15. Using the GHC WebAssembly backend

15.1. What does the WebAssembly “backend” mean

In order to compile Haskell to wasm, you need a custom GHC build that targets wasm. There isn’t a GHC option which allows you to use a stock GHC installed via ghcup or stack to generate wasm. That’s because GHC is still a single-target compiler, so each GHC build is only capable of compiling code that runs on a single architecture and operating system.

So, the term GHC wasm backend isn’t the same sense as the unregisterised/LLVM/NCG backends. It merely describes GHC’s support as a cross compiler that targets wasm, and more specifically, wasm32-wasi.

The generated wasm module makes use of a few post-MVP extensions that are supported by default in latest releases of Chrome/Firefox/Safari/wasmtime. The wasm module uses WASI as the system call layer, so it’s supported by any wasm engine that implements WASI (including browsers, which can provide the WASI layer via JavaScript).

15.2. Setting up the GHC wasm backend

The wasm backend is still a tech preview and not included in the official bindists yet. If you are using x86_64-linux, you can follow the “getting started” subsections in ghc-wasm-meta to quickly set up the GHC wasm backend using the nightly artifacts.

It’s also possible to build the GHC wasm backend manually, if your host system is one of {x86_64,aarch64}-{linux,darwin}. Refer to the ghc-wasm-meta readme for detailed instructions.

15.4. Running the GHC wasm backend’s output

Once you have a wasm module, you can run it with a dedicated wasm engine like wasmtime, or inside the browsers.

To run it with wasmtime, you can simply do:

$ wasmtime run foo.wasm

Just like native executables, you can pass command line arguments, and also RTS options, as long as it’s built with -rtsopts:

$ wasmtime run foo.wasm --bar +RTS --nonmoving-gc -RTS

You can also mount some host directory into it:

$ wasmtime run --mapdir /::$PWD foo.wasm

As long as the filesystem capability is provided, in addition to filesystem I/O in Haskell code, you can use the RTS eventlog and profiling functionality, then inspect the report files:

$ wasmtime run --mapdir /::$PWD foo.wasm +RTS -hc -l -RTS

15.5. JavaScript FFI in the wasm backend

The GHC wasm backend supports the JavaScript FFI feature. For Haskell projects that are meant to be run in JavaScript environments like browsers or nodejs, the JavaScript FFI enables:

  • Calling JavaScript from Haskell via foreign imports and vice versa via foreign exports.
  • Representing JavaScript values as first-class garbage collected Haskell values on the Haskell heap.
  • Blocking for asynchronous JavaScript computation in a single Haskell thread without blocking the entire runtime.
  • Not paying for JavaScript when not using it, the same toolchain still generates self-contained wasm32-wasi modules by default.

The JavaScript FFI as implemented in the GHC wasm backend is pioneered by the asterius project and heavily inspired by GHCJS, the predecessor of the GHC JavaScript backend. Despite some similarities, it still differs from the GHC JavaScript backend’s implementation significantly. The rest of this guide is a canonical reference for the GHC wasm backend’s JavaScript FFI, which we’ll now abbreviate as JSFFI.

15.5.1. Marshalable types and JSVal

JSFFI supports all boxed marshalable foreign types in C FFI:

  • Bool
  • Char
  • Int / Word
  • Int8 / Int16 / Int32 / Int64
  • Word8 / Word16 / Word32 / Word64
  • Ptr / FunPtr / StablePtr
  • Float / Double

The above types and their newtypes can be used as argument/result types in JSFFI. Some caveats to keep in mind:

  • Bool is marshaled to 0 / 1 instead of false / true in JavaScript. This is affected by implementation details of JSFFI, which is layered on top of C FFI and shares some characteristics of C FFI. It should be fine in most cases, since implicit conversion to boolean happens when it’s used as a boolean. It’s also fine to pass a JavaScript boolean into Haskell, since it’ll be implicitly converted to a number first.
  • Likewise, Char is marshaled to 32-bit integer that represents its Unicode code point. Do not pass a single character JavaScript string as Char, since implicit conversion to number results in NaN! If you absolutely need to use Char as a JSFFI argument/result type, you’re in charge of handling Chars as code points. Most likely you only need to marshal between Haskell String or Text and JavaScript strings, for which there already exist conversion functions.
  • 64-bit integer types are marshaled to JavaScript bigints. In JavaScript, mixing bigint and regular numbers in arithmetic results in type errors, so keep this in mind. As for Int / Word, they are 32-bit since the GHC wasm backend is based on wasm32 .
  • JSFFI doesn’t support unboxed foreign types like Int#, ByteArray#, etc, even when UnliftedFFITypes is enabled.

In addition to the above types, JSFFI supports the JSVal type and its newtypes as argument/result types. JSVal is defined in GHC.Wasm.Prim in ghc-experimental, which represents an opaque reference to a JavaScript value.

