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2025-05-19

· 2 min read

Language Updates​

The syntax of x..f(..) will be updated. In the case of .. chaining, the last return value will automatically be discarded if it’s not used. For example, the following code:​

impl[X : Show, Y : Show] Show for (X, Y) with output(self, logger) {
  logger
  ..write_string("(")
  ..write_object(self.0)
  ..write_string(", ")
  ..write_object(self.1)
  // Previously, omitting the final `..` could result in a type mismatch,
  // expecting `&Logger` but returning `unit`
  .write_string(")")
}

Can now be simplified as:

impl[X : Show, Y : Show] Show for (X, Y) with output(self, logger) {
  logger
  ..write_string("(")
  ..write_object(self.0)
  ..write_string(", ")
  ..write_object(self.1)
  // You can now write the full chain all the way to the end
  ..write_string(")")

However, if you're directly using the return value of x..f(..), this behavior will be removed. You must now explicitly save the value. For example:

let arr = []..push(1)..push(2)

Must now be rewritten as:

let arr = []
arr..push(1)..push(2)

Structs and enums now support field-level documentation comments, which will be displayed accordingly during doc generation:​

///| Location enum
struct Location {
  /// X coordinate
  x : Int
  /// y coordinate
  y : Int
}

///| Variant enum
enum Variant {
  /// Stirng constructor
  String(String)
  /// Number constructor
  Number(Double) 
}

@bytes.View and @string.View will now be compiled as value types in the C and wasm1 backends.​

This means that these two types will no longer involve heap allocation, leading to significantly improved memory layout and performance.

Toolchain Updates​

  • vscode now supports semantic token highlighting, which distinguishes between effectful functions (such as those that throw exceptions) and normal ones, as well as async functions. This gives each usage a different visual style.

  • The build system now supports the virtual package feature. By defining a package as virtual, you can specify a set of interfaces and allow users to choose actual implementations. If no implementation is provided, the system uses a default. This helps achieve clean decoupling and flexible substitution. Note: This feature is still under development. For more details, please refer to: Introducing virtual package in MoonBit

  • Now supports running test and debugging codelen directly on individual .mbt and .mbt.md files.