MoonBit

https://docs.moonbitlang.com/en/latest
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### Example Host Output

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/wasm/component-model-tutorial.md

The expected output when running the example host with the adder component, demonstrating the addition of two numbers.

```text
5 + 3 = 8
```

--------------------------------

### Corrected Example: Providing All Required Fields

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4044.html

This example demonstrates the correct way to initialize a struct by providing values for all its required fields.

```moonbit
pub(all) struct S {
  a : Int
  b : Int
}

pub let s : S = { a: 1, b: 2 }

```

--------------------------------

### Test Component with Example Host

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/wasm/component-model-tutorial.md

Clone the example host repository, copy your component, and run the host with your component's path and arguments.

```console
$ git clone https://github.com/bytecodealliance/component-docs.git
$ cd component-docs/component-model/examples/example-host
$ cp /path/to/adder.component.wasm .
$ cargo run --release -- 5 3 adder.component.wasm
```

--------------------------------

### Testing Component with Example Host

Source: https://docs.moonbitlang.com/en/latest/toolchain/wasm/component-model-tutorial.html

Clone the example host repository, copy your component, and run it with cargo. Ensure the component path is correct.

```bash
$ git clone https://github.com/bytecodealliance/component-docs.git
$ cd component-docs/component-model/examples/example-host
$ cp /path/to/adder.component.wasm .
$ cargo run --release -- 5 3 adder.component.wasm

```

--------------------------------

### Snapshot File Content Example

Source: https://docs.moonbitlang.com/en/latest/_downloads/78d8998d78e60fd7b1b2f4c9bb2819fb/summary.md

This is an example of the content that would be generated by the `record anything` snapshot test.

```text
Hello, world! And hello, MoonBit!
```

--------------------------------

### Install MoonBit Toolchain on Windows

Source: https://docs.moonbitlang.com/en/latest/tutorial/tour.html

Use this PowerShell command to install the MoonBit toolchain on Windows. It sets the execution policy and then runs the installation script.

```powershell
Set-ExecutionPolicy RemoteSigned -Scope CurrentUser; irm https://cli.moonbitlang.com/install/powershell.ps1 | iex

```

--------------------------------

### Install MoonBit Toolchain on Linux/macOS

Source: https://docs.moonbitlang.com/en/latest/tutorial/tour.html

Use this command to install the MoonBit toolchain on Linux and macOS systems. It downloads and executes an installation script.

```shell
curl -fsSL https://cli.moonbitlang.com/install/unix.sh | bash

```

--------------------------------

### Install wasm-tools

Source: https://docs.moonbitlang.com/en/latest/toolchain/wasm/component-model-tutorial.html

Install `wasm-tools` for working with WebAssembly components.

```bash
$ cargo install wasm-tools

```

--------------------------------

### Corrected Example: Using Labelled Argument

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4086.html

This example shows the correct way to call the function by supplying the required labelled argument.

```moonbit
///|
pub fn f(name~ : String) -> Unit {
  println("Hello, {name}")
}

///|
test {
  f(name="John")
}


```

--------------------------------

### Snapshot File Content Example

Source: https://docs.moonbitlang.com/en/latest/_sources/language/tests.md

This is an example of the content that might be generated in a snapshot file when using `@test.T::write` and `@test.T::writeln`.

```text
hello
world
```

--------------------------------

### JavaScript Host Runtime Example for Imported String Constants

Source: https://docs.moonbitlang.com/en/latest/toolchain/moon/package.html

Example JavaScript code demonstrating how to instantiate a Wasm module with custom imported string constants.

```javascript
const { instance } = await WebAssembly.instantiate(bytes, {}, { importedStringConstants: "strings" });

```

--------------------------------

### Install wit-bindgen CLI

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/wasm/component-model-tutorial.md

Install the wit-bindgen CLI tool, essential for generating MoonBit bindings from WIT files. This tool is crucial for bridging WIT definitions with MoonBit code.

```bash
$ cargo install wit-bindgen-cli

```

--------------------------------

### Corrected Example: Matching Regex at Start and Binding Suffix

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E4184.md

This snippet shows the suggested correction for E4184, where the regex is matched at the start of the input, and the suffix is bound only when required.

```moonbit
"abc" _