JSVals are first-class Haskell values on the Haskell heap. You can get them via foreign import results or foreign export arguments, store them in Haskell data structures and pass them between Haskell/JavaScript. They are garbage-collected by the GHC RTS:

  • There can be multiple JSVals that point to the same JavaScript value. As long as there’s at least one JSVal still alive on the Haskell heap, that JavaScript value will still be alive on the JavaScript heap.
  • If there’s no longer any live JSVal that points to the JavaScript value, then after Haskell garbage collection, the runtime no longer retain any reference to it, allowing the JavaScript runtime to eventually garbage collect it as well.

In addition to garbage collection, GHC.Wasm.Prim also exports freeJSVal :: JSVal -> IO (), allowing the user to drop the JavaScript reference from the runtime eagerly. You’re encouraged to make use of freeJSVal when you’re sure about a JSVal’s lifetime, especially for the temporary JSVals. This will help reducing the memory footprint at runtime.

Note that freeJSVal is not idempotent and it’s only safe to call it exactly once or not at all. Once it’s called, any subsequent usage of that JSVal results in a runtime panic.

15.5.2. Foreign imports

One can embed a JavaScript code snippet in a foreign import declaration and call that piece of JavaScript code by calling the foreign import function:

import GHC.Wasm.Prim

foreign import javascript unsafe "console.log($1)"
  js_print :: JSString -> IO ()

foreign import javascript unsafe "typeof $1 === 'object'"
  js_is_obj :: JSVal -> Bool

foreign import javascript unsafe "let acc = 1; for (let i = 1; i <= $1; ++i) acc *= i; return acc;"
  js_fac :: Word -> Word

A JSFFI import code snippet can be either a single JavaScript expression or a series of JavaScript statements as function body, in which case you can use return to return the import result value. The import code snippet has access to:

  • The import argument values, bound to arguments $1, $2, etc.
  • The __export binding, which contain all wasm module exports. For instance, you could use __exports.memory to access the WebAssembly.Memory object and use it to copy blobs between the Haskell/JavaScript side. The memory export exists by default.
  • The full Web API that exists in the JavaScript global scope.

There are two kinds of JSFFI imports: synchronous/asynchronous imports. unsafe indicates synchronous imports, which has the following caveats:

  • The calling thread as well as the entire runtime blocks on waiting for the import result.
  • If the JavaScript code throws, the runtime crashes with the same error. A JavaScript exception cannot be handled as a Haskell exception here, so you need to use a JavaScript catch explicitly shall the need arise.
  • Like unsafe C imports, re-entrance is not supported, the imported foreign code must not call into Haskell again. Doing so would result in a runtime panic.

When a JSFFI import is marked as safe / interruptible or lacks safety annotation, then it’s treated as an asynchronous import. The asynchronous JSFFI imports combine the Haskell concurrency model and the JavaScript event loop, allowing Haskell code to work with async JavaScript computation without blocking the entire runtime.

import Control.Exception

foreign import javascript safe "new Promise(res => setTimeout(res, $1))"
  js_sleep :: Int -> IO ()

sleep :: Int -> IO ()
sleep t = evaluate =<< js_sleep t

foreign import javascript safe "const r = await fetch($1); return r.text();"
  js_fetch :: JSString -> IO JSString

Asynchronous import code is wrapped in async JavaScript functions, therefore await is also supported. Async JavaScript functions always return Promises, and you can also explicitly create and return a Promise that resolves to the final result of the async computation.

When an asynchronous JSFFI import is called, the Haskell function returns immediately once the async JavaScript function returns a Promise. The value returned by the Haskell function is a thunk. When the thunk is evaluated later, the evaluating thread is suspended by the runtime, and resumed when the Promise actually resolves or rejects.

Compared to synchronous JSFFI imports, asynchronous JSFFI imports have the following notable pros/cons:

  • Waiting for the result only blocks a single Haskell thread, other threads can still make progress and garbage collection may still happen.
  • If the Promise rejects, Haskell code can catch JavaScript errors as JSExceptions.
  • Re-entrance is supported. The JavaScript code may call into Haskell again and vice versa.
  • Of course, it has higher overhead than synchronous JSFFI imports.

Using thunks to encapsulate Promise result allows cheaper concurrency without even needing to fork Haskell threads just for waiting for a bunch of async calls to return. Just like lazy I/O, the convenience comes with caveat, you need to take some care to force the result thunk before closing the underlying resource. And even if the result type is (), it’s still a thunk that needs to be explicitly forced to ensure the Promise has actually resolved, so you likely need to write a worker/wrapper function pair for cases like sleep.

