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MoonBit

MoonBit is an end-to-end programming language toolchain for cloud and edge computing using WebAssembly. The IDE environment is available at https://try.moonbitlang.com without any installation; it does not rely on any server either.

Status and aimed timeline

It is currently alpha, experimental. We expect MoonBit to reach beta-preview in 02/2024 and beta in 06/2024.

When MoonBit reaches beta, it means any backwards-incompatible changes will be seriously evaluated and MoonBit can be used in production(very rare compiler bugs). MoonBit is developed by a talented full time team who had extensive experience in building language toolchains, so we will grow much faster than the typical language ecosystem, you won't wait long to use MoonBit in your production.

Main advantages

  • Generate significantly smaller WASM output than any existing solutions.
  • Much faster runtime performance.
  • State of the art compile-time performance.
  • Simple but practical, data-oriented language design.

Overview

A MoonBit program consists of type definitions, function definitions, and variable bindings. The entry point of every package is a special init function. The init function is special in two aspects:

  1. There can be multiple init functions in the same package.
  2. An init function can't be explicitly called or referred to by other functions. Instead, all init functions will be implicitly called when initializing a package. Therefore, init functions should only consist of statements.
fn init {
  print("Hello world!") // OK
}

fn init {
  let x = 1
  // x // fail
  print(x) // success
}

MoonBit distinguishes between statements and expressions. In a function body, only the last clause should be an expression, which serves as a return value. For example:

fn foo() -> Int {
  let x = 1
  x + 1 // OK
}

fn bar() -> Int {
  let x = 1
  x + 1 // fail
  x + 2
}

fn init {
  print(foo())
  print(bar())
}

Expressions and Statements

Expressions include:

  • Value literals (e.g. Boolean values, numbers, characters, strings, arrays, tuples, structs)
  • Arithmetical, logical, or comparison operations
  • Accesses to array elements (e.g. a[0]) or struct fields (e.g r.x) or tuple components (e.g. t.0)
  • Variables and (capitalized) enum constructors
  • Anonymous local function definitions
  • match and if expressions

Statements include:

  • Named local function definitions
  • Local variable bindings
  • Assignments
  • While loops and related control constructs (break and continue)
  • return statements
  • Any expression whose return type is unit

Functions

Functions take arguments and produce a result. In MoonBit, functions are first-class, which means that functions can be arguments or return values of other functions.

Top-Level Functions

Functions can be defined as top-level or local. We can use the fn keyword to define a top-level function that sums three integers and returns the result, as follows:

fn add3(x: Int, y: Int, z: Int)-> Int {
x + y + z
}

Note that the arguments and return value of top-level functions require explicit type annotations.

Local Functions

Local functions can be named or anonymous. Type annotations can be omitted for local function definitions: they can be automatically inferred in most cases. For example:

fn foo() -> Int {
  fn inc(x) { x + 1 }  // named as `inc`
  fn (x) { x + inc(2) } (6) // anonymous, instantly applied to integer literal 6
}

fn init {
  print(foo())
}

Functions, whether named or anonymous, are lexical closures: any identifiers without a local binding must refer to bindings from a surrounding lexical scope. For example:

let y = 3
fn foo(x: Int) -> Unit {
  fn inc()  { x + 1 } // OK, will return x + 1
  fn four() { y + 1 } // Ok, will return 4
  print(inc())
  print(four())
}

fn init {
  foo(2)
}

Function Applications

A function can be applied to a list of arguments in parentheses:

add3(1, 2, 7)

This works whether add3 is a function defined with a name (as in the previous example), or a variable bound to a function value, as shown below:

fn init {
  let add3 = fn(x, y, z) { x + y + z }
  print(add3(1, 2, 7))
}

The expression add3(1, 2, 7) returns 10. Any expression that evaluates to a function value is applicable:

fn init {
  let f = fn (x) { x + 1 }
  let g = fn (x) { x + 2 }
  print((if true { f } else { g })(3)) // OK
}

Labelled arguments

Functions can declare labelled argument with the syntax ~label : Type. label will also serve as parameter name inside function body:

fn labelled(~arg1 : Int, ~arg2 : Int) -> Int {
arg1 + arg2
}

Labelled arguments can be supplied via the syntax label=arg. label=label can be abbreviated as ~label:

fn init {
let arg1 = 1
println(labelled(arg2=2, ~arg1)) // 3
}

Labelled function can be supplied in any order. The evaluation order of arguments is the same as the order of parameters in function declaration.