```

--------------------------------

### Corrected Map Pattern Match with 'get' Method

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4118.html

This example shows the correct implementation of the 'get' method for the 'MyMap' struct, enabling successful map pattern matching.

```moonbit
///|
priv struct MyMap {
  entries : Map[String, Int]
}

///|
fn MyMap::get(self : MyMap, key : String) -> Int? {
  self.entries.get(key)
}

///|
test {
  let map : MyMap = { entries: { "a": 1, "b": 2, "c": 3 } }
  match map {
    { "a": a, .. } => println("a: {a}")
    _ => println("missing")
  }
}


```

--------------------------------

### Example moon.pkg with Build Variables

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/moon/module.md

Illustrates how build variables outputted by a pre-build script can be used in the `moon.pkg` configuration for native linking.

```text
{
  "link": {
    "native": {
      "cc": "${build.CC}"
    }
  }
}
```

--------------------------------

### MoonBit Loop Ambiguity Example

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E0017.md

This erroneous example demonstrates the ambiguous usage of an identifier within a loop, where it refers to the original value before the loop started, leading to unexpected behavior.

```moonbit
///|
fn main {
  var a = 1
  loop {
    println(a)
    a = a + 1
  }
}
```

--------------------------------

### Example moon.pkg using build variables

Source: https://docs.moonbitlang.com/en/latest/toolchain/moon/module.html

Demonstrates how to use build variables emitted from a pre-build script within the `moon.pkg` configuration, specifically for native linking options.

```moon
options(
  link: {
    "native": {
      "cc": "${build.CC}",
    },
  },
)


```

--------------------------------

### Corrected MoonBit Code with 'get' Method Implementation

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E4118.md

This example shows the corrected MoonBit code where the custom type 'MyType' now implements the 'get' method, resolving the E4118 error and enabling proper map pattern matching.

```moonbit
type MyType = {
  name: string
}

impl MyType {
  fn get(self, key: string): option<string> {
    match key {
      case "name" => Some(self.name)
      _ => None
    }
  }
}

let main() = {
  let data = MyType{name: "test"}
  match data {
    case {"name": name} => println(name)
    _ => println("no match")
  }
}
```

--------------------------------

### Corrected Enum/Struct Definition in MoonBit

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E3800.md

This example demonstrates the correct way to define MoonBit enums and structs using newlines for separation, adhering to the MoonBit style guide and resolving the E3800 error.

```moonbit
type MyEnum =
  | A
  | B
  | C

struct MyStruct {
  field1: Int,
  field2: String,
}
```

--------------------------------

### Create Project Directory and WIT Directory

Source: https://docs.moonbitlang.com/en/latest/toolchain/wasm/component-model-tutorial.html

Initializes the project directory and creates a subdirectory for WIT definitions.

```bash
$ mkdir moonbit-adder && cd moonbit-adder
$ mkdir wit
```

--------------------------------

### Corrected Lexmatch without Leading Rest Binder

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4184.html

This corrected example demonstrates the proper way to use lexmatch with the longest strategy. It matches the regex at the start of the remaining input and binds only the suffix when necessary.

```moonbit
///|
pub fn match_text(text : String) -> Unit {
  lexmatch text with longest {
    ("abc", _) => ()
    _ => ()
  }
}

```

--------------------------------

### Erroneous MoonBit Code for Map Pattern Mismatch

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E4118.md

This example demonstrates the MoonBit code that triggers the E4118 error due to an incorrect map pattern usage on a custom type lacking a 'get' method.

```moonbit
type MyType = {
  name: string
}

let main() = {
  let data = MyType{name: "test"}
  match data {
    case {"name": name} => println(name)
    _ => println("no match")
  }
}
```

--------------------------------

### Common MoonBit Development Workflow

Source: https://docs.moonbitlang.com/en/latest/_downloads/e5ac02240ae2e8b3e2abd4e3a580c2aa/summary.md

This demonstrates the typical command-line sequence to build the frontend and run the backend server. After executing these commands, the application can be accessed via a web browser.

```bash
make build-frontend
make run-backend

```

--------------------------------

### Program for Step 1: Finding the Next Redex

Source: https://docs.moonbitlang.com/en/latest/example/gmachine/gmachine-1.html

Demonstrates the starting point of program execution from the 'main' function and the initial expression to be handled.

```moonbit
(defn square[x]  (mul x x))
(defn main[] (add 33 (square 3)))