There’s also a special kind of JSFFI import that allow converting a callable JSVal to a Haskell function:

type Logger = JSString -> IO ()

type JSFunction = JSVal

foreign import javascript unsafe "s => console.log(s)"
  js_logger :: JSFunction

foreign import javascript unsafe "dynamic"
  js_logger_to_hs :: JSFunction -> Logger

Much like foreign import ccall "dynamic" which wraps a C function pointer as a Haskell function, foreign import javascript "dynamic" wraps a JSVal that represent a JavaScript function as a Haskell function. The returned Haskell function retains the reference to that JSVal, and the unsafe / safe annotation indicates whether that JavaScript function is synchronous or asynchronous.

Of course, without foreign import javascript "dynamic", one could still easily implement similar functionality:

foreign import javascript unsafe "$1($2)"
  js_logger_to_hs :: JSFunction -> JSString -> IO ()

And that’s how it’s implemented under the hood. It’s handled as a JSFFI import with an auto-generated code snippet that calls the first argument, passing the rest of arguments.

15.5.3. Foreign exports

One can use foreign export javascript to export a top-level Haskell binding as a wasm module export which can be called in JavaScript:

foreign export javascript "my_fib"
  fib :: Word -> Word

Give fib :: Word -> Word, the above declaration exports fib as my_fib. It is a wasm module export function without any JavaScript wrapper, and as long as the wasm instance is properly initialized, you can call await instance.exports.my_fib(10) to invoke the exported Haskell function and get the result.

Unlike JSFFI imports which have synchronous/asynchronous flavors, JSFFI exports are always asynchronous. Calling them always return a Promise in JavaScript that needs to be awaited for the real result. If the Haskell function throws, the Promise is rejected with a WebAssembly.RuntimeError, and the message field contains a JavaScript string of the Haskell exception.

Above is the static flavor of JSFFI exports. It’s also possible to export a dynamically created Haskell function closure as a JavaScript function and obtain its JSVal:

type BinOp a = a -> a -> a

foreign import javascript "wrapper"
  js_func_from_hs :: BinOp Int -> IO JSVal

This is also much like foreign import ccall "wrapper", which wraps a Haskell function closure as a C function pointer. Note that unsafe / safe annotation is ignored here, since the JSVal that represent the exported function is always returned synchronously, but it is always an asynchronous JavaScript function, just like static JSFFI exports.

The JSVal callbacks created by dynamic JSFFI exports can be passed to the rest of JavaScript world to be invoked later. But wait, didn’t we say earlier that JSVals are garbage collected? Isn’t a use-after-free trap waiting ahead of the road, when the JSVal is collected in Haskell but the JavaScript callback is invoked later?

So, normal JSVals created by JSFFI import results or JSFFI export arguments only manage a single kind of resource: the JavaScript value it refers to. But JSVals created by dynamic JSFFI exports manage two kinds of resources: the JavaScript callback it refers to, as well as a stable pointer that retains the Haskell function closure. If this JSVal is garbage collected, the Haskell runtime no longer retains the JavaScript callback, but the JavaScript side may still hold that callback and intends to call it later, so the Haskell function closure is still retained by default.

Still, the runtime can gradually drop these retainers by using FinalizerRegistry to invoke the finalizers to free the underlying stable pointers once the JavaScript callbacks are recycled.

One last corner case is cyclic reference between the two heaps: if a JavaScript callback is retained only by JSVal and that JSVal is retained only by a Haskell function closure that gets exported, this creates a cyclic reference that can’t be automatically recycled. This is a fundamental limit of the GHC wasm backend today since the Haskell heap lives in the linear memory, distinct from the host JavaScript heap, and coordination between two heaps is always a non-trivial challenge. However, one can still use freeJSVal to break the cycle. When freeJSVal is applied to a JSVal that represents a JavaScript callback created by a dynamic JSFFI export, both kinds of resources are freed at once: the JavaScript callback, as well as the Haskell function closure.

15.5.4. Detect whether JSFFI is being used

If you’re writing a Haskell library, you may want to detect whether the final linked module involves JSFFI logic, or is it still a self-contained wasm32-wasi module. One obvious reason is wasi implementations in JavaScript environments are often incomplete and lack certain features (e.g. the poll_oneoff syscall), so it makes sense to dispatch code.

GHC.Wasm.Prim exports isJSFFIUsed :: Bool that can be used for this purpose. As long as there’s a single JSFFI import/export or anything involving JSVal linked into the final wasm module, it will be True. It is False if and only if the user code has absolutely no transitive dependency on anything related to JSFFI, in which case the linked wasm module will be a self-contained wasm32-wasi module. If something compiles fine with the GHC wasm backend before JSFFI feature is merged, but isJSFFIUsed is still True, then it’s definitely a bug.

