overhead
- Avoiding unnecessary copies of large sequences

Convert back with `.to_string()`, `.to_bytes()`, or `.to_array()` when you need ownership. (`moon ide doc StringView`)

## User defined types(`enum`, `struct`)

///| enum Tree[T] { Leaf(T) // Unlike Rust, no comma here Node(left~ : Tree[T], T, right~ : Tree[T]) // enum can use labels } derive(Debug, ToJson) // derive traits for Tree

///| pub fn Tree::sum(tree : Tree[Int]) -> Int { match tree { Leaf(x) => x // we don't need to write Tree::Leaf, when tree has a known type Node(left~, x, right~) => left.sum() + x + right.sum() // method invoked in dot notation } }

///| struct Point { x : Int y : Int } derive(Debug, ToJson) // derive traits for Point

///| pub fn Point::Point(x~ : Int, y~ : Int) -> Point { { x, y } }

///| test "user defined types: enum and struct" { json_inspect(Point(x=10, y=20), content={ "x": 10, "y": 20 }) debug_inspect( Point(x=10, y=20), content=( #|{ x: 10, y: 20 } ), ) }

## Functional `for` loop

///| pub fn binary_search(arr : ArrayView[Int], value : Int) -> Result[Int, Int] { let len = arr.length() // functional for loop: // initial state ; [predicate] ; [post-update] { // loop body with continue to update state //} nobreak { // exit block // } // predicate and post-update are optional for i = 0, j = len; i < j; { // post-update is omitted, we use continue to update state let h = i + (j - i) / 2 if arr[h] < value { continue h + 1, j // functional update of loop state } else { continue i, h // functional update of loop state } } nobreak { // exit of for loop if i < len && arr[i] == value { Ok(i) } else { Err(i) } } where { proof_invariant: 0 <= i && i <= j && j <= len, proof_invariant: i == 0 || arr[i - 1] < value, proof_invariant: j == len || arr[j] >= value, proof_reasoning: ( #|For a sorted array, the boundary invariants are witnesses: #| - arr[i-1] < value implies all arr[0..i) < value (by sortedness) #| - arr[j] >= value implies all arr[j..len) >= value (by sortedness) #| #|Preservation proof: #| - When arr[h] < value: new_i = h+1, and arr[new_i - 1] = arr[h] < value ✓ #| - When arr[h] >= value: new_j = h, and arr[new_j] = arr[h] >= value ✓ #| #|Termination: j - i decreases each iteration (h is strictly between i and j) #| #|Correctness at exit (i == j): #| - By invariants: arr[0..i) < value and arr[i..len) >= value #| - So if value exists, it can only be at index i #| - If arr[i] != value, then value is absent and i is the insertion point #| ), } }

///| test "functional for loop control flow" { let arr : Array[Int] = [1, 3, 5, 7, 9] inspect(binary_search(arr, 5), content="Ok(2)") // Array to ArrayView implicit conversion when passing as arguments inspect(binary_search(arr, 6), content="Err(3)") // for iteration is supported too for i, v in arr { println("\{i}: \{v}") // i is index, v is value } }

You are *STRONGLY ENCOURAGED* to use functional `for` loops instead of imperative loops *WHENEVER POSSIBLE*, as they are easier to read and reason about.

### Loop Invariants with `where` Clause

The `where` clause attaches **machine-checkable invariants** and **human-readable reasoning** to functional `for` loops. This enables formal verification thinking while keeping the code executable. Note for trivial loops, you are encouraged to convert it into `for.. in` so no reasoning is needed.

**Syntax:**

for ... { ... } where { invariant : <boolean_expr>, // checked at runtime in debug builds invariant : <boolean_expr>, // multiple invariants allowed reasoning : <string> // documentation for proof sketch }

**Writing Good Invariants:**

1. **Make invariants checkable**: Invariants must be valid MoonBit boolean expressions using loop variables and captured values.
2. **Use boundary witnesses**: For properties over ranges (e.g., "all elements in arr[0..i) satisfy P"), check only boundary elements. For sorted arrays, `arr[i-1] < value` implies all `arr[0..i) < value`.
3. **Handle edge cases with `||`**: Use patterns like `i == 0 || arr[i-1] < value` to handle boundary conditions where the check would be out of bounds.
4. **Cover three aspects in reasoning**:
  - **Preservation**: Why each `continue` maintains the invariants
  - **Termination**: Why the loop eventually exits (e.g., a decreasing measure)
  - **Correctness**: Why the invariants at exit imply the desired postcondition

## Label and Optional Parameters

Good example: use labeled and optional parameters

///| fn g( positional : Int, required~ : Int, optional? : Int, // no default => Option optional_with_default? : Int = 42, // default => plain Int ) -> String { // These are the inferred types inside the function body. let _ : Int = positional let _ : Int = required let _ : Int? = optional let _ : Int = optional_with_default "\{positional},\{required},\{optional},\{optional_with_default}" }

///| test { inspect(g(1, required=2), content="1,2,None,42") inspect(g(1, required=2, optional=3), content="1,2,Some(3),42") inspect(g(1, required=4, optional_with_default=100), content="1,4,None,100") }

Misuse: `arg: Type?` is not an optional parameter. Callers still must pass it (as `None`/`Some(...)`).

///| fn with_config(a : Int?, b : Int?, c : Int) -> String { "\{a},\{b},\{c}" }

///| test { inspect(with_config(None, None, 1), content="None,None,1") inspect(with_config(Some(5), Some(5), 1), content="Some(5),Some(5),1") }

Anti-pattern: `arg?: Type?` (no default => double Option). If you want a defaulted optional parameter, write `b?: Int = 1`, not `b?: Int? = Some(1)`.

///| fn f_misuse(a? : Int?, b? : Int = 1) -> Unit { let _ : Int?? = a // rarely intended let _ : Int = b } // How to fix: declare (a? : Int, b? : Int = 1) directly.

///| fn f_correct(a? : Int, b? : Int = 1) -> Unit { let _ : Int? = a let _ : Int = b }

///| test { f_misuse(b=3) f_misuse(a=Some(5), b=2) // works but confusing f_correct(b=2) f_correct(a=5) }

Bad example: `arg: APIOptions` (use labeled optional parameters instead)

///| /// Do not use struct to group options. struct APIOptions { width : Int? height : Int? }

///| fn not_idiomatic(opts : APIOptions, arg : Int) -> Unit {

}

///| test { // Hard to use in call site not_idiomatic({ width: Some(5), height: None }, 10) not_idiomatic({ width: None, height: None }, 10) }

## More details

For deeper syntax, types, and examples, read `references/moonbit-language-fundamentals.mbt.md`.