AssemblyScript

AssemblyScript defines a subset of TypeScript that it compiles to WebAssembly. The compiler itself is written in TypeScript and no binary dependencies are required to get started. Under the hood, it rewires TypeScript's compiler API to binaryen's compiler infrastructure.

Be warned that this is an early prototype and that it cannot do anything useful yet.

Example

/// <reference path="node_modules/assemblyscript/assembly.d.ts" />

export function add(a: int, b: double): short {
  return (a + (b as int)) as short;
}

Compiles to:

(module
 (type $iFi (func (param i32 f64) (result i32)))
 (memory $0 256)
 (export "memory" (memory $0))
 (export "add" (func $add))
 (func $add (type $iFi) (param $0 i32) (param $1 f64) (result i32)
  (return
   (i32.shl
    (i32.shr_s
     (i32.add
      (get_local $0)
      (i32.trunc_s/f64
       (get_local $1)
      )
     )
     (i32.const 16)
    )
    (i32.const 16)
   )
  )
 )
)

Usage

An AssemblyScript program is valid TypeScript syntactically, but not necessarily semantically.

WebAssembly-specific types are obtained by referencing assembly.d.ts:

Type	WASM type	Description
sbyte	i32	An 8-bit signed integer.
byte	i32	An 8-bit unsigned integer.
short	i32	A 16-bit signed integer.
ushort	i32	A 16-bit unsigned integer.
int	i32	A 32-bit signed integer.
uint	i32	A 32-bit unsigned integer.
long	i64	A 64-bit signed integer.
ulong	i64	A 64-bit unsigned integer.
bool	i32	A 1-bit unsigned integer.
uintptr	i32 / i64	A 32-bit unsigned integer when targeting WASM32 respectively a 64-bit unsigned integer when targeting WASM64.
float	f32	A 32-bit float.
double	f64	A 64-bit float.
void	none	No return type.
Ptr<T>	i32 / i64	A pointer of type uintptr with underlying type T.

WebAssembly-specific operations are available as built-in functions that translate to the respective opcode directly:

Function	OpCode
rotl(value: int, shift: int): int	i32.rotl
rotll(value: long, shift: long): long	i64.rotl
rotr(value: int, shift: int): int	i32.rotr
rotrl(value: long, shift: long): long	i64.rotr
clz(value: int): int	i32.clz
clzl(value: long): long	i64.clz
ctz(value: int): int	i32.ctz
ctzl(value: long): long	i64.ctz
popcnt(value: int): int	i32.popcnt
popcntl(value: long): long	i64.popcnt
abs(value: double): double	f64.abs
absf(value: float): float	f32.abs
ceil(value: double): double	f64.ceil
ceilf(value: float): float	f32.ceil
floor(value: double): double	f64.floor
floorf(value: float): float	f32.floor
sqrt(value: double): double	f64.sqrt
sqrtf(value: float): float	f32.sqrt
trunc(value: double): double	f64.trunc
truncf(value: float): float	f32.trunc
nearest(value: double): double	f64.nearest
nearestf(value: float): float	f32.nearest
min(left: double, right: double): double	f64.min
minf(left: float, right: float): float	f32.min
max(left: double, right: double): double	f64.max
maxf(left: float, right: float): float	f32.max

Type coercion requires an explicit cast where precision is lost respectively is implicit where precision is maintained. For example, to cast a double to an int:

function example(value: double): int {
  return value as int; // translates to the respective opcode(s)
}

Global WebAssembly imports are declared and WebAssembly exports exported from the entry file (the file specified when calling asc or Compiler.compile). Aside from that, imports and exports work just like in TypeScript.

// entry.ts

import { myOtherExportThatDoesntBecomeAWebAssemblyExport } from "./imported";

declare function myImport(): void;

export function myExport(): void {
  myOtherExportThatDoesntBecomeAWebAssemblyExport();
}

Currently, imports can also be pulled from different namespaces by separating the namespace and the function with a $ character.

declare function Math$random(): double;

Naming a function start with no arguments and a void return type will automatically make it the start function that is being called on load even before returning to the embedder.

function start(): void {
  ...
}

Command line

The command line compiler is named asc following TypeScript's tsc.

Syntax: asc [options] [file ...]

Options:
 -o, --out, --outFile   Specifies the output file name.
 -v, --validate         Validates the module.
 -O, --optimize         Runs optimizing binaryen IR passes.
 -t, --text             Emits text format instead of a binary.

---

That's it for now. Feel free to experiment. PRs welcome!

License: Apache License, Version 2.0