The MoonBit Update——0722

Announcement
1

Language Updates

Breaking Changes

  • Error Handling Syntax: The syntax for error handling has changed from f(x)! to f!(x). This change facilitates IDE auto-completion, allowing the function name and ! to be completed together and enabling special rendering for f!.

  • Local Functions: If a local function can return an error type, it must be annotated. For example:

fn toplevel() -> Unit!String {
  fn local1() -> Unit!String { raise "err" }  // local function
  fn local2() -> _!_ { raise "err" }          // local function
  apply!(fn (_x) -> Int!String { raise "err" }, 42) |> ignore // anonymous function
}

fn apply(f: (Int) -> Int!String, x: Int) -> Int!String {
  f!(x)
}

Additionally, the fn! and fn local!() syntax can be used to indicate that a function might return an error and to let the compiler infer the error type. The former is used for lambda and anonymous functions, and the latter for regular local functions. For example:

fn toplevel() -> Unit!String {
  fn local!() { raise "err" }                    // local function
  apply!(fn! (_x) { raise "err" }, 42) |> ignore // anonymous function
  apply!(fn! { _ => raise "err" }, 42) |> ignore // lambda function
}

Try Expression Enhancements

The try expression has been improved. The complete usage of the try expression is as follows:

fn test_try() -> Unit {
  fn f() -> Int!String {
    raise "err"
  }
  try { 
    println("this is try body")
    f!()
  } except {
    err => println(err)
  } else {
    val => println(val)
  }
}
  • The except keyword is used to handle errors returned by the try body.
  • The else keyword is used to handle normal returns from the try body.

Simplified cases for the try expression:

  • The else branch can be omitted if no handling of the return value is needed.
  • The try body can be a simple expression.
  • The except keyword can be omitted.

For example:

fn test_try() -> Result[Int, String] {
  fn f() -> Int!String {
    raise "err"
  }
  try Ok(f!()) { err => Err(err) }
}

Integer Literal Overloading in Pattern Matching

Integer literal overloading has been added to pattern matching:

fn is_42(x : Double) -> Bool {
  match x {
    42 => true
    _ => false
  }
 }

Upcoming Breaking Changes

  • Map Pattern Matching: The semantics of { "key" : value } in map patterns will be changed this week.

    Previously, in a map pattern, value was of type Option[_] and would match regardless of whether "key" existed.

    In the future, { "key": value } will match only if "key" exists, and value will directly match the actual content of "key".

    To match both the presence and absence of "key", use the new syntax { "key"? : value_or_none }, where value_or_none is of type Option[_].

  • Trait Implementation Syntax: The old syntax for explicit trait implementation fn Trait::method(...) will be removed this week. Existing code should use the impl Trait for SelfType with method(...) syntax. The fn Trait::method(...) syntax will be repurposed for defining methods on trait object types in the future.

IDE Updates

  • Run | Debug Codelens for the main function: The main function will now have a Run | Debug Codelens for quick execution and debugging. Currently, quick debugging only supports the JavaScript backend.

ide
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  • AI-Generated Tests for Top-Level Functions: Top-level functions now support generating tests through AI via Codelens. * Please note that this feature is still under development and may not be stable.

    1. Click the “Gen test” button.

step1

  1. A new window will appear on the right side, displaying the generated test.

step2
step2956×484 29.2 KB

  1. Click “Reject” to remove unwanted tests, or “Insert” to add the test into the source file.

Core Updates

  • Show Trait Redefinition: The Show trait has been redefined for more accurate behavior. The new definition of the Show trait is as follows:
pub trait Show {
  // The `output` function is for implementing `Show` for composite types.
  // It writes a string representation of `Self` to a `Logger`.
  output(Self, Logger) -> Unit
  // The `to_string` method is for `Show` users.
  // It can be used for string interpolation and `println`.
  // `to_string` has a default implementation using `output`.
  // Some types, like `String`, will override `to_string` to modify its behavior.
  to_string(Self) -> String
}

The new Show resolves the incorrect behavior of String. Previously, if a composite type contained a String, the Show implementation would not escape this String, leading to incorrect results. Conversely, String used directly in string interpolation and println should not be escaped. The new Show definition and its core implementation address both needs.

For Show implementers, please keep in mind:

  • Implement the output method, not the to_string method.

  • When recursively calling the Show implementation of substructures (e.g., struct fields), use Show::output(..., logger) instead of to_string.

  • The default implementation of Show::to_string is not a method, so it cannot be called with a dot. To support .to_string() syntax, add a helper definition like this:

pub fn to_string(self: ...) -> String {
  Show::to_string(self)
}

  • Upcoming Breaking Change: Deprecation of Debug Trait: Following the redefinition of the Show trait, the functionality of the Debug trait is now completely covered by Show. Therefore, the Debug trait is deprecated and will be removed soon.

  • Data Structure Type Renaming: Data structure type names have been changed to T. For example:

// old
// let m : @hashmap.HashMap[Int, Int] = @hashmap.new()

// new
let m : @hashmap.T[Int, Int] = @hashmap.new()

The following data structures have been renamed to T:

  • deque
  • hashmap
  • hashset
  • immut/array
  • immut/hashmap
  • immut/hashset
  • immut/list
  • immut/priority_queue
  • immut/sorted_map
  • immut/sorted_set
  • queue
  • priorityqueue
  • rational
  • sorted_map
  • sorted_set

Build System Updates

  • Build Output Adjustment: The build process output has been adjusted to only print a command if that command fails.

  • Target Specification for moon check|build|test|bundle: Previously, the --target option only supported three choices: wasm|wasm-gc|js. Now it supports specifying multiple backends to run in parallel. The supported options have changed to wasm|wasm-gc|js|all, and they can be combined with commas. The --serial option changes the parallel execution of different backends to serial execution, but the same backend will still execute concurrently. For example:

    • moon test --target all runs tests for all backends.
    • moon test --target js,wasm runs tests for js and wasm backends.
    • moon test --target js,wasm --serial is equivalent to running moon test --target js; moon test --target wasm sequentially.

  • Memory Configuration in moon.pkg.json: Added heap-start-address and import-memory configuration for ["link"]["wasm"|"wasm-gc"] in moon.pkg.json:

{ 
  "link": {
    "wasm": {
      "heap-start-address": 65536,
      "import-memory": {
        "module": "xxx",
        "name": "yyy"
      }
    }
  }
}
  • Default Protocol for moon new: The default protocol for moon new has been changed to Apache-2.0.
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