The MoonBit Update——0513


MoonBit is a Rust-like programming language (with GC support) and toolchain optimized for WebAssembly.

Language Update

  • Support mutable fields in the payload of the constructor, here is how to use it:
enum E {
  C(mut ~x : Int, mut ~y : Int)
} derive(Debug)

fn swap_xy(x : E) -> Unit {
  match x {
    // `~y` will bind to the value of the `y` field in the `C` before pattern matching.
    // When encountering a pattern like `C(..) as c`, the compiler will know that `c` must be the constructor,
    // so `c.x` and `c.y` can be directly accessed within the branch to access the fields of `C`.
    C(~y, ..) as c => {
      // `c.x` and `c.y` can be used to modify/read the latest values in `C`.
      c.y = c.x
      c.x = y

fn init {
  let e : E = C(x=1, y=2)
  debug(e) // C(x=1, y=2)
  debug(e) // C(x=2, y=1)

In this example, the swap_xy function swaps the values of the x and y fields of the constructor C, and this swap is done in-place without introducing additional memory allocation.

  • Array literals by default construct vectors. Array literal syntax has been overloaded. Now, array literals can be used to construct Vector and Array, with the specific type determined by the context. If the type is ambiguous in the context, Vector is used by default, for example:
fn init {
  let v = [1, 2, 3]             // v has type @vec.Vec[Int]
  let a : Array[_] = [1, 2, 3]  // a has type Array[Int]

In the next two weeks, we plan to rename Array to FixedArray and Vec to Array to accurately reflect the characteristics of the data structures and avoid confusion with vectors.

  • Error messages now include error codes, for example:
./vec/sort.mbt:68:16-68:23 [E4021] The value identifier minimum is unbound.
./vec/sort_by.mbt:90:16-90:23 [E4021] The value identifier minimum is unbound.
./vec/vec.mbt:962:37-962:50 [E4020] Package "iter" not found in the loaded packages.
./vec/vec.mbt:963:3-963:13 [E4024] The type/trait @iter.Iter is not found.

IDE Update

  • Support functionalities like gotodef/gotoref/rename for the labels of labeled fields in constructors, for example:

Build System Update

  • Support configuring the warn list and alert list at the package level.
    • Configuration in moon.pkg.json is as follows, where you can disable corresponding warns and alters during compilation. (Here, 2, alter_1, and alert_2 are respectively the compiler’s predefined warn id for Unused variable and user-defined alert id.)
  "warn_list": "-2",
  "alert_list": "-alert_1-alert_2"
  • - represents closing the corresponding id’s warn and alter. Check the predefined warns with moonc build-package -warn-help.

  • The default backend for moon check|build|run|test has been switched from wasm to wasm-gc

  • The default execution mode for moon test has been changed from release to debug.

  • moon check|build|run|test now supports automatic dependency installation without the need to manually execute moon install

  • moon doc --serve now supports specifying the address and port.

-b, --bind <BIND> [default:]
-p, --port <PORT> [default: 3000]
  • Optimize moon size.
Platform Before After
macOS arm64 7.3 MiB 3.6 MiB
macOS x86_64 8.2 MiB 4.1 MiB
Ubuntu x86_64 14.0 MiB 9.6 MiB
Windows x86_64 9.4 MiB 4.9 MiB

Toolchain Update

  • moonrun now supports printing backtrace.
    Create a new project with moon new hello, and replace main/main.mbt with:
  fn bar() -> Unit {

fn foo() -> Unit {

fn main {

Execute moon run main --debug, and you will see output similar to:

error: RuntimeError: unreachable
    at $username/hello/ (wasm://wasm/6fe99e5e:wasm-function[3]:0xe6)
    at $username/hello/ (wasm://wasm/6fe99e5e:wasm-function[4]:0xea)
    at *init*/3 (wasm://wasm/6fe99e5e:wasm-function[5]:0xef)
    at <anonymous>:9:22