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JavaScript FFI

There is a prototype implementation of foreign import javascript right now, check the jsffi/teletype unit tests for details. The syntax is like:

import Asterius.Types

foreign import javascript "new Date()" current_time :: IO JSVal

foreign import javascript "console.log(${1})" js_print :: JSVal -> IO ()

The source text of foreign import javascript should be a valid JavaScript expression. You can use ${n} to refer to the nth function parameter starting from 1. By using the IIFE(Immediately Invoked Function Expression) design pattern, it’s even possible to define local variables and write for loops.

Supported basic types are:

  • Ptr
  • FunPtr
  • StablePtr
  • Bool
  • Int
  • Word
  • Char
  • Float
  • Double
  • JSVal/JSArrayBuffer/JSString/JSArray

For the lifted basic types, the result can be wrapped in IO (or not). There’s also limited support for unlifted FFI types:

  • StablePtr# a
  • Addr#
  • ByteArray#
  • MutableByteArray# s
  • Char#
  • Int#
  • Word#
  • Float#
  • Double#

These unlifted FFI types can be used in a foreign import javascript clause (but not export.) The results can’t be wrapped in IO.

JSVal is defined in Asterius.Types in the patched ghc-prim package. In the Haskell land, JSVal is first-class and opaque: you can pass it around, put it in a data structure, etc, but under the hood it’s just a handle. The runtime maintains mappings from handles to real JavaScript objects.

Normally, the Haskell FFI mechanism permits defining newtypes to the marshallable basic types, and the wrapping/unwrapping is done automatically. However, this doesn’t work yet due to the way we implement JSFFI right now. You can define a newtype to JSVal/JSWhatever, but in a foreign import javascript/foreign export javascript declaration, you still must use one of the builtin JS* types, and when using imported functions, you need to manually coerce them (coerce works when Asterius.Types is imported).

Also, a prototype of foreign export javascript is implemented, check jsffi for details. The syntax is roughly:

foreign export javascript "mult_hs" (*) :: Int -> Int -> Int

In a Haskell module, one can specify the exported function name (must be globally unique), along with its Haskell identifier and type. One can specify ahc-link --export-function=mult_hs to make the linker include the relevant bits in final WebAssembly binary, and export mult_hs as a regular WebAssembly export function. After calling hs_init to initialize the runtime, one can call mult_hs just like a regular JavaScript function.

Converting between Haskell and JavaScript types

The Asterius.Types/Asterius.ByteString modules provide some high-level functions for converting between Haskell and JavaScript types:

fromJSString :: JSString -> [Char]
toJSString :: [Char] -> JSString

fromJSArray :: JSArray -> [JSVal]
toJSArray :: [JSVal] -> JSArray

byteStringFromJSArrayBuffer :: JSArrayBuffer -> ByteString
byteStringToJSArrayBuffer :: ByteString -> JSArrayBuffer

It’s possible to define them just by using the basic JSFFI mechanism, but those functions are backed by special runtime interfaces which makes them a lot faster. Most notably, the fromJS* functions directly traverse the JavaScript value and build a fully-evaluated Haskell data structure on the heap in one pass.

What’s permitted in foreign import javascript

In a foreign import javascript declaration, you can access all properties of the global object (window in browsers, global in node.js), so all functionalities of standard JavaScript is permitted.

Additionally, the __asterius_jsffi object is in scope; it is initialized before instantiating the WebAssembly instance, and contains the runtime interfaces used to support the JSFFI features (e.g. manipulation of JSVals). You may check rts/rts.js to see what __asterius_jsffi contains, but we don’t recommend using it in your code since it’s intended to be an implementation detail; shall you feel the need to access it, please file an issue instead and we’ll add your missing functionality as proper Haskell/JavaScript interfaces instead.


This subsection presents a high-level overview on the implementation of JSFFI, based on the information flow from syntactic sugar to generated WebAssembly/JavaScript code. It’s not a required reading for users of the JSFFI feature.

Syntactic sugar

As documented in previous sections, one can write foreign import javascript or foreign export javascript clauses in a .hs module. How are they processed? The logic resides in Asterius.JSFFI.

First, there is addFFIProcessor, which given a Compiler (defined in ghc-toolkit), returns a new Compiler and a callback to fetch a stub module. The details of Compiler’s implementation are not relevant here, just think of it as an abstraction layer to fetch/modify GHC IRs without dealing with all the details of GHC API.

addFFIProcessor adds one functionality to the input Compiler: rewrite parsed Haskell AST and handle the foreign import javascript/foreign export javascript syntactic sugar. After rewriting, JavaScript FFI is really turned into C FFI, so type-checking/code generation proceeds as normal.

After the parsed AST is processed, a “stub module” of type AsteriusModule is generated and can be later fetched given an AsteriusModuleSymbol. It contains JSFFI related information of type FFIMarshalState. Both AsteriusModule and FFIMarshalState types has Semigroup instance so they can be combined later at link-time.


Adding a JSFFI basic type

Look at the following places:

  • Asterius.JSFFI module. All JavaScript reference types are uniformly handled as FFI_JSVAL, while value types are treated as FFI_VAL. Assuming we are adding a value type. Add logic to:

    • marshalToFFIValueType: Recognize the value type in parsed AST, and translate to FFI_VAL
  • Asterius.Builtins module. Add the corresponding rts_mkXX/rts_getXX builtin functions. They are required for stub functions of foreign export javascript.