Optional arguments

A labelled argument can be made optional by supplying a default expression with the syntax ~label : Type = default_expr. If this argument is not supplied at call site, the default expression will be used:

fn optional(~opt : Int = 42) -> Int {
  opt
}

fn init {
  println(optional()) // 42
  println(optional(opt=0)) // 0
}

The default expression will be evaluated everytime it is used. And the side effect in the default expression, if any, will also be triggered. For example:

fn incr(~counter : Ref[Int] = { val: 0 }) -> Ref[Int] {
  counter.val = counter.val + 1
  counter
}

fn init {
  println(incr()) // 1
  println(incr()) // still 1, since a new reference is created everytime default expression is used
  let counter : Ref[Int] = { val: 0 }
  println(incr(~counter)) // 1
  println(incr(~counter)) // 2, since the same counter is used
}

If you want to share the result of default expression between different function calls, you can lift the default expression to a toplevel let declaration:

let default_counter : Ref[Int] = { val: 0 }

fn incr(~counter : Ref[Int] = default_counter) -> Int {
  counter.val = counter.val + 1
  counter.val
}

fn init {
  println(incr()) // 1
  println(incr()) // 2
}

Default expression can depend on the value of previous arguments. For example:

fn sub_array[X](xs : Array[X], ~offset : Int, ~len : Int = xs.length() - offset) -> Array[X] {
... // take a sub array of [xs], starting from [offset] with length [len]
}

fn init {
println(sub_array([1, 2, 3], offset=1)) // [2, 3]
println(sub_array([1, 2, 3], offset=1, len=1)) // [2]
}

Autofill arguments

MoonBit supports filling specific types of arguments automatically at different call site, such as the source location of a function call. To declare an autofill argument, simply declare an optional argument with _ as default value. Now if the argument is not explicitly supplied, MoonBit will automatically fill it at the call site.

Currently MoonBit supports two types of autofill arguments, SourceLoc, which is the source location of the whole function call, and ArgsLoc, which is a array containing the source location of each argument, if any:

fn f(_x : Int, _y : Int, ~loc : SourceLoc = _, ~args_loc : ArgsLoc = _) -> Unit {
println("loc of whole function call: \(loc)")
println("loc of arguments: \(args_loc)")
}

fn init {
f(1, 2)
// loc of whole function call: <filename>:7:3-7:10
// loc of arguments: [Some(<filename>:7:5-7:6), Some(<filename>:7:8-7:9), None, None]
}

Autofill arguments are very useful for writing debugging and testing utilities.

Control Structures

Conditional Expressions

A conditional expression consists of a condition, a consequent, and an optional else clause.

if x == y {
expr1
} else {
expr2
}

if x == y {
expr1
}

The else clause can also contain another if-else expression:

if x == y {
expr1
} else if z == k {
expr2
}

Curly brackets are used to group multiple expressions in the consequent or the else clause.

Note that a conditional expression always returns a value in MoonBit, and the return values of the consequent and the else clause must be of the same type.

Functional loop

Functional loop is a powerful feature in MoonBit that enables you to write loops in a functional style.

A functional loop consumes arguments and returns a value. It is defined using the loop keyword, followed by its arguments and the loop body. The loop body is a sequence of clauses, each of which consists of a pattern and an expression. The clause whose pattern matches the input will be executed, and the loop will return the value of the expression. If no pattern matches, the loop will panic. Use the continue keyword with arguments to start the next iteration of the loop. Use the break keyword with arguments to return a value from the loop. The break keyword can be omitted if the value is the last expression in the loop body.

fn sum(xs: List[Int]) -> Int {
  loop xs, 0 {
    Nil, acc => break acc // break can be omitted
    Cons(x, rest), acc => continue rest, x + acc
  }
}

fn init {
  println(sum(Cons(1, Cons(2, Cons(3, Nil)))))
}

While loop

In MoonBit, while loop can be used to execute a block of code repeatedly as long as a condition is true. The condition is evaluated before executing the block of code. The while loop is defined using the while keyword, followed by a condition and the loop body. The loop body is a sequence of statements. The loop body is executed as long as the condition is true.

while x == y {
expr1
}

The while statement doesn't yield anything; it only evaluates to () of unit type. MoonBit also provides the break and continue statements for controlling the flow of a loop.

let mut i = 0
let mut n = 0

while i < 10 {
i = i + 1
if (i == 3) {
continue
}

if (i == 8) {
break
}
n = n + i
}
// n = 1 + 2 + 4 + 5 + 6 + 7
println(n) // outputs 25

Built-in Data Structures

Boolean

MoonBit has a built-in boolean type, which has two values: true and false. The boolean type is used in conditional expressions and control structures.

let a = true
let b = false
let c = a && b
let d = a || b
let e = not(a)

Number

MoonBit have integer type and floating point type:

typedescription
Int32-bit signed integer
Int6464-bit signed integer
Double64-bit floating point, defined by IEEE754

MoonBit also supports numeric literals, including decimal, binary, octal, and hexadecimal numbers.

