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GHCJS Foreign Function Interface(FFI) enables users to

  • Interface with javascript functions
  • Emulate C dependencies in javascript so that existing haskell modules that depend on C code can be ported to GHCJS.

Haskell concurrency on GHCJS

A basic understanding of haskell concurency on GHCJS is essential for understanding foreign function interface(FFI) on GHCJS.

A javascript runtime environment has a single thread. Haskell RunTimeSystem(RTS) implements its own threads that are executed on the single javascript thread. Haskell RTS schedules thread execution to ensure smooth transition between haskell threads and javascript code.

Calling Javascript Functions from Haskell

GHCJS can import javascript functions with foreign import javascript calling convention.

Basic JavaScript FFI Syntax

The basic syntax is

foreign import javascript unsafe|safe|interruptible
  "javascript expression or javascript statement"
  functionName :: ArgumentType1 -> ArgumentType2 -> ... -> ArgumentTypeN -> ReturnType -- N >= 0

unsafe, safe, and interruptible are safety levels. The safety levels will be explained in a separate section. ArgumentType1 is represented as $1, ArgumentType2 as $2, and so on in the string part of FFI.

foreign import javascript unsafe "$1 + $2" add :: Int -> Int -> Int

If you insert a simple javascript expression such as $1 + $2 in the string part, you don't have to use $r, but if javascript statements are in the string portion, you have to assign the result to $r.

foreign import javascript unsafe "$r = $1 + $2;" add :: Int -> Int -> Int

Don't be surprised by the preceding example. Semicolon, ; turns a javascript expression into a javascript statement. You can use loops, local variables and other JavaScript constructs. Thanks to JMacro, local variable names in the import are converted to hygienic names, so you don’t need to worry about existing local variables in the code.

import GHCJS.Types (JSVal)

foreign import javascript unsafe
  "try { $r = $1 / $2; } catch (e) { $r = "error"; }"
  divide :: Double -> Double -> JSVal
import GHCJS.Types (JSVal)

foreign import javascript unsafe
   "for(var i=0;i<$3;i++) { $1[i] = $2[i]; }"
   copyArray :: JSVal -> JSVal -> Int -> IO ()

In the preceding examples, JSVal represents any javascript type. Also, the return type of an imported javascript function should be IO a if the function incurs side effects such as mutation, database I/O, file I/O, etc, ... It also needs to return IO a if the function cannot reliably return the same output for the same input.

Argument Types supported in Javascript FFI

Primitive Types

Types Size Javascript Type
Int(32-bit), Int8, Int16, Int32 1 number
Word(32-bit), Word8, Word16, Word32 1 number
Int64 2 number x number
Word64 2 number x number
Char 1 number
Bool 1 boolean
Float 1 number
Double 1 number
Ptr a, FunPtr a, StablePtr a 2 object x number
GHCJS.Types.JSVal 1 Any Javascript type

Types of size 2 are called size-2 types. A separate section will explain how to handle size-2 types.


  • newtype wrappers to the primitive types or other newtype wrappers
    • newtype JSString = JSString JSVal
    • newtype Window a b c = Window JSVal
    • newtype NewInt = NewInt Int
    • newtype NewIntA a = NewIntA Int
    • newtype NewNewInt = NewNewInt NewInt
    • ...

Type Synonyms

  • type synonyms to the preceding types or other type synonyms
    • type Bingo a = NewIntA a
    • type Bingo a b = JSString
    • type OkInt = Int
    • type OkInt2 = OkInt
    • ...

Return Types supported in Javascript FFI

  • Any argument type
    • JSString
    • Int
    • ...
  • IO AnyArgumentType
    • IO JSString
    • IO Int
    • ...
  • IO ()

Javascript FFI Safety Levels


The imported code is run directly. returning an incorrectly typed value leads to undefined behaviour. JavaScript exceptions in the foreign code kill the Haskell thread.


