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weekly 2024-04-22

· 4 min read

MoonBit is a Rust-like language (with GC support) and toolchain optimized for WebAssembly experience. This is our recent update:

Language Update

  • The standard library has added the Iter type, which efficiently accesses elements in a container and optimizes the access process into a loop. Here's an example of how to use it:
test "iter" {
let sum = Iter::[1, 2, 3, 4, 5, 6]
.filter(fn { x => x % 2 == 1 })
.map(fn { x => x * x})
.reduce(fn (x, y) { x + y }, 0)
inspect(sum, content="10")?
  • A VecView type has been added to the standard library, and you can obtain its VecView[T] for values of type Vec[T] using the following method.
test "view" {
let v = Vec::[1, 2, 3, 4, 5]
let vv1 = v[..]
let vv2 = v[1..]
let vv3 = v[..4]
let vv4 = v[1..4]
inspect(vv1, content="VecView::[1, 2, 3, 4, 5]")?
inspect(vv2, content="VecView::[2, 3, 4, 5]")?
inspect(vv3, content="VecView::[1, 2, 3, 4]")?
inspect(vv4, content="VecView::[2, 3, 4]")?
  • The array pattern supports the form [a, .. as rest, b], where rest will be bound to a VecView.
test "view_pattern" {
fn is_palindrome(s: VecView[Int]) -> Bool {
match s {
[] => true
[_] => true
[x, .. as rest, y] => x == y && is_palindrome(rest)
let v1 = Vec::[1, 2, 3, 4, 5]
let v2 = Vec::[1, 2, 3, 2, 1]
  • Allow the omission of the tilde ~ inside tags when calling tag parameters, for example:
inspect(1, content="1")
  • Constructor now supports labelled arguments:
enum Tree[X] {
Branch(X, ~left : Tree[X], ~right : Tree[X])

fn leftmost[X](self : Tree[X]) -> Option[X] {
loop self {
Nil => None
// use `label=pattern` to match labelled arguments of constructor
Branch(top, left=Nil, right=Nil) => Some(top)
// `label=label` can be abbreviated as `~label`
Branch(_, left=Nil, ~right) => continue right
// use `..` to ignore all remaining labelled arguments
Branch(_, ~left, ..) => continue left

fn init {
// syntax for creating constructor with labelled arguments is the same as callig labelled function
let t: Tree[Int] = Branch(0, right=Nil, left=Branch(1, left=Nil, right=Nil))
println(t.leftmost()) // `Some(1)`
  • The default value of optional parameters can depend on preceding parameters, for example:
fn f(~x: Int = 1, ~y: Int = x) -> Int {
x + y
  • Byte literals support octal escapes.


IDE Update

  • IDE supports test codelens for local environments and automatic updates for expect tests.


  • Fixed an issue where the online IDE couldn't find core on Windows.

  • Support for identifying test_import and *_test.mbt files.

Toolchain Update

  • moonfmt

    • Fixed an issue where escape sequences in String and Char literals were not formatted correctly.
    • Adjusted formatting for { ..record }
  • moon info and moon coverage now support Windows.

  • moon info supports shortening type names when it doesn't cause ambiguity, making the generated results cleaner. - Hide the builtin package name prefix when there is no type definition with the same name. - Use the last part of the package name instead of the full package name when there is no ambiguity. Before:

package moonbitlang/core/map

// -- snip --

type Map
fn Map::debug_write[K : @moonbitlang/core/builtin.Debug, V : @moonbitlang/core/builtin.Debug](Map[K, V], @moonbitlang/core/builtin.Buffer) -> Unit
fn Map::keys_set[K : @moonbitlang/core/builtin.Compare + @moonbitlang/core/builtin.Eq, V](Map[K, V]) -> @moonbitlang/core/immutable_set.ImmutableSet[K]
fn Map::lookup[K : @moonbitlang/core/builtin.Compare + @moonbitlang/core/builtin.Eq, V](Map[K, V], K) -> Option[V]
fn Map::to_vec[K, V](Map[K, V]) -> @moonbitlang/core/vec.Vec[Tuple[K, V]]


package moonbitlang/core/map

alias @moonbitlang/core/immutable_set as @immutable_set
alias @moonbitlang/core/vec as @vec

// -- snip --

type Map
fn Map::debug_write[K : Debug, V : Debug](Map[K, V], Buffer) -> Unit
fn Map::keys_set[K : Compare + Eq, V](Map[K, V]) -> @immutable_set.ImmutableSet[K]
fn Map::lookup[K : Compare + Eq, V](Map[K, V], K) -> Option[V]
fn Map::to_vec[K, V](Map[K, V]) -> @vec.Vec[Tuple[K, V]]

Build System Update

  • moon test adds options related to test filtering:
-p, --package <PACKAGE_NAME>  Run test in the specified package
-f, --file <FILE> Run test in the specified file, only valid when --package is specified
-i, --index <INDEX> Run the index-nth test in the specified file, only valid when --file is specified

Command example: moon test -p username/hello/A -f hello.mbt -i 0 (runs the 0th test block in the hello.mbt file in the username/hello/A package); -f and -i are effective only when specified with -p; if -f is not specified, all tests in the package specified by -p will be run, and if -i is not specified, all tests in the file specified by -f will be run.

  • moon check|build|test adds the -sort-input option for generating stable build orders.

  • The ~content= generated by expect tests omits the ~ and is changed to content=.