To improve readability, you may place underscores in the middle of numeric literals such as 1_000_000. Note that underscores can be placed anywhere within a number, not just every three digits.

  • There is nothing surprising about decimal numbers.
let a = 1234
let b = 1_000_000 + a
let large_num = 9_223_372_036_854_775_807L // Integers of the Int64 type must have an 'L' as a suffix
  • A binary number has a leading zero followed by a letter "B", i.e. 0b/0B. Note that the digits after 0b/0B must be 0 or 1.
let bin =  0b110010
let another_bin = 0B110010
  • An octal number has a leading zero followed by a letter "O", i.e. 0o/0O. Note that the digits after 0o/0O must be in the range from 0 through 7:
let octal = 0o1234
let another_octal = 0O1234
  • A hexadecimal number has a leading zero followed by a letter "X", i.e. 0x/0X. Note that the digits after the 0x/0X must be in the range 0123456789ABCDEF.
let hex = 0XA
let another_hex = 0xA

String

String holds a sequence of UTF-16 code units. You can use double quotes to create a string, or use #| to write a multi-line string.

let a = "兔rabbit"
println(a[0]) // output: 兔
println(a[1]) // output: r
let b =
#| Hello
#| MoonBit
#|

In double quotes string, a backslash followed by certain special characters forms an escape sequence:

escape sequencesdescription
\n,\r,\t,\bNew line, Carriage return, Horizontal tab, Backspace
\\Backslash
\x41Hexadecimal escape sequence
\o102Octal escape sequence
\u5154,\u{1F600}Unicode escape sequence

MoonBit supports string interpolation. It enables you to substitute variables within interpolated strings. This feature simplifies the process of constructing dynamic strings by directly embedding variable values into the text.

fn init {
  let x = 42
  print("The answer is \(x)")
}

Variables used for string interpolation must support the to_string method.

Char

Char is an integer representing a Unicode code point.

let a : Char = 'A'
let b = '\x41'
let c = '🐰'

Byte

A byte literal in MoonBit is either a single ASCII character or a single escape enclosed in single quotes ', and preceded by the character b. Byte literals are of type Byte. For example:

fn init {
  let b1 : Byte = b'a'
  println(b1.to_int())
  let b2 = b'\xff'
  println(b2.to_int())
}

Tuple

A tuple is a collection of finite values constructed using round brackets () with the elements separated by commas ,. The order of elements matters; for example, (1,true) and (true,1) have different types. Here's an example:

fn pack(a: Bool, b: Int, c: String, d: Double) -> (Bool, Int, String, Double) {
    (a, b, c, d)
}
fn init {
    let quad = pack(false, 100, "text", 3.14)
    let (bool_val, int_val, str, float_val) = quad
    println("\(bool_val) \(int_val) \(str) \(float_val)")
}

Tuples can be accessed via pattern matching or index:

fn f(t : (Int, Int)) -> Unit {
  let (x1, y1) = t // access via pattern matching
  // access via index
  let x2 = t.0
  let y2 = t.1
  if (x1 == x2 && y1 == y2) {
    print("yes")
  } else {
    print("no")
  }
}

fn init {
  f((1, 2))
}

Array

An array is a finite sequence of values constructed using square brackets [], with elements separated by commas ,. For example:

let numbers = [1, 2, 3, 4]

You can use numbers[x] to refer to the xth element. The index starts from zero.

fn init {
  let numbers = [1, 2, 3, 4]
  let a = numbers[2]
  numbers[3] = 5
  let b = a + numbers[3]
  print(b) // prints 8
}

Variable Binding

A variable can be declared as mutable or immutable using let mut or let, respectively. A mutable variable can be reassigned to a new value, while an immutable one cannot.

let zero = 0

fn init {
  let mut i = 10
  i = 20
  print(i + zero)
}

Data Types

There are two ways to create new data types: struct and enum.

