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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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Last modified 11 October 2025

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