15.5.5. Interaction with async exception

When a thread is blocked waiting for an async JSFFI import call to return, it can be interrupted by a Haskell async exception, with some caveats:

  • The async exception would not magically cancel the Promise. In general, JavaScript Promises aren’t cancelable anyway. There do exist third-party Promise libraries to provide a “cancel” interface, which only guarantees all .then() continuations registered on that Promise will no longer be invoked. For simplicity of implementation, we aren’t using those for the time being.
  • Normally, throwTo would block until the async exception has been delivered. In the case of JSFFI, throwTo would always return successfully immediately, while the target thread is still left in a suspended state. The target thread will only be waken up when the Promise actually resolves or rejects, though the Promise result will be discarded at that point.

The current way async exceptions are handled in JSFFI is subject to change though. Ideally, once the exception is delivered, the target thread can be waken up immediately and continue execution, and the pending Promise will drop reference to that thread and no longer invoke any continuations.

15.5.6. Interaction with C FFI

User code can take advantage of JSFFI and C FFI together, and make use of third party C/C++ code as long as they work on wasm32-wasi. However, there is an important limitation to keep in mind when it comes to the interaction between JSFFI and C FFI:

A Haskell thread cannot force an async JSFFI import thunk when it represents a Haskell function exported via C FFI. Doing so would throw WouldBlockException.

For example, suppose we’re using the Haskell bindings of a certain C library, and some of the C functions expect callers to pass a C function pointer as the callback argument. Yes, we can use foreign import ccall "wrapper" to wrap a Haskell function closure and pass it to that C function. The wrapped Haskell function can even call sync JSFFI imports, but it cannot call an async JSFFI import and block on the result.

The other direction is fine. Regardless of whether a C import is unsafe or safe, it can be called in a Haskell thread that represents a Haskell function exported to JavaScript. Do keep in mind that we’re using the single-threaded runtime at the moment, so other than supporting re-entrancy, safe C calls don’t offer extra advantage than unsafe.

As mentioned before, JSFFI is layered on top of C FFI under the hood, and they share the same C symbol namespace. In most cases, the JSFFI related symbols are auto-generated so they can’t collide, but for each foreign export javascript "my_func", there will be a my_func externally visible C symbol, so you need to take a bit of care not to duplicate symbol with the C side.

15.5.7. The JavaScript API

When linking a wasm module that makes use of JSFFI, correct link-time arguments must be passed to GHC and this needs to be adjusted on a per-project basis:

ghc -no-hs-main -optl-mexec-model=reactor -optl-Wl,--export=my_func


Consider the case ghc hello.hs where hello.hs is just a good old main = putStrLn "hello world". By default, ghc exports main as a C function, compiles and links a little C stub file that contain the actual main that initializes the runtime and call the Haskell main, and clang would link everything as a wasm32-wasi command module.

Conceptually, a wasm32-wasi command module is like an executable, with a single entry point and meant to be invoked only once and then exits. But certain link-time arguments can tell clang to target a wasm32-wasi reactor module instead. A reactor module is a bit like a shared library: it has internal functions as well as user-defined entry points, and once it’s initialized, the entry points can be called as many times as the user wants.

Given the nature of JSFFI, if a project uses JSFFI, then it surely is meant to target a wasm32-wasi reactor module. And there’s no sensible default entry point, not even main, you need to explicitly pass the export names you need via link-time arguments, otherwise those exports will be absent from the resulting wasm module due to linker dead code elimination.

Now, suppose a wasm module has already been linked and it makes use of JSFFI. This wasm module will contain ghc_wasm_jsffi custom sections, and the section payloads include information like JSFFI import code snippets and function arities. The next step is calling a small post-linker script, which will parse the wasm module and emit a JavaScript module. The post-linker is written in nodejs, though the resulting JavaScript module has nothing nodejs specific and work in browsers as well.

$ $(wasm32-wasi-ghc --print-libdir)/post-link.mjs -i hello.wasm -o hello.js

The generated JavaScript module contains a default export which is a function. The function takes an __exports argument and generates the ghc_wasm_jsffi wasm imports. There is knot-tying going on here: only after a wasm module is instantiated, you can access its exports, but the ghc_wasm_jsffi imports required for instantiation need access to the exports! JavaScript is not a lazy language, but we can achieve knot-tying less elegantly by using mutation:

let __exports = {};

const { instance } = await WebAssembly.instantiateStreaming(
    ghc_wasm_jsffi: (await import(js_url)).default(__exports),
    wasi_snapshot_preview1: ...

Object.assign(__exports, wasm_instance.exports);

This way, the ghc_wasm_jsffi imports will have access to all exports of the wasm instance.

After the wasm instance is created, initialization needs to be done:


The wasm32-wasi reactor module ABI defines the _initialize export function, which is auto generated at link time and it must be called exactly once before any other wasm exports can be called. The correct way to call it depends on the wasi implementation provider in JavaScript.

Finally, in JavaScript, you can use await __exports.my_func() to call your exported my_func function and get its result, pass arguments, do error handling, etc etc.