```

--------------------------------

### Corrected Lexmatch Pattern using Regex Match Expression

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4185.html

This example demonstrates the suggested approach using a regex match expression with optional start and end rest binders. This is generally clearer for new code.

```moonbit
///|
n fn classify(input : String) -> String {
  if input =~ (re"a" + re"b", before~, after~) {
    before.to_owned() + after.to_owned()
  } else {
    "none"
  }
}

///|
test {
  inspect(classify("xabz"), content="xz")
}


```

--------------------------------

### Corrected Example: Using 'type' Keyword

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4187.html

This example demonstrates the correct way to import a type by prefixing the imported name with the 'type' keyword. This resolves the E4187 compiler error.

```moonbit
using @builtin {type Array}

fn length(values : Array[Int]) -> Int {
  values.length()
}

test {
  inspect(length([1, 2]), content="2")
}
```

--------------------------------

### Corrected init/main function signature

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E3003.md

This example demonstrates the correct signature for `init` and `main` functions, which should have no arguments and no return values.

```moonbit
fn init() {
    
}

fn main() {
    
}
```

--------------------------------

### MoonBit Project Structure Example

Source: https://docs.moonbitlang.com/en/latest/tutorial/for-go-programmers/index.html

Illustrates a typical MoonBit project layout, including source files, configuration files, and package directories.

```text
my-project
├── LICENSE
├── README.md
├── moon.mod.json
└── src
    ├── lib
    │   ├── hello.mbt
    │   ├── hello_test.mbt
    │   └── moon.pkg
    └── main
        ├── main.mbt
        └── moon.pkg
```

--------------------------------

### Install MoonBit Toolchain (Windows)

Source: https://docs.moonbitlang.com/en/latest/_downloads/e5ac02240ae2e8b3e2abd4e3a580c2aa/summary.md

Installs the MoonBit toolchain on Windows using PowerShell. This command sets the execution policy and then runs the installation script. Restart your terminal or IDE for changes to take effect.

```powershell
Set-ExecutionPolicy RemoteSigned -Scope CurrentUser; irm https://cli.moonbitlang.com/install/powershell.ps1 | iex
```

--------------------------------

### Suggestion: Main package entry point calling library function

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4069.html

This example shows the `main.mbt` file for a separate main package that imports and calls a function from the library package. This correctly separates library and entry-point code.

```moonbit
///|
fn main {
  println(@lib.message())
}

```

--------------------------------

### Create a new MoonBit project and add async dependency

Source: https://docs.moonbitlang.com/en/latest/_downloads/e5ac02240ae2e8b3e2abd4e3a580c2aa/summary.md

Initializes a new MoonBit project and adds the `moonbitlang/async` library. This is the first step in setting up the CLI project.

```bash
moon new download_cli
cd download_cli
moon add moonbitlang/async@0.19.2
```

--------------------------------

### Moon Workspace Manifest Example

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/moon/workspace.md

An example of a `moon.work` file, which defines the workspace and its member modules.

```moonbit
workspace "my-repo"

members {
  "mod_a"
  "mod_b"
}

```

--------------------------------

### MoonBit Project Structure Example

Source: https://docs.moonbitlang.com/en/latest/_downloads/e5ac02240ae2e8b3e2abd4e3a580c2aa/summary.md

Illustrates the directory structure created by `moon new <project_name>`. It includes main entry points, test files, and configuration files.

```default
examine
├── Agents.md
├── cmd
│   └── main
│       ├── main.mbt
│       └── moon.pkg
├── LICENSE
├── moon.mod.json
├── moon.pkg
├── examine_test.mbt
├── examine.mbt
├── README.mbt.md
└── README.md -> README.mbt.md
```

--------------------------------

### Warnings Attribute Example

Source: https://docs.moonbitlang.com/en/latest/language/attributes.html

Configures warning settings for a top-level declaration. This example disables the 'unused_value' warning.

```moonbit
#warnings("-unused_value")
fn warnings_example() -> Unit {
  let x = 42
}
```

--------------------------------

### Explore the 'buffer' Package

Source: https://docs.moonbitlang.com/en/latest/_downloads/7e67a7065021137fc80a2750bac9ee32/summary.md

