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Build Status

A compiler to generate JavaScript code from Haskell.

It even has a website and a mailing list.


  • Seamless, type-safe single program framework for client-server communication
  • Support for modern web technologies such as WebSockets, WebStorage and Canvas
  • Simple JavaScript interoperability
  • Generates small, fast programs
  • Supports all GHC extensions except Template Haskell
  • Uses standard Haskell libraries
  • Cabal integration
  • Simple, one-step build; no need for error prone Rube Goldberg machines of Vagrant, VirtualBox, GHC sources and other black magic
  • Concurrency and MVars with Haste.Concurrent
  • Unboxed arrays, ByteArrays, StableNames and other low level features
  • Low-level DOM base library
  • Easy integration with Google's Closure compiler
  • Works on Windows, GNU/Linux and Mac OS X


You have three options for getting Haste: installing from Hackage, from Github or from one of the pre-built binary packages. In the first two cases, you need to add add Cabal's bin directory, usually ~/.cabal/bin, to your $PATH if you haven't already done so. When installing from the Mac, portable Windows or generic Linux package, you may want to add path/to/haste-compiler/bin to your $PATH. The Debian package as well as the Windows installer and the optional install script included in the generic Linux package take care of this automatically.

Or, you can install the latest stable version from Hackage:

$ cabal install haste-compiler
$ haste-boot

Building from Github source is equally easy. After checking out the source, cd to the source tree and run:

$ cabal install
$ haste-boot --force --local

Alternatively, you may also build from Github source using Stack:

$ stack install
$ haste-boot --force --local

See doc/building.md for more information about build requirements and procedures for the various platforms.

If you are having problems with the haste-cabal installed by haste-boot, you can try building it from scratch and then passing the --no-haste-cabal flag to haste-boot:

$ git clone https://github.com/valderman/cabal.git
$ cd cabal && git checkout haste-cabal
$ cd Cabal && cabal install
$ cd ../cabal-install && cabal install

When installing Haste from GitHub, you should probably run the test suite first, to verify that everything is working. To do that, execute ./runtests.sh in the Haste root directory. You may also run only a particular test by executing ./runtests.sh NameOfTest. The test suite uses the nodejs interpreter by default, but this may be modified by setting the JS environment variable as such: JS=other-js-interpreter ./runtests.sh. Other JavaScript interpreters may or may not work. runtests.sh isn’t downloaded when installing from Hackage. You would have to download it from GitHub.

To build the patched Closure compiler used when compiling using --opt-minify, get the Closure source, apply patches/closure-argument-removal.patch and build it as you normally would. This is not usually necessary however, as haste-boot fetches a pre-compiled Closure binary when run.

For more detailed build instructions, see doc/building.md.

Haste has been tested to work on Windows and OSX platforms, but is primarily developed on GNU/Linux. As such, running on a GNU/Linux platform will likely get you less bugs.


To compile your Haskell program to a JavaScript blob ready to be included in an HTML document or run using a command line interpreter:

$ hastec myprog.hs

This is equivalent to calling ghc --make myprog.hs; Main.main will be called as soon as the JS blob has finished loading.

You can pass the same flags to hastec as you'd normally pass to GHC:

$ hastec -O2 -fglasgow-exts myprog.hs

Haste also has its own set of command line arguments. Invoke it with --help to read more about them. In particular --opt-all, --opt-minify, --start and --with-js should be fairly interesting.

If you want your package to compile with both Haste and, say, GHC, you might want to use the CPP extension for conditional compilation. Haste defines the preprocessor symbol __HASTE__ in all modules it compiles. This symbol may also be used to differentiate between Haste versions, since it is defined as an integer representation of the current Haste version. Its format is MAJOR*10 000 + MINOR*100 + MICRO. Version 1.2.3 would thus be represented as 10203, and 0.4.3 as 403.