Struct

In MoonBit, structs are similar to tuples, but their fields are indexed by field names. A struct can be constructed using a struct literal, which is composed of a set of labeled values and delimited with curly brackets. The type of a struct literal can be automatically inferred if its fields exactly match the type definition. A field can be accessed using the dot syntax s.f. If a field is marked as mutable using the keyword mut, it can be assigned a new value.

struct User {
  id: Int
  name: String
  mut email: String
}

fn init {
  let u = { id: 0, name: "John Doe", email: "john@doe.com" }
  u.email = "john@doe.name"
  println(u.id)
  println(u.name)
  println(u.email)
}

Constructing Struct with Shorthand

If you already have some variable like name and email, it's redundant to repeat those names when constructing a struct:

fn init{
  let name = "john"
  let email = "john@doe.com"
  let u = { id: 0, name: name, email: email }
}

You can use shorthand instead, it behaves exactly the same.

fn init{
  let name = "john"
  let email = "john@doe.com"
  let u = { id: 0, name, email }
}

Struct Update Syntax

It's useful to create a new struct based on an existing one, but with some fields updated.

struct User {
  id: Int
  name: String
  email: String
} derive(Debug)

fn init {
  let user = { id: 0, name: "John Doe", email: "john@doe.com" }
  let updated_user = { ..user, email: "john@doe.name" }
  debug(user)         // output: { id: 0, name: "John Doe", email: "john@doe.com" }
  debug(updated_user) // output: { id: 0, name: "John Doe", email: "john@doe.name" }
}

Enum

Enum types are similar to algebraic data types in functional languages. Users familiar with C/C++ may prefer calling it tagged union.

An enum can have a set of cases (constructors). Constructor names must start with capitalized letter. You can use these names to construct corresponding cases of an enum, or checking which branch an enum value belongs to in pattern matching:

// An enum type that represents the ordering relation between two values,
// with three cases "Smaller", "Greater" and "Equal"
enum Relation {
  Smaller
  Greater
  Equal
}

// compare the ordering relation between two integers
fn compare_int(x: Int, y: Int) -> Relation {
  if x < y {
    // when creating an enum, if the target type is known, you can write the constructor name directly
    Smaller
  } else if x > y {
    // but when the target type is not known,
    // you can always use `TypeName::Constructor` to create an enum unambiguously
    Relation::Greater
  } else {
    Equal
  }
}

// output a value of type `Relation`
fn print_relation(r: Relation) -> Unit {
  // use pattern matching to decide which case `r` belongs to
  match r {
    // during pattern matching, if the type is known, writing the name of constructor is sufficient
    Smaller => println("smaller!")
    // but you can use the `TypeName::Constructor` syntax for pattern matching as well
    Relation::Greater => println("greater!")
    Equal => println("equal!")
  }
}

fn init {
  print_relation(compare_int(0, 1)) // smaller!
  print_relation(compare_int(1, 1)) // equal!
  print_relation(compare_int(2, 1)) // greater!
}

Enum cases can also carry payload data. Here's an example of defining an integer list type using enum:

enum List {
  Nil
  // constructor `Cons` carries additional payload: the first element of the list,
  // and the remaining parts of the list
  Cons (Int, List)
}

fn init {
  // when creating values using `Cons`, the payload of by `Cons` must be provided
  let l: List = Cons(1, Cons(2, Nil))
  println(is_singleton(l))
  print_list(l)
}

fn print_list(l: List) -> Unit {
  // when pattern-matching an enum with payload,
  // in additional to deciding which case a value belongs to
  // you can extract the payload data inside that case
  match l {
    Nil => print("nil")
    // Here `x` and `xs` are defining new variables instead of referring to existing variables,
    // if `l` is a `Cons`, then the payload of `Cons` (the first element and the rest of the list)
    // will be bind to `x` and `xs
    Cons(x, xs) => {
      print(x)
      print(",")
      print_list(xs)
    }
  }
}

// In addition to binding payload to variables,
// you can also continue matching payload data inside constructors.
// Here's a function that decides if a list contains only one element
fn is_singleton(l: List) -> Bool {
  match l {
    // This branch only matches values of shape `Cons(_, Nil)`, i.e. lists of length 1
    Cons(_, Nil) => true
    // Use `_` to match everything else
    _ => false
  }
}

Constructor with labelled arguments

Enum constructors can have labelled argument:

enum E {
  // `x` and `y` are alabelled argument
  C(~x : Int, ~y : Int)
}

// pattern matching constructor with labelled arguments
fn f(e : E) -> Unit {
  match e {
    // `label=pattern`
    C(x=0, y=0) => println("0!")
    // `~x` is an abbreviation for `x=x`
    // Unmatched labelled arguments can be omitted via `..`
    C(~x, ..) => println(x)
  }
}

// creating constructor with labelled arguments
fn init {
  f(C(x=0, y=0)) // `label=value`
  let x = 0
  f(C(~x, y=1)) // `~x` is an abbreviation for `x=x`
}