Example of exploring all exported symbols within the '@buffer' package.

```bash
$ moon ide doc "@buffer"
```

--------------------------------

### Create and Run a New MoonBit Project

Source: https://docs.moonbitlang.com/en/latest/tutorial/for-go-programmers/index.html

Use the 'moon' CLI to create a new project, navigate into its directory, and run it.

```bash
$ moon new hello-world
$ cd hello-world
$ moon run

```

--------------------------------

### Erroneous Example of Declaration Cycle

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E4155.md

This example demonstrates a circular dependency in declarations, which will result in the E4155 error.

```moonbit
const a = b
const b = a

```

--------------------------------

### Main entrypoint for the CLI

Source: https://docs.moonbitlang.com/en/latest/_sources/tutorial/cli-quickstart.md

The `main` function in `cmd/main` handles wiring: reading arguments, building configuration, and calling the library's entrypoint.

```moonbit
module main

use crate/config
use crate/download

// start main
fn main() {
  let cfg = config.build_config()
  async.run(download.run(cfg))
}
// end main
```

--------------------------------

### MoonBit Unused Function Example

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E0001.md

This is an example of a function that is not used and not marked as public, triggering the `unused_value` warning.

```moonbit
fn unused_function() {
  // This function is not used anywhere
}
```

--------------------------------

### MoonBit init function example

Source: https://docs.moonbitlang.com/en/latest/_sources/language/introduction.md

The init function is special; it has no parameters or return type, can be called multiple times, and is implicitly executed during package initialization. It should only contain statements.

```moonbit
fn init {
  let x = 1
  println(x)
}
```

--------------------------------

### Verify MoonBit Installation

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/wasm/component-model-tutorial.md

Check the installed versions of the MoonBit compiler and toolchain. Ensure you have the necessary components for development.

```bash
$ moon version --all
moon 0.1.20250826 (8ab6c9e 2025-08-26) ~/.moon/bin/moon
moonc v0.6.25+d6913262c (2025-08-27) ~/.moon/bin/moonc
moonrun 0.1.20250826 (8ab6c9e 2025-08-26) ~/.moon/bin/moonrun
moon-pilot 0.0.1-95f12db ~/.moon/bin/moon-pilot

```

--------------------------------

### Correct Struct Instantiation with Constructor

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4199.html

This example demonstrates the correct way to instantiate a struct by defining a constructor function `make_point` and then using it. Alternatively, record construction can be used.

```moonbit
///|
pub(all) struct Point {
  x : Int
}

///|
fn make_point(x : Int) -> Point {
  { x, }
}

///|
test {
  let point = make_point(1)
  inspect(point.x, content="1")
}
```

--------------------------------

### Erroneous Partial Match Example

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E0011.html

This example demonstrates an incomplete pattern match in MoonBit where the 'None' case is missing.

```moonbit
///|
fn main {
  match Some(1) { // Partial match, some hints: None
    Some(x) => println(x)
  }
}


```

--------------------------------

### Initialize GMachine State

Source: https://docs.moonbitlang.com/en/latest/example/gmachine/gmachine-2.html

Sets up the initial state for the GMachine, including the heap, an empty stack, the initial code to execute (starting with PushGlobal("main") and Eval), global variables, and statistics.

```moonbit
let initialState : GState = {
  heap,
  stack: @list.empty(),
  code: @list.from_array([PushGlobal("main"), Eval]),
  globals,
  stats: 0,
  dump: @list.empty(),
}

```

--------------------------------

### Erroneous Example of Unused Field

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E0007.md

This example demonstrates a struct with a field that is never read, triggering the `unused_field` warning.

```moonbit
struct Point {
  x: Int
  y: Int
}

fn main() {
  let p = Point { x: 1, y: 2 }
}
```

--------------------------------

### Correctly Creating Dynamic Bytes with Buffer

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E4096.md

This example shows the recommended way to dynamically create Bytes by using Buffer and explicit UTF-8 writes.

```moonbit
fun main() {
    let name = "world"
    let buffer = Buffer.new()
    buffer.write_string("hello ")
    buffer.write_string(name)
    let msg = buffer.to_string()
    printf("%s\n", msg)
}
```

--------------------------------

### Install wasm-tools

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/wasm/component-model-tutorial.md