Haste also comes with wrappers for cabal and ghc-pkg, named haste-cabal and haste-pkg respectively. You can use them to install packages just as you would with vanilla GHC and cabal:

$ haste-cabal install mtl

Finally, you can interact with JavaScript code using the Haste.Foreign module in the bundled haste-lib library. See doc/js-externals.txt for more information about that. This library also contains all sorts of functionality for DOM manipulation, event handling, preemptive multitasking, canvas graphics, native JS string manipulation, etc.

For more information on how Haste works, see the Haste Report, though beware that parts of Haste may have changed quite a bit.

You should also have a look at the documentation and/or source code for haste-lib, which resides in the libraries/haste-lib directory, and the small programs in the examples directory, to get started.

Interfacing with JavaScript

When writing programs you will probably want to use some native JavaScript in your program; bindings to native libraries, for instance. The preferred way of doing this is the Haste.Foreign module:

{-# LANGUAGE OverloadedStrings #-}
import Haste.Foreign

addTwo :: Int -> Int -> IO Int
addTwo = ffi "(function(x, y) {return x + y;})"

The ffi function is a little bit safer than the GHC FFI in that it enforces some type invariants on values returned from JS, and is more convenient. Performance-wise, it is roughly as fast as the GHC FFI except for complex types (lists, records, etc.) where it is an order of magnitude faster.

If you do not feel comfortable throwing out your entire legacy JavaScript code base, you can export selected functions from your Haste program and call them from JavaScript:


{-# LANGUAGE OverloadedStrings #-}
import Haste.Foreign
import Haste.Prim (toJSStr)

fun :: Int -> String -> IO String
fun n s = return $ "The number is " ++ show n ++ " and the string is " ++ s

main = do
  export "fun" fun


function mymain() {
  console.log(Haste.fun(42, "hello"));

...then compile with:

$ hastec '--start=$HASTE_MAIN(); mymain();' --with-js=legacy.js fun.hs

fun.hs will export the function fun when its main function is run. Our JavaScript obviously needs to run after that, so we create our "real" main function in legacy.js. Finally, we tell the compiler to start the program by first executing Haste's main function (the $HASTE_MAIN gets replaced by whatever name the compiler chooses for the Haste main) and then executing our own mymain.

The mechanics of Haste.Foreign are described in detail in this paper.

Effortless type-safe client-server communication

Using the framework from the Haste.App module hierarchy, you can easily write web applications that communicate with a server without having to write a single line of AJAX/WebSockets/whatever. Best of all: it's completely type safe.

In essence, you write your web application as a single program - no more forced separation of your client and server code. You then compile your program once using Haste and once using GHC, and the two compilers will magically generate client and server code respectively.

You will need to have the same libraries installed with both Haste and vanilla GHC (unless you use conditional compilation to get around this). haste-compiler comes bundled with all of haste-lib, so you only need to concern yourself with this if you're using third party libraries. You will also need a web server, to serve your HTML and JS files; the binary generated by the native compilation pass only communicates with the client part using WebSockets and does not serve any files on its own.

Examples of Haste.App in action is available in examples/haste-app and examples/chatbox.

For more information about how exactly this works, see this paper.

Base library and documentation

You can build your own set of docs for haste-lib by running cabal haddock in the Haste base directory as with any other package.

Or you could just look at the online docs.


Haste is able to use standard Haskell libraries. However, some primitive operations are still not implemented which means that any code making use of them will give you a compiler warning, then die at runtime with an angry error. Some libraries also depend on external C code - if you wish to use such a library, you will need to port the C bits to JavaScript yourself (perhaps using Emscripten) and link them into your program using --with-js.

Known issues

  • Not all GHC primops are implemented; if you encounter an unimplemented primop, please report it together with a small test case that demonstrates the problem.

  • Template Haskell is still broken.

  • Generated code is not compatible with the vanilla Closure compiler's ADVANCED_OPTIMIZATIONS, as it is not guaranteed to preserve Function.length. haste-boot bundles a compatibility patched version of Closure which does preserve this property. Invoking hastec with the --opt-minify option will use this patched version to minify the generated code with advanced optimizations.