It is also possible to access labelled arguments of constructors like accessing struct fields in pattern matching:

enum Object {
  Point(~x : Double, ~y : Double)
  Circle(~x : Double, ~y : Double, ~radius : Double)
}

fn distance_with(self : Object, other : Object) -> Double {
  match (self, other) {
    // For variables defined via `Point(..) as p`,
    // the compiler knows it must be of constructor `Point`,
    // so you can access fields of `Point` directly via `p.x`, `p.y` etc.
    (Point(_) as p1, Point(_) as p2) => {
      let dx = p2.x - p1.x
      let dy = p2.y - p1.y
      (dx * dx + dy * dy).sqrt()
    }
    (Point(_), Circle(_)) | (Circle(_) | Point(_)) | (Circle(_), Circle(_)) => abort("not implemented")
  }
}

fn init {
  let p1 : Point = Point(x=0, y=0)
  let p2 : Point = Point(x=3, y=4)
  println(p1.distance_with(p2)) // 5.0
}

Constructor with mutable fields

It is also possible to define mutable fields for constructor. This is especially useful for defining imperative data structures:

// A mutable binary search tree with parent pointer
enum Tree[X] {
  Nil
  // only labelled arguments can be mutable
  Node(mut ~value : X, mut ~left : Tree[X], mut ~right : Tree[X], mut ~parent : Tree[X])
}

// A set implemented using mutable binary search tree.
struct Set[X] {
  mut root : Tree[X]
}

fn Set::insert[X : Compare](self : Set[X], x : X) -> Unit {
  self.root = self.root.insert(x, parent=Nil)
}

// In-place insert a new element to a binary search tree.
// Return the new tree root
fn Tree::insert[X : Compare](self : Tree[X], x : X, ~parent : Tree[X]) -> Tree[X] {
  match self {
    Nil => Node(value=x, left=Nil, right=Nil, ~parent)
    Node(_) as node => {
      let order = x.compare(node.value)
      if order == 0 {
        // mutate the field of a constructor
        node.value = x
      } else if order < 0 {
        // cycle between `node` and `node.left` created here
        node.left = node.left.insert(x, parent=node)
      } else {
        node.right = node.right.insert(x, parent=node)
      }
      // The tree is non-empty, so the new root is just the original tree
      node
    }
  }
}

Newtype

MoonBit supports a special kind of enum called newtype:

// `UserId` is a fresh new type different from `Int`, and you can define new methods for `UserId`, etc.
// But at the same time, the internal representation of `UserId` is exactly the same as `Int`
type UserId Int
type UserName String

Newtypes are similar to enums with only one constructor (with the same name as the newtype itself). So, you can use the constructor to create values of newtype, or use pattern matching to extract the underlying representation of a newtype:

fn init {
let id: UserId = UserId(1)
let name: UserName = UserName("John Doe")
let UserId(uid) = id // the type of `uid` is `Int`
let UserName(uname) = name // the type of `uname` is `String`
println(uid)
println(uname)
}

Besides pattern matching, you can also use .0 to extract the internal representation of newtypes:

fn init {
let id: UserId = UserId(1)
let uid: Int = id.0
println(uid)
}

Pattern Matching

We have shown a use case of pattern matching for enums, but pattern matching is not restricted to enums. For example, we can also match expressions against Boolean values, numbers, characters, strings, tuples, arrays, and struct literals. Since there is only one case for those types other than enums, we can pattern match them using let binding instead of match expressions. Note that the scope of bound variables in match is limited to the case where the variable is introduced, while let binding will introduce every variable to the current scope. Furthermore, we can use underscores _ as wildcards for the values we don't care about, use .. to ignore remaining fields of struct or elements of array.

let id = match u {
{ id: id, name: _, email: _ } => id
}
// is equivalent to
let { id: id, name: _, email: _ } = u
// or
let { id: id, ..} = u
let ary = [1,2,3,4]
let [a, b, ..] = ary // a = 1, b = 2
let [.., a, b] = ary // a = 3, b = 4

There are some other useful constructs in pattern matching. For example, we can use as to give a name to some pattern, and we can use | to match several cases at once. A variable name can only be bound once in a single pattern, and the same set of variables should be bound on both sides of | patterns.

match expr {
Lit(n) as a => ...
Add(e1, e2) | Mul(e1, e2) => ...
_ => ...
}

Map Pattern

MoonBit allows convenient matching on map-like data structures:

match map {
// matches if any only if "b" exists in `map`
{ "b": Some(_) } => ..
// matches if and only if "b" does not exist in `map` and "a" exists in `map`.
// When matches, bind the value of "a" in `map` to `x`
{ "b": None, "a": Some(x) } => ..
// compiler reports missing case: { "b": None, "a": None }
}
  • To match a data type T using map pattern, T must have a method op_get(Self, K) -> Option[V] for some type K and V.
  • Currently, the key part of map pattern must be a constant
  • Map patterns are always open: unmatched keys are silently ignored
  • Map pattern will be compiled to efficient code: every key will be fetched at most once