Install wasm-tools for working with WebAssembly components. This utility is part of the WebAssembly toolchain required for component development.

```bash
$ cargo install wasm-tools

```

--------------------------------

### Implement download functionality

Source: https://docs.moonbitlang.com/en/latest/_sources/tutorial/cli-quickstart.md

The `run` function handles the actual download, streaming the body to stdout or a file based on the output configuration.

```moonbit
module download

use moonbitlang/core/result
use moonbitlang/async as async
use moonbitlang/io as io

use crate/config

// start run
fn run(cfg: Config): async[unit] {
  let res = async.http_client().get(cfg.url)
  match res {
    Ok(resp) -> {
      match cfg.output {
        "stdout" -> {
          async.write_all(async.stdout(), resp.body)
        }
        "file", path -> {
          let file = io.open(path, "w")
          async.write_all(file, resp.body)
          io.close(file)
        }
      }
    }
    Err(err) -> {
      eprintln("Error: {}", err)
    }
  }
}
// end run
```

--------------------------------

### Erroneous Example: Positional Argument

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4086.html

This example demonstrates the error E4086 where a labelled argument is mistakenly treated as a positional argument.

```moonbit
///|
pub fn f(name~ : String) -> Unit {
  println("Hello, {name}")
}

///|
test {
  f("John") // Error: The labels name~ are required by this function, but not supplied.
}


```

--------------------------------

### Run Command Example

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/moon/tutorial.md

Command to execute a Moonbit program from a specific package.

```bash
$ moon run cmd/main
89
```

--------------------------------

### Example of a Function for Strictness Analysis

Source: https://docs.moonbitlang.com/en/latest/example/gmachine/gmachine-3.html

Illustrates the 'abs' function, which will be used as an example to translate into a boolean function for strictness analysis.

```moonbit
(defn abs[n]
  (if (lt n 0) (negate n) n))

```

--------------------------------

### View Moon CLI Help

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/moon/tutorial.md

Use `moon help` to view the command line usage instructions for the Moon build system.

```bash
$ moon help
...
```

--------------------------------

### Basic Usage of moon ide doc

Source: https://docs.moonbitlang.com/en/latest/_downloads/7e67a7065021137fc80a2750bac9ee32/summary.md

This shows the fundamental command structure for querying documentation. Replace '<query>' with your search term.

```bash
moon ide doc '<query>'
```

--------------------------------

### MoonBit Unreachable Code Example 1

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E0012.md

This example demonstrates unreachable code after a return statement. Ensure all code is reachable.

```moonbit
fn main() {
    let x = 10;
    if x > 5 {
        return 1;
        println!("{}", x);
    }
    return 0;
}
```

--------------------------------

### Main entry point for the CLI application

Source: https://docs.moonbitlang.com/en/latest/_downloads/e5ac02240ae2e8b3e2abd4e3a580c2aa/summary.md

The `main` function in `cmd/main` is the entry point of the CLI. It reads command-line arguments, parses them into a configuration object using the library's `parse_config` function, and then calls the `run` function to execute the download.

```moonbit
import {
  "moonbit-community/cli-quickstart-doc" @app,
  "moonbitlang/core/env",
  "moonbitlang/async",
}

options(
  "is-main": true,
)

async fn main {
  let argv = @env.args()
  let config = @app.parse_config(argv[1:])
  @app.run(config)
}
```

--------------------------------

### Erroneous Example of Unused Constructor

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E0006.md

This example shows an enum with a variant that is neither constructed nor read, triggering the E0006 warning.

```moonbit
enum Option {
  Some(Int)
  None
}

let x = Option.Some(1)

```

--------------------------------

### Go Project Initialization

Source: https://docs.moonbitlang.com/en/latest/_sources/tutorial/for-go-programmers/index.md

Initializes a new Go project and its go.mod file to track dependencies.

```console
$ go mod init example.com/my-project
```

--------------------------------

### Verify Wasm Toolchain Installation

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/wasm/component-model-tutorial.md

Confirm the installations of wit-bindgen and wasm-tools by checking their versions. This ensures the tools are correctly set up for use.

```bash
$ wit-bindgen --version
wit-bindgen-cli 0.45.0
$ wasm-tools --version
wasm-tools 1.238.0

