weekly 2024-04-22

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 })
  .take(2)
  .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]
    @assertion.assert_false(is_palindrome(v1[..]))?
    let v2 = Vec::[1, 2, 3, 2, 1]
    @assertion.assert_true(is_palindrome(v2[..]))?
}

• 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] {
  Nil
  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]]

After:

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=.