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2025-02-10

· 3 min read

Language Updates​

New is Expression​

  1. The syntax for this expression is expr is pat. The expression is a Bool type and evaluates to true when expr matches the pattern pat. For example:
fn use_is_expr(x: Int?) -> Unit {
  if x is Some(i) && i >= 10 { ... }
}
  1. The pattern can introduce new binders, which can be used in the following cases:
  • In e1 && e2, if e1 is an is expression, the binders introduced by the pattern can be used in e2.
  • In if e1 && e2 && ... { if_branch } else { ... }, binders introduced by is expressions in the &&-chained conditions like e1 & e2 can be used in the if_branch.

String Construction and Pattern Matching​

  1. New support for constructing strings using array spread syntax, for example:
fn string_spread() -> Unit {
  let s = "hello🤣😂😍"
  let sv = s[1:6]
  let str : String = ['x', ..s, ..sv, '😭']
  println(str) // xhello🤣😂😍ello🤣😭
}

In an array spread, individual elements are Char values. You can use .. to insert a String or a @string.View segment. This syntax is equivalent to using StringBuilder to construct a string.

  1. Support for pattern matching on strings using array patterns, which can be mixed with array literal patterns, for example:
fn match_str(s: String) -> Unit {
  match s {
    "hello" => ... // string literal pattern
    [ 'a' ..= 'z', .. ] => ... // array pattern
    [ .., '😭' ] => ... // array pattern with unicode
    _ => ...
  }
}

New Compiler Warnings​

  • The compiler now warns about unused guard statements and missing cases in guard let ... else ....
fn main {
  guard let (a, b) = (1, 2)
  ^^^^^^^^^ ----- useless guard let
  println(a + b)
}

moonfmt Fixes​

  • Fixed formatting errors related to async code in moonfmt.
  • Adjusted insertion rules for ///| markers.

Package Updates​

  • moonbitlang/x/sys now supports the native backend and fixes inconsistencies across different operating systems.

  • The fs package in moonbitlang/x has been updated with improved error handling.

  • String-related operations are being reorganized. The string package will provide more Unicode-safe APIs while deprecating some APIs that expose UTF-16 implementation details. During this transition, string methods may become unstable. It is recommended to use iter methods or pattern matching to access string elements.

  • Refactored ArrayView/StringView/BytesView types by moving them from the @builtin package to their respective type-related packages. Their names have been updated accordingly to @array.View/@string.View/@bytes.View.

IDE Updates​

  • Added code action support for filling in missing cases in pattern matching.

  • Enabled inline autocompletion for all cases in empty pattern matches.

  • Fixed a bug in trait method "Go to Reference".

  • Fixed missing autocompletion for variables introduced in guard let ... else ... and improved pattern completion in else branches.

Build System Updates​

  • Fixed a bug in moon test where panic tests were being skipped on the native backend.

Documentation Updates​