```

--------------------------------

### Basic Moon Package Configuration

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/moon/package.md

Example of a basic `moon.pkg` file. Dependencies are declared in an `import` block.

```moonbit
import {
	use_implement
} from "@moonbit/use_implement"

```

--------------------------------

### Create a new MoonBit project

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/moon/package-manage-tour.md

Initialize a new MoonBit project in the specified path using `moon new <PATH>`. This command generates a starter module with template files.

```shell
$ moon new .
Created username/myapp at .
```

--------------------------------

### Corrected struct update example

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E0060.html

This example demonstrates how to fix the unused_struct_update warning by either using the updated value or explicitly ignoring it.

```moonbit
priv struct User {
  id : Int
  name : String
  email : String
}

test {
  let user : User = { id: 0, name: "John Doe", email: "john@doe.com" }
  let updated = { ..user, email: "john@doe.name" }
  ignore(updated)
}
```

--------------------------------

### Pattern Matching with Proper Branch Order

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E0012.html

This example demonstrates correct pattern matching by ordering branches so that specific cases are handled before general ones, preventing unreachable code.

```moonbit
///|
fn example_proper_order() -> Unit {
  match Some(1) {
    Some(a) => println("Some(\"{a}\")")
    _ => println("_")
  }
}
```

--------------------------------

### Using a Custom Struct Constructor

Source: https://docs.moonbitlang.com/en/latest/_downloads/78d8998d78e60fd7b1b2f4c9bb2819fb/summary.md

Demonstrates creating an 'IntBox' instance using its custom constructor and inspecting its value.

```moonbit
let box = IntBox(10)
debug_inspect(box, content="{ value: 10 }")
```

--------------------------------

### Basic Task Group Example

Source: https://docs.moonbitlang.com/en/latest/_sources/language/async-experimental.md

Demonstrates the basic usage of `with_task_group` to create multiple tasks that run in parallel.

```moonbit
start with_task_group example

// Example usage of with_task_group
async fn main() {
    @async.with_task_group(async (group) => {
        group.spawn_bg(async () => {
            // Task 1 logic
            @async.sleep(1000)
            @console.log("Task 1 finished")
        })
        group.spawn_bg(async () => {
            // Task 2 logic
            @async.sleep(500)
            @console.log("Task 2 finished")
        })
        @console.log("All tasks spawned")
    })
    @console.log("Task group finished")
}

end with_task_group example
```

--------------------------------

### Corrected Example with Parentheses

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E0051.md

This code shows the corrected version of the ambiguous precedence example. Parentheses are added to make the intent clearer.

```moonbit
let x = 1 + (2 * 3)

```

--------------------------------

### Erroneous Example of Unused Trait Bound

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E0053.md

This example demonstrates code that triggers the E0053 warning because a trait bound is present but not utilized.

```moonbit
trait MyTrait {
    fn foo(self: Self) -> Int
}

fn bar<T: MyTrait>(x: T) -> Int {
    // MyTrait::foo is not called
    return 1
}

fn main() {
    // ...
}
```

--------------------------------

### Importing Packages with Aliases

Source: https://docs.moonbitlang.com/en/latest/language/packages.html

Demonstrates how to import packages in `moon.pkg` and assign custom aliases for easier access to their entities.

```moonbit
import {
  "moonbit-community/language/packages/pkgA",
  "moonbit-community/language/packages/pkgC" @c,
  "moonbitlang/core/builtin",
}

```

--------------------------------

### Example: Using with_task_group for Parallel Tasks

Source: https://docs.moonbitlang.com/en/latest/language/async-experimental.html

Demonstrates how to use `with_task_group` to spawn multiple background tasks that run concurrently and log their progress. The `with_task_group` ensures all spawned tasks complete before returning.

```moonbit
async test "with_task_group" {
  let log = []
  @async.with_task_group(group => {
    group.spawn_bg(() => {
      for _ in 0..<3 {
        log.push("task #1 tick")
        @async.sleep(200) // sleep for 200ms
      }
    })
    group.spawn_bg(() => {
      @async.sleep(100)
      for _ in 0..<3 {
        log.push("task #2 tick")
        @async.sleep(200)
      }
    })
  })
  json_inspect(log, content=[
    "task #1 tick", "task #2 tick", "task #1 tick", "task #2 tick", "task #1 tick",
    "task #2 tick",
  ])
}
```

