2025-05-06

Language Updates

• [Breaking Change] The way traits are implemented will change. In the future, traits must be explicitly implemented for a type A using impl T for A { ... }. Simply defining methods on A with the same signature as those in trait T will no longer be considered as implementing the trait T for A. This change was previously accompanied by a compiler warning and will soon take effect officially.

• A new syntactic sugar has been introduced to allow the use of _ as a placeholder for omitted parameters when creating anonymous functions, simplifying their syntax. For example, f(a, _, c, _) is equivalent to fn(b, d) { f(a, b, c, d) }. Supported usage scenarios include:

  • Constructor(args, _), lhs |> Constructor(args, _)
  • function(args, _), lhs |> function(args, _)
  • object.method(args, _) (Note: _.method(args) is not supported yet)

• fnalias now supports creating aliases for types and trait methods:

  trait Floating {
    sin(Self) -> Self
    cos(Self) -> Self
  }
  
  // You can now use `sin` and `cos` directly without qualifying with `Floating::`
  pub fnalias Floating::(sin, cos)

• All pragmas have been removed and will be completely replaced by attributes going forward.

• The #internal attribute has been implemented to provide warnings for external users of a public API:

  /// in moonbitlang/core
  #internal(unsafe, "message")
  pub fn unsafe_get(args){...}
  
  /// in user/module
  fn main {
    unsafe_get(...) // warning!
  }

  Users can disable these warnings by setting the alert option in moon.pkg.json.

• A new warning has been added for potentially ambiguous use of loop arguments inside a loop block:

  fn main {
    let a = "asdf"
    loop a {
      [k, .. b] => continue a // warning
      [k, .. b] as a => continue a // suggested
      [] => ()
    }
  }

• Implicit type conversions are now supported from Array to ArrayView, and from Bytes to @bytes.View.

Toolchain Updates

• The moon CLI now supports the bench subcommand for running performance benchmarks.

  Use a test block with a b : @bench.T parameter to define a benchmark. You can use b.keep() to prevent the compiler from optimizing away side-effect-free computations:

  fn fib(n : Int) -> Int {
    if n < 2 {
      return n
    }
    return fib(n - 1) + fib(n - 2)
  }
  
  test (b : @bench.T) {
    b.bench(fn() { b.keep(fib(20)) })
  }

  Run benchmarks using:

  $ moon bench
  [..]
  time (mean ± σ)         range (min … max)
    21.67 µs ±   0.54 µs    21.28 µs …  23.14 µs  in 10 ×   4619 runs

  For more detailed usage instructions, refer to https://docs.moonbitlang.com/en/latest/language/benchmarks.html.