Weekly 2024-09-03

Language Updates

• Breaking Change: String interpolation syntax changed from \() to \{} where more expression types within \{} are supported. For example:

fn add(i: Int) -> Int {
  i + 1
}

fn main {
  println("\{add(2)}") // cannot invoke function in \() before
}

• New Optional Argument: Supported a new optional argument syntax where the compiler automatically inserts the Some constructor. Optional arguments of type Option are common, with a default value of None, and explicitly passed arguments use the Some constructor. The new syntax ~label? : T simplifies this process:

fn image(~width?: Int, ~height?: Int, ~src: String) -> Unit

Previously, we had to manually insert lots of Some constructors with the default value, which is quite troublesome.

image(width=Some(300), height=Some(300), src="img.png")

Therefore, MoonBit introduced a new optional argument syntax ~label? : T, which is quite similar to ? in map pattern:

fn image(~width?: Int, ~height?: Int, ~src: String) -> Unit

At this point, width and height are optional arguments with a default value of None, and their types in the function body are Int?. When calling image, if you want to provide values for width and height, you don't need to manually insert Some; the compiler will automatically do it for you:

image(width=300, height=300, src="img.png")

If you need to pass an Int? type value directly to image, you can use the following syntax:

fn image_with_fixed_height(~width? : Int, ~src : String) -> Unit {
  // `~width?` is shorthand for `width?=width`
  image(~width?, height=300, src="img.png")
 }

• Range Operators: Added support for range operators ..< (upper bound exclusive) and ..= (upper bound inclusive) to simplify for loops:

fn main {
  for i in 1..<10 {
    println(i)
  }
}

Currently, the range operators only support the built-in types Int, UInt, Int64, and UInt64, and they can only be used within for .. in loops. In the future, these limitations may be relaxed.

• expr._ replaces <expr>.0: Introduced expr._ syntax for accessing newtypes. The previous syntax <expr>.0 will be deprecated in the future, and currently, it will trigger a warning. The purpose of this change is to simplify the usage of newtype when wrapping a struct or tuple type by automatically forwarding field access to the underlying type, making it easier to use newtype.

• Trait Implementation Consistency: Implementing a trait now includes consistency checks to ensure that all impl signatures match. For example:

trait MyTrait {
  f1(Self) -> Unit
  f2(Self) -> Unit
}

// The signatures of these two `impl` blocks are inconsistent,
// with `f2` being more generic. However, because `impl` is used
// to implement a specific trait, this generality in `f2` is meaningless.
// All `impl` blocks for the same trait and type should have consistent signatures.
impl[X : Show] MyTrait for Array[X] with f1(self) { .. }
impl[X] MyTrait for Array[X] with f2(self) { ... }

Core Update

• Deprecate xx_exn: Functions named xx_exn have been renamed to unsafe_xx (e.g., unsafe_pop, unsafe_nth, unsafe_peek).

• Breaking Change: Converting floating-point numbers to strings now conforms to the ECMAScript standard.

• Function Update: The op_as_view function type signature has been updated for compatibility with the new optional argument syntax.

Before:

fn op_as_view[T](self : Array[T], ~start : Int, ~end : Int) -> ArrayView[T]

Now:

fn op_as_view[T](self : Array[T], ~start : Int, ~end? : Int) -> ArrayView[T]

This allows the Iter type to implement the op_as_view method, enabling slice syntax:

fn main {
  let it: Iter[Int] = [1, 2, 3, 4].iter()[1:2] // slice syntax on iter

  for x in it {
    println(x)  // 2
  }
}

As the new optional argument syntax ~end? : Int is backward compatible, all previous ways of calling op_as_view still work and maintain the same semantics.

• Renaming: Int::to_uint and UInt::to_int have been renamed to Int::reinterpret_as_uint and UInt::reinterpret_as_int.

• Removal and Fixes: The BigInt::lsr function was removed, and bug fixes and performance improvements were made to BigInt.

Toolchain Updates

• Breaking Change: The Diagnostic information text for moon {build,check,test,run} (such as printed errors and warnings) has been moved from stdout to stderr to avoid pollution of stdout output with error and warning details when running moon {test,run}. If your tools rely on stdout text-format diagnostics, please update your code accordingly.

  JSON mode output is unaffected.

• MoonBit AI: Supports batch generation of tests and documentation.

• New Feature: Snapshot testing is now supported, similar to inspect!, but results are written to a file. For example, when running moon test -u, the following test block will generate a file 001.txt in the __snapshot__ folder:

test (it : @test.T) {
    it.write(".")
    it.writeln("..")
    it.snapshot!(filename="001.txt")
}

Note that snapshot testing ignores LF and CRLF differences.

• Build Process Update: moon build now supports building projects even without is-main or link fields in moon.pkg.json. These packages will only generate a core file without linking to wasm/js.

• Formatting Update: moon fmt now supports incremental formatting, initially formatting all .mbt files, then only formatting changed files in subsequent runs.

IDE Updates

• Project-Level Goto References: The IDE now supports project-level goto references. For example, you can find all references to a function like inspect within the core. In the pic, all references calling inspect are found:

• Test Block Debugging: Test blocks allows for quick debugging through codelens.