Returned values are replaced with a default value if they have the wrong type. JavaScript exceptions are caught and propagated as Haskell exceptions (JSException), so they can be handled with the standard Control.Exception machinery.


The import involves asynchronous callbacks. The calling Haskell thread sleeps until the foreign code calls the $c JavaScript function with the result. The thread is in interruptible state while blocked, so it can receive asynchronous exceptions.

If the callback doesn't accept an argument, the return type is IO ().

foreign import javascript interruptible "setTimeout($c, $1);"
  delay :: Int -> IO ()

If the callback receives an argument of type a, the return type is IO a.

foreign import javascript interruptible "setTimeout($c, $1, param1);"
  delayWithOneParam :: Int -> IO TypeOfParam1

If the callback receives more than one argument, the return type is IO (a, b, ...), and you have to explicitly pass arguments to $c by wrapping $c in another function as below.

import GHCJS.Types (JSString, JSVal)

foreign import javascript interruptible
  "require('fs').stat($1, function (err, stat) { $c(err, stat); });" -- node.js API
  js_fsStat :: JSString -> IO (JSVal, JSVal) -- IO (TypeOfErr, TypeOfStat)

A caveat with interruptible is that it doesn't allow a simple javascript expression in the string part. Thus, a simple function call should be followed by a semicolon, ;. A semicolon turns a javascript expression into a javascript statement.

Size-2 types in Javascript FFI

You won't need or want to use size-2 types in most cases, but it's explained here for completeness. In Javascript FFI, arguments of size-2 types are represented as $1_1, $1_2, $2_1, $2_2, ... Result of size-2 type is represented as $r1 and $r2.

For Int64 and Word64, $1_1 represents the high 32-bits of the first argument. $1_2 represents the low 32 bits of the first argument. As you can see below, it's better to not use Word64 and Int64 in Javascript FFI unless you want to split them into two parts and manipulate the parts separately.

foreign import javascript unsafe
  "$r1=$1_1; $r2=$1_2" idInt64 :: Int64 -> Int64

foreign import javascript unsafe
  "$r1=$1_1; $r2=$1_2" idWord64 :: Word64 -> Word64

The pointer types(Ptr a, StablePtr a, and FunPtr a) are used mostly to interface with GHC constructs that depend on C code. For the pointer types, $r1 is a javascript object, and $r2 is an offset within the object. Examples of dealing with the pointer types are listed below.

foreign import javascript unsafe "$r1 = $1; $r2 = 0;"
  js_mkPtr :: JSVal -> Ptr a

foreign import javascript unsafe "$r = $1_1;"
  js_ptrVal  :: Ptr a -> JSVal

Caveats on Javascript FFI

  • Int64 and Word64 can be manipulated naturally in haskell code, and Int64 and Word64 can be passed into and returned from javascript functions via Javascript FFI. However, since javascript doesn't support Int64 and Word64, they are represented as size-2 numerical types in javascript, and it'll be very very difficult to manipulate size-2 numerical types in javascript functions. Therefore, it's strongly recommended that you use Double as 53-bit integer instead of Int64 and Word64 in Javascript FFI.
  • It's ok to pass Float values into Javascript FFI, and Float can be returned from Javascript FFI. But, since javascript has only Double. Javascript FFI can return Double values exceeding the limitation of Float. Thus, it's recommended that you return Double or JSVal instead of Float in Javascript FFI.
  • You won't need to use Ptr a, FunPtr a, or StablePtr a in Javascript FFI in most cases because they exist to interface with C code.

Calling Haskell From Javascript

It is possible to call haskell from javascript. But, it is not as straightforward as calling Javascript from Haskell.

TODO : Document how to call haskell from javascript

However, it's relatively straightforward to pass a haskell callback to javascript functions. Refer to README on for details.

Right now, if you want to write javascript modules or javascript plugins in haskell, you should consider PureScript. It is similar to haskell and interfaces seamlessly with javascript.

