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MoonBit is an end-to-end programming language toolchain for cloud and edge computing using WebAssembly. The language is like Rust with GC support. The fast compiler is optimized to produce very compact WASM binaries.

Getting Started

As of 6 Oct 2025, installation is via VSCode extension ("Moonbit Language") or manual (via curl): curl -fsSL https://cli.moonbitlang.com/install/unix.sh | bash (macOS/Linux) or Set-ExecutionPolicy RemoteSigned -Scope CurrentUser; irm https://cli.moonbitlang.com/install/powershell.ps1 | iex (POSH)

Examples

///|
fn main {
  // explicit type annotation
  let a1 : Int = 10
  println((10 : Int))

  // Variable with type inference (Int inferred from value 20)
  let b = 20
  println("a + b = \{a1 + b}")

  // Mutable variable - can be modified
  let mut c = 10
  println("c before: \{c}")
  c = c + 1
  println("c after: \{c}")

  // Immutable variable - cannot be modified
  let d = 20
  println("d = \{d}")
  // d = d + 1  // Uncommenting this line will cause a compilation error
}

Language Tour

• Type annotations (and inferred if not present)
• let bindings immutable; mut bindings mutable
• Basic types: Int, Double, Bool, Char (Unicode), String (immutable), Unit
• Escape sequences, \u{...} for Unicode
• String interpolation via \{variable}
• Functions
  • use fn keyword
  • parens to designate parameters; -> for return type (Unit assumed if not present)
  • parameters
    • must be type-annotated
    • names suffixed with ~ are labeled, and callers must use param name
    • names suffixed with ? have default value, and callers must use param name
    • labeled/optional arguments can be passed in any order
  • return value designated via return or final expression
• Block
  • namespace/binding scope
  • an expression
  • containing sequence of statements and an optional expression
  • yields final expression or Unit (()) result
• Array
  • homogenous
  • literal syntax: [1, 2, 3]
  • Array::make function; Array::make(4,1) == [1, 1, 1, 1]
  • ..arrayname is "array spread"; expands the array's contents in-place
  • array[start:end] obtains a view of the array from index start to index end (exclusive); reference
  • elements inside the array are implicitly mutable
• Tuple
  • accessed via indexed field names: tuple.0, tuple.1, etc
  • destructured via let (a, b) = tuple
• Map
  • key-value pairs; mutable collection
  • {"key1": value1, "key2" : value2}
  • Map::of function; Map::of([("key1", 1), ("key2", 2), ("key3", 3)]) == {"key1":1, "key2":2, "key3":3}
  • accessed via map[key]

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Tags: language   web assembly   browser  

Last modified 14 December 2025

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