package binaryen

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OCaml bindings for Binaryen


Dune Dependency





OCaml bindings for Binaryen.

Binaryen is a compiler and toolchain infrastructure for WebAssembly. It makes compilation to WebAssembly pretty darn easy.

Here's Binaryen's hello world test in OCaml form:

open Binaryen

let wasm_mod = Module.create ()

(* Create function type for i32 (i32, i32) *)
let params = Type.create [| Type.int32; Type.int32 |]
let results = Type.int32

(* Get arguments 0 and 1, add them *)
let x = Expression.Local_get.make wasm_mod 0 Type.int32
let y = Expression.Local_get.make wasm_mod 1 Type.int32
let add = Expression.Binary.make wasm_mod Op.add_int32 x y

(* Create the add function *)
(* Note: no additional local variables *)
let adder = Function.add_function wasm_mod "adder" params results [||] add

let _ = Module.print wasm_mod
let _ = Module.dispose wasm_mod

Feature Parity

This project aims to provide full feature parity with the Binaryen C API. It's fairly complete, but a few things still need bindings:

  • SIMD instructions

  • Tags

  • Atomics

  • Query operations on expressions

  • Query operations on functions

None of these are particularly challenging to create bindings for—they just haven't been written yet. If you need anything that's missing, feel free to open a PR.

MacOS C++ Compiler

When including this library in your dune MacOS executables, you'll need to specify -cc clang++ in your (ocamlopt_flags) stanza. This is required because Binaryen will throw errors for itself to catch and using clang++ is the only way to handle them correctly. You can find more info on this ocaml issue.

Your stanza could look something like this:

  (name example)
  (public_name example)
  (package example)
+ (ocamlopt_flags -cc clang++)
  (modules example)
  (libraries binaryen))

These flags likely won't work on other operating systems, so you'll probably need to use dune-configurator to vary the flags per platform. You can see an example of this in our tests/.

Static Linking

If you are planning to create portable binaries for Windows, it will try to find Cygwin/MinGW locations in your PATH. To avoid this, you probably want to add this to your (executable) stanzas:

  (name example)
  (public_name example)
  (package example)
+ (flags (:standard -ccopt -- -ccopt -static))
  (modules example)
  (libraries binaryen))

These flags might not work on other operating systems (like MacOS), so you'll probably need to use dune-configurator to vary the flags per platform.


When compiling to JavaScript with js_of_ocaml, you'll need to add the --disable share flag. This is needed until we can depend on the bug fix from ocsigen/js_of_ocaml#1249. In the meantime, you can change your dune file:

  (name example)
  (public_name example)
   (byte js))
+ (js_of_ocaml
+  (flags --disable share))
  (modules example)
  (libraries binaryen))


You'll need Node.js and esy to build this project.

dune will take care of compiling the C stubs, so to build the project you'll only need to run:


This will take a while. Once it's done, you can run the tests:

esy test

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