Adding global javascript functions to Cabal Packages

This section assumes that you have a basic understanding of cabal. GHCJS FFI has access to the global javascript variables. If you want to add more javascript functions to the global scope, you can add javascript files to js-sources in .cabal files as below.

  js-sources:      jsbits/array.js

When you upload your GHCJS library or program to hackage, the javascript files in js-sources will be uploaded to hackage, too. Currently, it seems that GHCJS doesn't detect duplicate names in the global scope.

It's like c-sources for including snippets of C code in cabal packages. A CPP (C preprocessor) is used in js-sources, so if you wish, you can use header files and macros in your JS sources. The header file ghcjs/rts.h contains some useful macros that make constructing Haskell values in javascript a little easier.

/* */
#include <ghcjs/rts.h>

   convert an array to a Haskell list, wrapping each element in a
   JSVal constructor
function h$fromArray(a) {
    var r = HS_NIL;
    for(var i=a.length-1;i>=0;i--) r = MK_CONS(MK_JSVAL(a[i]), r);
    return a;

Following is the included header file ghcjs/rts.h

/* */
#ifndef __GHCJS_RTS_H_
#define __GHCJS_RTS_H_

#include "constants.h"

 * low-level heap object manipulation macros

#define MK_TUP2(x1,x2)                           (h$c2(h$ghczmprimZCGHCziTupleziZLz2cUZR_con_e,(x1),(x2),h$currentThread?h$currentThread.ccs:h$CCS_SYSTEM))
#define MK_TUP3(x1,x2,x3)                        (h$c3(h$ghczmprimZCGHCziTupleziZLz2cUz2cUZR_con_e,(x1),(x2),(x3),h$currentThread?h$currentThread.ccs:h$CCS_SYSTEM))
/* ... */

I don't know if it's a good idea to use CPP in js-sources. Use it at your discretion.

Emulating C code with foreign import ccall

This section assumes that you already learned foreign import ccall from GHC FFI. If you don't know GHC FFI already, first read

Currently, GHCJS doesn't support wrapper and dynamic properly. Don't use them until #536 is fixed.

When do we need to emulate C code? When we want to use haskell libraries or modules that depend on C libraries or c-sources in .cabal files. GHCJS emulates foreign import ccall with javascript.

Let's assume that we have an imported C function, add3.

foreign import ccall "add3" add3 :: Int -> Int

With GHCJS, add3 should be available as a javascript function in the global scope. Thus, you have to add a javascript file containing h$add3 to js-sources in the .cabal file. Beware that h$ is prefixed to C function names in javascript.

For example, add jsbits/anything.js to js-sources in .cabal file.

  js-sources: jsbits/anything.js

The content of jsbits/anything.js is

function h$add3(a) {
  return a + 3;

Types supported in foreign import ccall and specify the types supported by foreign import ccall, but I'm going to give a brief description.

With the exception that foreign import ccall doesn't support JSVal, argument types and return types supported by foreign import ccall are (probably) the same as those supported by foreign import javascript. Actually, foreign import javascript was modeled after foreign import ccall and supports every type supported by foreign import ccall and an additional type, JSVal.

Using size-2 types with foreign import ccall

foreign import ccall convention doesn't support $r1, $r2, $1_1, $1_2, and so on. Thus, size-2 types are passed as two arguments to javascript functions imported by foreign import ccall, and if a return value is of a size-2 type, the second part of the return value should be stored in the global variable, h$ret1.

Here is an example for size-2 numerical types. If you have the following function,

foreign import ccall "idInt64" idInt64 :: Int64 -> Int64

add jsbits/anything.js to js-sources in the project's .cabal file.

  js-sources: jsbits/anything.js
/* jsbits/anything.js */
function h$idInt64(highInt, lowInt) {
  h$ret1 = lowInt;
  return highInt;

Here is another example for pointer types.

foreign import ccall "idPtr" idPtr :: Ptr a -> Ptr a
/* jsbits/anything.js */
function h$idPtr(data, offset) {
  h$ret1 = offset;
  return data;