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A utility library for the purescript foreign function interface

This library is primarily intended to alleviate the unique challenge of javascript to purescript translation (and vice versa). Thus it is intended only for purescript's default javascript backend, and there are currently no intentions to support alternate backends.

Accessing a module

You can access a module the same way you do in node:

import FFI.Util (require)

-- Require the fs module
foreign import data FileSystemM :: *

fs :: FileSystemM
fs = require "fs"

-- Require the buffer module
foreign import data BufferM :: *

buffer :: BufferM
buffer = require "buffer"

Since the require function returns the module object, you likely should give it a new data type, suffixed here with M so that it's clearer that this is a module object.

Calling a function

You can call functions using the call0, call1, etc, up to call7 functions. The number refers to the number of arguments for that function. For each function, you need to pass the object and method to call first.

In this example, the Buffer function in the buffer module is called in order to create a new buffer. The Buffer function only takes one parameter, in this case a String, so it calls call1:

import FFI.Util.Function (call1)

toBuffer :: String -> Buffer
toBuffer s = call1 buffer "Buffer" s

However there is an issue with this signature for toBuffer. Buffers in JS are mutable, so it should be an effectful function. You can write effectful functions using callEff:

toBuffer :: forall e. String -> Eff (err :: EXCEPTION, buffer :: BUFFER | e) Buffer
toBuffer s = callEff1 buffer "Buffer" s

Calling async functions

Calling an async function is similar to calling a regular function, except that it requires one extra step, which is to determine what the callback function returns. The async helper functions are written like callAff2r1, which means that the function takes two arguments (excluding the callback), and the callback returns one argument (excluding the error argument).

For example, for the fs.readFile function in the fs module, it takes the signature fs.readFile(file[, options], callback) and the callback is passed two arguments (err, data), where data is the contents of the file, so it should use the callAff2r1 helper function.

This implementation just passes a String for options to determine the output encoding of the file:

import FFI.Util.Function (callAff2r1)

readFile :: forall e. String -> Aff (fs :: FS | e) String
readFile file = callAff2r1 fs "readFile" file "utf8"

JS has a convention where the first argument of a callback is a potential error, and the remaining arguments are the return values. The callAff* functions assume that this is the case. In some cases the format is a little different, where either an error is never returned, or the first argument is either an error or the result type. In those cases you should use the callAff*' versions that have a suffixed tick in the name.

For callbacks that return more than one argument, the result that Aff returns is in the form of { res1 :: r1, res2 :: r2, ...} where res1 corresponds to the first argument, res2 for the second, and so on.


You can define listeners by using the functions that are in the form of listen0, listenTo0, and listenToEff0. The listenTo form takes an additional String for the listener event name, for JS callbacks like

someObject.on('eventname', function() { ... })

The listenToEff form returns the callback as an Eff function, which is convenient for defining coroutines. Coroutines are a nicer way of using listeners in purescript. See the library tests for an example on how to use them.

Optional function parameters

All of the call functions (ie. call1 or callAff2r1) can take optional arguments in the form of Maybe types. If you pass in a Maybe type as a parameter to any of these functions, these functions will automatically handle the type so that Just a returns a and Nothing returns null, similar to how toNullable works from the purescript-nullable library.


You can get properties using property and propertyPath and set properties with setProperty and setPropertyPath. There are two shortcut functions property' and propertyPath' which get a property from the global or window object (whichever is available).

Tagged sums to untagged sums

Sometimes javascript functions arguments can be more than one type, often written in APIs like String | Number. However, these sum types are untagged, and in purescript you can only have tagged sums. There are convenience functions to untag a tagged sum for the JS FFI, or to tag an untagged sum from the JS FFI. See the library tests for an example on how to use them.

Complementary libraries

There are other useful libraries for working with the FFI that complement this one, such as:

puescript-foreign, purescript-nullable, purescript-undefinable, purescript-aff, purescript-coroutines, purescript-aff-coroutines, purescript-functions, purescript-eff-functions, purescript-argonaut, purescript-generics, etc.


  • The current plan is to convert this library into a meta-package, and to have all the utilities in separate libraries
  • With purescript > v0.11 row constraints, it's now possible to rewrite some of the functions in a more robust way
  • A longer description of how to use all of the utilities, pros/cons, etc
  • Resurrecting easy-ffi