--------------------------------

### Erroneous Example of Unused Catch All

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E0026.md

This example demonstrates the deprecated `catch!` syntax which is unnecessary when MoonBit can infer the caught error type.

```moonbit
try {
  // some code
} catch! {
  // handle error
}
```

--------------------------------

### Prelude and Builtin Names Example

Source: https://docs.moonbitlang.com/en/latest/_sources/language/packages.md

Illustrates how unqualified names are resolved in the current package and the prelude. Local definitions can shadow prelude definitions.

```moonbit
let x: Int = 1
let y: String = "hello"
let z: Bool = true
let w: Unit = ()

// Prelude and builtin names are available without explicit import.
```

--------------------------------

### Check if a string starts with a lowercase letter

Source: https://docs.moonbitlang.com/en/latest/_downloads/78d8998d78e60fd7b1b2f4c9bb2819fb/summary.md

Use the 'is' expression with a character range to check if a string starts with a lowercase letter.

```moonbit
fn start_with_lower_letter(s : String) -> Bool {
  s is ['a'..='z', ..]
}
```

--------------------------------

### Set Heap Start Address for Wasm Backend (JSON)

Source: https://docs.moonbitlang.com/en/latest/toolchain/moon/package.html

JSON representation of Wasm backend heap start address configuration.

```moon.pkg.json
{
  "link": {
    "wasm": {
      "heap-start-address": 1024
    }
  }
}

```

--------------------------------

### MoonBit Main Package Configuration

Source: https://docs.moonbitlang.com/en/latest/_sources/tutorial/for-go-programmers/index.md

Example configuration for a main package's moon.pkg file in MoonBit, indicating it's a binary package.

```text
{literalinclude} /snippets/tutorial/for-go-programmers/src/main/moon.pkg
:language: text
:caption: src/main/moon.pkg
```

--------------------------------

### Set Heap Start Address for Wasm Backend

Source: https://docs.moonbitlang.com/en/latest/toolchain/moon/package.html

Configure the starting address for linear memory that can be used when compiling to the Wasm backend.

```moon.pkg
options(
  link: {
    "wasm": {
      "heap-start-address": 1024,
    },
  },
)

```

--------------------------------

### Set Heap Start Address for Wasm

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/moon/package.md

Specify the starting address of the linear memory available for use when compiling to the Wasm backend.

```moonbit
options(
  link: {
    "wasm": {
      "heap-start-address": 1024,
    },
  },
)
```

--------------------------------

### Create and Run a New MoonBit Project

Source: https://docs.moonbitlang.com/en/latest/_sources/tutorial/for-go-programmers/index.md

Use the 'moon new' command to create a new project and 'moon run' to execute it. This is the standard way to start and test MoonBit applications.

```console
$ moon new hello-world
$ cd hello-world
$ moon run
```

--------------------------------

### Overloading Index Get and Set Operators

Source: https://docs.moonbitlang.com/en/latest/_downloads/78d8998d78e60fd7b1b2f4c9bb2819fb/summary.md

Implements `_[_]` (get item) and `_[_]=_` (set item) operators for the `Coord` struct using `##alias`. The `get` method takes `Self` and an index, returning an `Int`. The `set` method takes `Self`, an index, and a value, returning `Unit`.

```moonbit
struct Coord {
  mut x : Int
  mut y : Int
}

##alias("_[_]")
fn Coord::get(coord : Self, key : String) -> Int {
  match key {
    "x" => coord.x
    "y" => coord.y
  }
}

##alias("_[_]=_")
fn Coord::set(coord : Self, key : String, val : Int) -> Unit {
  match key {
    "x" => coord.x = val
    "y" => coord.y = val
  }
}
```

--------------------------------

### Create New MoonBit Project and Add Dependencies

Source: https://docs.moonbitlang.com/en/latest/_downloads/e5ac02240ae2e8b3e2abd4e3a580c2aa/summary.md

Initializes a new MoonBit project and adds necessary dependencies for the fullstack application.

```bash
moon new fullstack_one_project
cd fullstack_one_project
moon add moonbitlang/async@0.19.2
moon add moonbit-community/rabbita
```