Generics

Generics are supported in top-level function and data type definitions. Type parameters can be introduced within square brackets. We can rewrite the aforementioned data type List to add a type parameter T to obtain a generic version of lists. We can then define generic functions over lists like map and reduce.

enum List[T] {
Nil
Cons(T, List[T])
}

fn map[S, T](self: List[S], f: (S) -> T) -> List[T] {
match self {
Nil => Nil
Cons(x, xs) => Cons(f(x), map(xs, f))
}
}

fn reduce[S, T](self: List[S], op: (T, S) -> T, init: T) -> T {
match self {
Nil => init
Cons(x, xs) => reduce(xs, op, op(init, x))
}
}

Access Control

By default, all function definitions and variable bindings are invisible to other packages; types without modifiers are abstract data types, whose name is exported but the internals are invisible. This design prevents unintended exposure of implementation details. You can use the pub modifier before type/enum/struct/let or top-level function to make them fully visible, or put priv before type/enum/struct to make it fully invisible to other packages. You can also use pub or priv before field names to obtain finer-grained access control. However, it is important to note that:

  • Struct fields cannot be defined as pub within an abstract or private struct since it makes no sense.
  • Enum constructors do not have individual visibility so you cannot use pub or priv before them.
struct R1 {       // abstract data type by default
x: Int // implicitly private field
pub y: Int // ERROR: `pub` field found in an abstract type!
priv z: Int // WARNING: `priv` is redundant!
}

pub struct R2 { // explicitly public struct
x: Int // implicitly public field
pub y: Int // WARNING: `pub` is redundant!
priv z: Int // explicitly private field
}

priv struct R3 { // explicitly private struct
x: Int // implicitly private field
pub y: Int // ERROR: `pub` field found in a private type!
priv z: Int // WARNING: `priv` is redundant!
}

enum T1 { // abstract data type by default
A(Int) // implicitly private variant
pub B(Int) // ERROR: no individual visibility!
priv C(Int) // ERROR: no individual visibility!
}

pub enum T2 { // explicitly public enum
A(Int) // implicitly public variant
pub B(Int) // ERROR: no individual visibility!
priv C(Int) // ERROR: no individual visibility!
}

priv enum T3 { // explicitly private enum
A(Int) // implicitly private variant
pub B(Int) // ERROR: no individual visibility!
priv C(Int) // ERROR: no individual visibility!
}

Another useful feature supported in MoonBit is pub(readonly) types, which are inspired by private types in OCaml. In short, values of pub(readonly) types can be destructed by pattern matching and the dot syntax, but cannot be constructed or mutated in other packages. Note that there is no restriction within the same package where pub(readonly) types are defined.

// Package A
pub(readonly) struct RO {
field: Int
}
fn init {
let r = { field: 4 } // OK
let r = { ..r, field: 8 } // OK
}

// Package B
fn print(r : RO) -> Unit {
print("{ field: ")
print(r.field) // OK
print(" }")
}
fn init {
let r : RO = { field: 4 } // ERROR: Cannot create values of the public read-only type RO!
let r = { ..r, field: 8 } // ERROR: Cannot mutate a public read-only field!
}

Access control in MoonBit adheres to the principle that a pub type, function, or variable cannot be defined in terms of a private type. This is because the private type may not be accessible everywhere that the pub entity is used. MoonBit incorporates sanity checks to prevent the occurrence of use cases that violate this principle.

pub struct S {
x: T1 // OK
y: T2 // OK
z: T3 // ERROR: public field has private type `T3`!
}

// ERROR: public function has private parameter type `T3`!
pub fn f1(_x: T3) -> T1 { T1::A(0) }
// ERROR: public function has private return type `T3`!
pub fn f2(_x: T1) -> T3 { T3::A(0) }
// OK
pub fn f3(_x: T1) -> T1 { T1::A(0) }

pub let a: T3 // ERROR: public variable has private type `T3`!

Method system

MoonBit supports methods in a different way from traditional object-oriented languages. A method in MoonBit is just a toplevel function associated with a type constructor. Methods can be defined using the syntax fn TypeName::method_name(...) -> ...:

enum MyList[X] {
Nil
Cons(X, MyList[X])
}

fn MyList::map[X, Y](xs: MyList[X], f: (X) -> Y) -> MyList[Y] { ... }
fn MyList::concat[X](xs: MyList[MyList[X]]) -> MyList[X] { ... }

As a convenient shorthand, when the first parameter of a function is named self, MoonBit automatically defines the function as a method of the type of self:

fn map[X, Y](self: MyList[X], f: (X) -> Y) -> List[Y] { ... }
// equivalent to
fn MyList::map[X, Y](xs: MyList[X], f: (X) -> Y) -> List[Y] { ... }

