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Fay programming language

Build Status

See the Fay web site for documentation.


A proper subset of Haskell that compiles to JavaScript, Fay is a small programming language which has the following properties:

  • A proper syntactic and semantic subset of Haskell
  • Statically typed
  • Lazy
  • Pure by default
  • Compiles to JavaScript
  • Has fundamental data types (Double, String, etc.) based upon what JS can support
  • Outputs minifier-aware code for small compressed size
  • Has a trivial foreign function interface to JavaScript

Install and run

To install:

$ cabal install

To run tests from within this directory (you need nodejs installed):

$ cabal install
$ fay-tests

To generate documentation (from within this directory):

$ fay-docs

Try it out:

$ fay examples/console.hs
$ node examples/console.js
Hello, World!

Cabal-dev also works:

$ cabal-dev install
$ cabal-dev/bin/fay-tests
$ cabal-dev/bin/fay-docs

$ cabal-dev/bin/fay --no-ghc examples/console.hs
$ node examples/console.js
Hello, World!

If you only installed with cabal-dev then you will probably get a 'no package fay' error from GHC, so you can tell it where to get the package from with an environment variable:

HASKELL_PACKAGE_SANDBOX=cabal-dev/packages-7.4.1.conf cabal-dev/bin/fay examples/alert.hs


If you intend on submitting a pull request, whichever branch you choose to submit a pull request on, please ensure that it is properly rebased against master. This will ensure that the merge is clean and that you have checked that your contribution still works against the recent master. A typical workflow might be:

Have a remote setup that we can pull proper changes from:

$ git remote add fay git://

Start our topic branch:

$ git branch my-topic-branch

Hack hack hack! Once changes committed, run git pull on master and try to rebase onto it to check whether your work is out of date.

$ git commit -a -m "My topic done."
$ git fetch fay
$ git rebase fay/master -i

Install with cabal install and run fay-tests. If there are no warnings and all tests pass, move on.

If there are any conflicts, resolve them. Install and run fay-tests again. Push changes to your Github fork:

$ git push origin my-topic-branch

Make a pull request on Github for my-topic-branch. Pull request gets merged in. Pull from the official Github remote:

$ git pull haskell master

Delete your topic branch:

$ git branch -D my-topic-branch

I'm all done!

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