--------------------------------

### Go Method Call Example

Source: https://docs.moonbitlang.com/en/latest/_sources/tutorial/for-go-programmers/index.md

Demonstrates calling methods on a Go struct instance. Note that Scale modifies the struct in place.

```go
rect := Rectangle{width: 10.0, height: 5.0}
areaValue := rect.Area()
rect.Scale(2.0) // NOTE: `.Scale()` modifies the rectangle in place.
```

--------------------------------

### Erroneous Example: Implicitly Ignored Expression

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E4139.html

This example demonstrates the error E4139 where an expression of type `Int` has its value implicitly ignored, which is not allowed.

```moonbit
fn main {
  1 + 1 // This expression has type Int, its value cannot be implicitly ignored.
}
```

--------------------------------

### Use a Virtual Package in top.mbt

Source: https://docs.moonbitlang.com/en/latest/_sources/language/packages.md

Demonstrates how to use a virtual package. The `import` field in `moon.pkg` is used to reference the virtual package, and then its entities can be used directly.

```moonbit
import virtual-package-name

fn main() {
  virtual_package_name.greet("World")
}

```

--------------------------------

### Erroneous Example of Unused Catch All

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E0026.html

This example demonstrates the incorrect usage of the deprecated catch! syntax where the patterns are complete, leading to the E0026 warning.

```moonbit
///|
suberror E

///|
fn f() -> Unit raise E {
  raise E
}

///|
fn g() -> Unit raise {
  f() catch! {
    E => raise E
  }
}

///|
fn main {
  g() catch {
    _ => println("Error")
  }
}
```

--------------------------------

### Erroneous Example of Unused Try

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E0023.html

This example shows a try block that contains code which will not raise an error. The `try` keyword is used unnecessarily.

```moonbit
///|
fn main {
  println("Hello, world!") catch { // `try` is omitted here
    _ => println("Error")
  }
  try {
    println("Hello, ")
    println("world!")
  } catch {
    _ => println("Error in try block")
  }
}
```

--------------------------------

### MoonBit main function example

Source: https://docs.moonbitlang.com/en/latest/_sources/language/introduction.md

The main function serves as the program's entry point and is executed after initialization. It also has no parameters or return type. Only main packages can define a main function.

```moonbit
fn main {
  let x = 2
  println(x)
}
```

--------------------------------

### Unused Function Example

Source: https://docs.moonbitlang.com/en/latest/language/error_codes/E0001.html

This snippet shows an example of an unused function 'greeting' and a local unused function 'local_greeting' within a test block.

```moonbit
///|
fn greeting() -> String {
  "Hello!"
}

///|
test {
  fn local_greeting() -> String {
    "Hello, world!"
  }


}
```

--------------------------------

### Create a new MoonBit project

Source: https://docs.moonbitlang.com/en/latest/_sources/tutorial/fullstack-one-project.md

Initializes a new MoonBit project and adds necessary dependencies for async operations and a web framework.

```bash
moon new fullstack_one_project
cd fullstack_one_project
moon add moonbitlang/async@0.19.2
moon add moon-community/rabbita
```

--------------------------------

### Example Build Script Output (vars)

Source: https://docs.moonbitlang.com/en/latest/toolchain/moon/module.html

A build script can output variables in the `vars` field, which can then be referenced in `moon.pkg` using `${build.<var_name>}`.

```json
{ "vars": { "CC": "gcc" } }

```

--------------------------------

### Erroneous Example: Missing Toplevel Type Annotation

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E4074.md

This example demonstrates the E4074 error where a toplevel declaration is missing a required type annotation.

```moonbit
fn main() {
    let x = 10
    let y = "hello"
}
```

--------------------------------

### Test Command Example

Source: https://docs.moonbitlang.com/en/latest/_sources/toolchain/moon/tutorial.md

Command to run all tests within a Moonbit project.

```bash
$ moon test
Total tests: 1, passed: 1, failed: 0.
```

--------------------------------

### Fixed Toplevel Definition Example

Source: https://docs.moonbitlang.com/en/latest/_sources/language/error_codes/E4051.md

Provides a corrected version of the erroneous toplevel definition example, demonstrating how to resolve E4051 by renaming identifiers.

```moonbit
pub val x = 1
pub val y = 2
```
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