2025-04-21

Language Updates

• Async function syntax: The call syntax for async functions has changed to match the error style: f!(...). The old f!!(...) syntax will now trigger a warning.

• Operator overloading migration to traits: Operator overloading is moving from method-based to trait-based. Going forward, to overload an operator, you must impl the corresponding trait from @moonbitlang/core/builtin. You can find the list of operator-related traits in the language docs and operators.mbt.

  Migration details:

  • Old method-based operator overloading still works, but will raise a compiler warning.

  • Migrate by replacing op_xxx methods with the appropriate impl of the trait.

    • Operator traits enforce stricter type signatures—for example, the - operator requires the return type to be the same as the input type. If an operator doesn’t match the expected signature, it must be rewritten as a regular method instead of using operator syntax.

  • If a trait defines op_xxx methods, they can be removed and replaced by adding the operator trait to the super trait list. For example:

    // Old version
    trait Number {
      from_int(Int) -> Self
      op_add(Self, Self) -> Self
      op_sub(Self, Self) -> Self
    }
    
    // Migrated version
    trait Number : Add + Sub {
      from_int(Int) -> Self
    }

• New = _ syntax for traits: You can now mark whether a method in a trait has a default implementation using = _:

  trait Hash {
    hash_combine(Self, Hasher) -> Unit
    hash(Self) -> Int = _ // indicates that `hash` has a default implementation
  }

  a. If a method is marked with = _, a matching default impl Trait with some_method(..) must exist. If a method has a default impl but is not marked with = _, the compiler will warn you.

  b. This is mainly for readability: now you can see from the trait definition which methods have defaults. Why not include the default code directly in the trait? To keep the trait definition short and focused on method signatures.

• Implicit conversion from String to @string.View is now supported. Also, slicing with [:] now returns a full view again. General slicing like [i:j] is still under design.

  fn f(v : @string.View) -> Unit {
    ignore(v)
  }
  
  fn main {
    f("hello")
    f("world"[:])
  }

• Pattern matching on @string.View/@bytes.View now supports directly matching against string/byte literals:

  test {
    let s = "String"
    inspect!(s.view(end_offset=3) is "Str", content="true")
    let s : Bytes = "String"
    inspect!(s[:3] is "Str", content="true")
  }

• [Breaking Change] Updates to the @string core package:

  • Many APIs now use @string.View instead of String for parameters.
  • Some return types also changed from String to View.

  Notable changes:

  Old Signature	New Signature
  self.replace(old~: String, new~: String) -> String	self.replace(old~: View, new~: View) -> String
  self.trim(charset: String) -> String	self.trim(charset: View) -> View
  self.split(substr: String) -> Iter[String]	self.split(substr: View) -> Iter[View]
  self.index_of(substr: String, from~: Int) -> Int	self.find(substr: View) -> Option[Int]
  self.last_index_of(substr: String, from~: Int) -> Int	self.rev_find(substr: View) -> Option[Int]
  self.starts_with(substr: String) -> Bool	self.has_prefix(substr: View) -> Bool
  self.ends_with(substr: String) -> Bool	self.has_suffix(substr: View) -> Bool

• Core Json type will become read-only: In the future, enum constructors for Json will no longer be available. Instead, helper functions are provided as replacements:

  test {
    let num = Json::number(3.14)
    let str = Json::string("Hello")
    let obj = Json::object({ "hello": num, "world": str })
  }

Toolchain Updates

• IDE drops support for moonbit: true front matter in Markdown:

  • Now, only files with the .mbt.md extension will be treated as MoonBit Markdown files in the IDE.

• IDE supports setting debug breakpoints in .mbt.md directly:

  • You no longer need to enable Debug: Allow Breakpoint Everywhere in VSCode settings.

• New scripts field in moon.mod.json for build scripts:

  • Currently supports a postadd script: if a module contains a postadd field, it will be automatically executed after moon add.
    • To skip execution, set the MOON_IGNORE_POSTADD environment variable.

  Example:

  {
    "scripts": {
      "postadd": "python3 build.py"
    }
  }

• Improvements to .mbt.md Markdown support in the moon CLI:

  • moon check now includes Markdown checking automatically.
  • moon test now includes Markdown tests by default. (The --md flag will be removed in the future.)