Methods are just regular functions owned by a type constructor. So when there is no ambiguity, methods can be called using regular function call syntax directly:

fn init {
let xs: MyList[MyList[_]] = ...
let ys = concat(xs)
}

Unlike regular functions, methods support overloading: different types can define methods of the same name. If there are multiple methods of the same name (but for different types) in scope, one can still call them by explicitly adding a TypeName:: prefix:

struct T1 { x1: Int }
fn T1::default() -> { { x1: 0 } }

struct T2 { x2: Int }
fn T2::default() -> { { x2: 0 } }

fn init {
  // default() is ambiguous!
  let t1 = T1::default() // ok
  let t2 = T2::default() // ok
}

When the first parameter of a method is also the type it belongs to, methods can be called using dot syntax x.method(...). MoonBit automatically finds the correct method based on the type of x, there is no need to write the type name and even the package name of the method:

// a package named @list
enum List[X] { ... }
fn List::length[X](xs: List[X]) -> Int { ... }

// another package that uses @list
fn init {
let xs: @list.List[_] = ...
debug(xs.length()) // always work
debug(@list.List::length(xs)) // always work, but verbose
debug(@list.length(xs)) // simpler, but only possible when there is no ambiguity in @list
}

Operator Overloading

MoonBit supports operator overloading of builtin operators via methods. The method name corresponding to a operator <op> is op_<op>. For example:

struct T {
  x:Int
} derive(Debug)

fn op_add(self: T, other: T) -> T {
  { x: self.x + other.x }
}

fn init {
  let a = { x: 0 }
  let b = { x: 2 }
  debug(a + b)
}

Another example about op_get and op_set:

struct Coord {
  mut x: Int
  mut y: Int
} derive(Debug)

fn op_get(self: Coord, key: String) -> Int {
  match key {
    "x" => self.x
    "y" => self.y
  }
}

fn op_set(self: Coord, key: String, val: Int) -> Unit {
    match key {
    "x" => self.x = val
    "y" => self.y = val
  }
}

fn init {
  let c = { x: 1, y: 2 }
  debug(c)
  debug(c["y"])
  c["x"] = 23
  debug(c)
  debug(c["x"])
}

Currently, the following operators can be overloaded:

operator namemethod name
+op_add
-op_sub
*op_mul
/op_div
%op_mod
-(unary)op_neg
_[_](get item)op_get
_[_] = _(set item)op_set

Pipe operator

MoonBit provides a convenient pipe operator |>, which can be used to chain regular function calls:

fn init {
x |> f // equivalent to f(x)
x |> f(y) // equivalent to f(x, y)

// Chain calls at multiple lines
arg_val
|> f1 // equivalent to f1(arg_val)
|> f2(other_args) // equivalent to f2(f1(arg_val), other_args)
}

Trait system

MoonBit features a structural trait system for overloading/ad-hoc polymorphism. Traits declare a list of operations, which must be supplied when a type wants to implement the trait. Traits can be declared as follows:

trait I {
method(...) -> ...
}

In the body of a trait definition, a special type Self is used to refer to the type that implements the trait.

To implement a trait, a type must provide all the methods required by the trait. However, there is no need to implement a trait explicitly. Types with the required methods automatically implements a trait. For example, the following trait:

trait Show {
to_string(Self) -> String
}

is automatically implemented by builtin types such as Int and Double.

When declaring a generic function, the type parameters can be annotated with the traits they should implement, allowing the definition of constrained generic functions. For example:

trait Number {
op_add(Self, Self) -> Self
op_mul(Self, Self) -> Self
}

fn square[N: Number](x: N) -> N {
x * x
}

Without the Number requirement, the expression x * x in square will result in a method/operator not found error. Now, the function square can be called with any type that implements Number, for example:

fn init {
debug(square(2)) // 4
debug(square(1.5)) // 2.25
debug(square({ x: 2, y: 3 })) // (4, 9)
}

struct Point {
x: Int
y: Int
} derive(Debug)

fn op_add(self: Point, other: Point) -> Point {
{ x: self.x + other.x, y: self.y + other.y }
}

fn op_mul(self: Point, other: Point) -> Point {
{ x: self.x * other.x, y: self.y * other.y }
}

Methods of a trait can be called directly via Trait::method. MoonBit will infer the type of Self and check if Self indeed implements Trait, for example:

fn init {
  println(Show::to_string(42))
  debug(Compare::compare(1.0, 2.5))
}

MoonBit provides the following useful builtin traits:

trait Eq {
op_equal(Self, Self) -> Bool
}

trait Compare {
// `0` for equal, `-1` for smaller, `1` for greater
op_equal(Self, Self) -> Int
}

trait Hash {
hash(Self) -> Int
}

trait Show {
to_string(Self) -> String
}

trait Default {
default() -> Self
}

trait Debug {
// write debug information of [self] to a buffer
debug_write(Self, Buffer) -> Unit
}

Access control of methods and extension methods

To make the trait system coherent (i.e. there is a globally unique implementation for every Type: Trait pair), and prevent third-party packages from modifying behavior of existing programs by accident, only the package that defines a type can define methods for it. So one cannot define new methods or override old methods for builtin and foreign types.

