Creating and Using a New Package

Creating a New Module

To create a new module, enter the moon new command in the terminal, and you will see the module creation wizard. By using all the default values, you can create a new module named hello in the my-project directory.

$ moon new
Enter the path to create the project (. for current directory): my-project
Select the create mode: exec
Enter your username: username
Enter your project name: hello
Enter your license: Apache-2.0
Created my-project

Creating and Using a New Package

First, create a new directory named fib under lib:

$ mkdir lib/fib

Now, you can create new files under lib/fib:

a.mbt:

pub fn fib(n : Int) -> Int {
  match n {
    0 => 0
    1 => 1
    _ => fib(n - 1) + fib(n - 2)
  }
}

b.mbt:

pub fn fib2(num : Int) -> Int {
  fn aux(n, acc1, acc2) {
    match n {
      0 => acc1
      1 => acc2
      _ => aux(n - 1, acc2, acc1 + acc2)
    }
  }

  aux(num, 0, 1)
}

moon.pkg.json:

{}

After creating these files, your directory structure should look like this:

my-project
├── README.md
├── lib
│   ├── fib
│   │   ├── a.mbt
│   │   ├── b.mbt
│   │   └── moon.pkg.json
│   ├── hello.mbt
│   ├── hello_test.mbt
│   └── moon.pkg.json
├── main
│   ├── main.mbt
│   └── moon.pkg.json
└── moon.mod.json

In the main/moon.pkg.json file, import the username/hello/lib/fib package and customize its alias to my_awesome_fibonacci:

{
  "is_main": true,
  "import": [
    "username/hello/lib",
    {
      "path": "username/hello/lib/fib",
      "alias": "my_awesome_fibonacci"
    }
  ]
}

This line imports the fib package, which is part of the lib package in the hello module. After doing this, you can use the fib package in main/main.mbt. Replace the file content of main/main.mbt to:

fn init {
  let a = @my_awesome_fibonacci.fib(10)
  let b = @my_awesome_fibonacci.fib2(11)
  println("fib(10) = \(a), fib(11) = \(b)")

  println(@lib.hello())
}

To execute your program, specify the path to the main package:

$ moon run main
fib(10) = 55, fib(11) = 89
Hello, world!

Package Importing

In the MoonBit’s build system, a module’s name is used to reference its internal packages. To import the lib package in main/main.mbt, you need to specify it in main/moon.pkg.json:

{
  "is_main": true,
  "import": [
    "username/hello/lib"
  ]
}

Here, username/hello/lib specifies importing the username/hello/lib package from the username/hello module, so you can use @lib.hello() in main/main.mbt.

Note that the package name imported in main/moon.pkg.json is username/hello/lib, and @lib is used to refer to this package in main/main.mbt. The import here actually generates a default alias for the package name username/hello/lib. In the following sections, you will learn how to customize the alias for a package.

peter-jerry-ye/memory/

the directory generated by a normal moon new hello looks like this.

hello/
├── README.md
├── lib
│   ├── hello.mbt
│   ├── hello_test.mbt
│   └── moon.pkg.json
├── main
│   ├── main.mbt
│   └── moon.pkg.json
└── moon.mod.json

At first, I thought this structure would be directly published as a library, but it was a bit different when I tried it.

When making a library, the code generated with —lib is easier to understand.

$ moon new mylib --lib
$ tree mylib -I target
mylib
├── README.md
├── lib
│   ├── hello.mbt
│   ├── hello_test.mbt
│   └── moon.pkg.json
├── moon.mod.json
├── moon.pkg.json
└── top.mbt

pub in top.mbt becomes the function exported outside the module.

However, there’s no syntax like TypeScript’s re-export (export {} from ”…”), so if you want to publish the implementation of lib functions as is, you need to write a function that just wraps the same function signature.

Actually, this top.mbt is like this.

pub fn greeting() -> Unit {
  println(@lib.hello())
}

Is this redundant? Looking at packages that are published, many simply implement at the top level.

$ tree ~/repo/peter-jerry-ye/memory/
├── LICENSE
├── README.md
├── memory.mbt
├── moon.mod.json
└── moon.pkg.json

It’s simple, so I think this way is fine.

In this way, I was wondering how to place debug use examples, and I figured it out by digging a main/ and then pointing to the parent library name in main/moon.pkg.json.

├── main
│   ├── main.mbt
│   ├── moon.pkg.json
│   └── run_test.ts
├── mod.mbt
├── mod.ts
├── moon.mod.json
├── moon.pkg.json
└── tsconfig.json

{
  "link": {
    "wasm-gc": {
      "exports": ["write_bytes_test"]
    }
  },
  "import": [
    {
      "path": "mizchi/js_io", // Parent mod name
      "alias": "js_io"
    }
  ]
}

I think this alias should be unnecessary, but currently, it doesn’t resolve names correctly without it. I’d like to know a good way to resolve this.

Another problem I’m having is that there’s no .npmignore equivalent currently, so everything gets uploaded. Publishing this unnecessary main could potentially trigger compilation errors for some reason.

If I could make a request, I’d like moon.pkg.json to be optional. From what I’ve tried, most cases just involve placing an empty {} file.

Creating and Using a New Package

Creating a New Module