However, it is often useful to extend the functionality of an existing type. So MoonBit provides a mechanism called extension method, defined using the syntax fn Trait::method_name(...) -> .... Extension methods extend the functionality of an existing type by implementing a trait. For example, to implement a new trait ToMyBinaryProtocol for builtin types, one can (and must) use extension methods:

trait ToMyBinaryProtocol {
to_my_binary_protocol(Self, Buffer) -> Unit
}

fn ToMyBinaryProtocol::to_my_binary_protocol(x: Int, b: Buffer) -> Unit { ... }
fn ToMyBinaryProtocol::to_my_binary_protocol(x: Double, b: Buffer) -> Unit { ... }
fn ToMyBinaryProtocol::to_my_binary_protocol(x: String, b: Buffer) -> Unit { ... }

When searching for the implementation of a trait, extension methods have a higher priority, so they can be used to override ordinary methods with undesirable behavior. Extension methods can only be used to implement the specified trait. They cannot be called directly like ordinary methods. Furthermore, only the package of the type or the package of the trait can implement extension methods. For example, only @pkg1 and @pkg2 are allowed to implement an extension method @pkg1.Trait::f for type @pkg2.Type. This restriction ensures that MoonBit's trait system is still coherent with the extra flexibility of extension methods.

To invoke an extension method directly, use the Trait::method syntax.

trait MyTrait {
  f(Self) -> Unit
}

fn MyTrait::f(self: Int) -> Unit {
  println("Got Int \(self)!")
}

fn init {
  MyTrait::f(42)
}

Automatically derive builtin traits

MoonBit can automatically derive implementations for some builtin traits:

struct T {
  x: Int
  y: Int
} derive(Eq, Compare, Debug, Default)

fn init {
  let t1 = T::default()
  let t2 = { x: 1, y: 1 }
  debug(t1) // {x: 0, y: 0}
  debug(t2) // {x: 1, y: 1}
  debug(t1 == t2) // false
  debug(t1 < t2) // true
}

Trait objects

MoonBit supports runtime polymorphism via trait objects. If t is of type T, which implements trait I, one can pack the methods of T that implements I, together with t, into a runtime object via t as I. Trait object erases the concrete type of a value, so objects created from different concrete types can be put in the same data structure and handled uniformly:

trait Animal {
  speak(Self)
}

type Duck String
fn Duck::make(name: String) -> Duck { Duck(name) }
fn speak(self: Duck) -> Unit {
  println(self.0 + ": quak!")
}

type Fox String
fn Fox::make(name: String) -> Fox { Fox(name) }
fn Fox::speak(_self: Fox) -> Unit {
  println("What does the fox say?")
}

fn init {
  let duck1 = Duck::make("duck1")
  let duck2 = Duck::make("duck2")
  let fox1 = Fox::make("fox1")
  let animals = [ duck1 as Animal, duck2 as Animal, fox1 as Animal ]
  let mut i = 0
  while i < animals.length() {
    animals[i].speak()
    i = i + 1
  }
}

Not all traits can be used to create objects. "object-safe" traits' methods must satisfy the following conditions:

  • Self must be the first parameter of a method
  • There must be only one occurrence of Self in the type of the method (i.e. the first parameter)

The question operator

MoonBit features a convenient ? operator for error handling. The ? postfix operator can be applied to expressions of type Option or Result. When applied to expression t : Option[T], t? is equivalent to:

match t {
None => { return None }
Some(x) => x
}

When applied to expression t: Result[T, E], t? is equivalent to:

match t {
Err(err) => { return Err(err) }
Ok(x) => x
}

The question operator can be used to combine codes that may fail or error elegantly:

fn may_fail() -> Option[Int] { ... }

fn f() -> Option[Int] {
let x = may_fail()?
let y = may_fail()?.lsr(1) + 1
if y == 0 { return None }
Some(x / y)
}

fn may_error() -> Result[Int, String] { ... }

fn g() -> Result[Int, String] {
let x = may_error()?
let y = may_error()? * 2
if y == 0 { return Err("divide by zero") }
Ok(x / y)
}

MoonBit's build system

The introduction to the build system is available at MoonBit's Build System Tutorial.