How to write a Haskell program

A developers' guide to creating a new Haskell project or program, and working in the Haskell developer ecosystem.

Note: for learning the Haskell language itself we recommend these resources.

Recommended tools

Almost all new Haskell projects use the following tools. Each is intrinsically useful, but using a set of common tools also helps everyone by increasing productivity, and you're more likely to get patches.

Revision control

Use git or darcs unless you have a specific reason not to. Both are lightweight distributed revision control system. Darcs is written in Haskell. They are the two most popular DVCSes in the Haskell world. If you want to encourage contributions from other Haskell hackers then git is best. For git, github is very popular. Darcs hosting is available on hub.darcs.net.

Build system

Built with Cabal

Use Cabal. You should read at least the start of section 2 of the Cabal User's Guide.

You should use cabal-install as a front-end for installing your Cabal library. Cabal-install provides commands not only for building libraries but also for installing them from, and uploading them to, Hackage. As a bonus, for almost all programs, it's faster than using Setup.hs scripts directly, since no time is wasted compiling the scripts. (This does not apply for programs that use custom Setup.hs scripts, since those need to be compiled even when using cabal-install.)

cabal-install is widely available, as a binary distribution or as part of the Haskell Platform, so you can probably assume your users will have it too.

Documentation

For libraries, use Haddock. Haddock generates nice markup, with links to source.

Testing

Typical unit/spec based testing, particularly with impure code, can be done with HSpec and HUnit.

You can use QuickCheck or SmallCheck to test pure code. These libraries work best when you have known invariants in your code's behavior. See this Cabal file for an example of how to include tests in your Cabal package.

To get started, try Introduction to QuickCheck. For a slightly more advanced introduction, Simple Unit Testing in Haskell is a blog article about creating a testing framework for QuickCheck using some Template Haskell. For HUnit, see HUnit 1.0 User's Guide

Distribution

The standard mechanism for distributing Haskell libraries and applications is Hackage. Hackage can host your cabalised tarball releases, and link to any library dependencies your code has. Users will find and install your packages via "cabal install", and your package will be integrated into Haskell search engines, like hoogle

Target Environment

If possible, depend on libraries that are provided by the current stable Stackage package versions or the Haskell Platform. Ideally your package should be able to build with users that keep current with Stackage or users of the Platform.

Structure of a simple project

The basic structure of a new Haskell project can be adopted from HNop, the minimal Haskell project. It consists of the following files, for the mythical project "haq".

• Haq.hs -- the main haskell source file
• haq.cabal -- the cabal build description
• Setup.hs -- build script itself
• .git -- revision control
• README -- info
• LICENSE -- license

Of course, you can elaborate on this, with subdirectories and multiple modules. See Structure of a Haskell project for an example of a larger project's directory structure.

Here is a transcript that shows how you'd create a minimal git and cabalised Haskell project for the cool new Haskell program "haq", build it, install it and release.

Note: The new tool "cabal init" automates all this for you, but you should understand all the parts even so.

We will now walk through the creation of the infrastructure for a simple Haskell executable. Advice for libraries follows after.

Create a directory

Create somewhere for the source:

$ mkdir haq
$ cd haq

Write some Haskell source

Write your program:

$ cat > Haq.hs
--
-- Copyright (c) 2006 Don Stewart - http://www.cse.unsw.edu.au/~dons/
-- GPL version 2 or later (see http://www.gnu.org/copyleft/gpl.html)
--
import System.Environment

-- | 'main' runs the main program
main :: IO ()
main = getArgs >>= print . haqify . head

haqify s = "Haq! " ++ s

Stick it in version control

Place the source under revision control (you may need to enter your e-mail address first, to identify you as maintainer of this source):

$ git init
$ git add Haq.hs 
$ git commit -am "my commit message"

Add a build system

Create a .cabal file describing how to build your project:

$ cabal init

that will ask a few questions about your project and generate a file similar to the example. -- Initial scratch.cabal generated by cabal init. For further -- documentation, see http://haskell.org/cabal/users-guide/

name: haq version: 0.1.0.0 description: Super cool mega lambdas license: GPL license-file: LICENSE author: Don Stewart maintainer: dons@cse.unsw.edu.au build-type: Simple cabal-version: >=1.10

executable haq

 main-is:           Haq.hs 
 build-depends:     base >=4.5 && <4.8
 default-language:  Haskell2010

(If your package uses other packages, e.g. text, you'll need to add them to the build-depends: field as a comma separated list.)

Cabal will also generate a Setup.hs file that will do the actual building. You will rarely need to modify it.

If you specifed a known license, it will also add a LICENSE file.

You might like to add a README file to tell what your project is about.

Record your changes:

$ git add haq.cabal Setup.hs LICENSE README
$ git commit -am "Add a build system"

Build your project

Now build it! There are two methods of accessing Cabal functionality: through your Setup.hs script or through cabal-install. cabal-install is now the preferred method.

Building using cabal-install and sandboxes, always use sandboxes in your projects unless you are an expert and know what you are doing! Demonstrated here:

$ cabal sandbox init
$ cabal install -j

This will install your newly minted haq program in $PROJECT_DIR/.cabal-sandbox/bin.

Run it

And now you can run your cool project:

$ .cabal-sandbox/bin/haq me
"Haq! me"

Since our program doesn't rely on any other assets we can just copy it to a directory in our path if we want to be able to use it from anywhere without referencing the sandbox directory. Like so:

$ sudo cp .cabal-sandbox/bin/haq /usr/local/bin/haq
$ haq me
"Haq! me"

Build some haddock documentation

Generate some API documentation into dist/doc/*

Using cabal install:

$ cabal haddock

Using cabal install if you planned to upload your haq package to Hackage: $ cabal haddock --hyperlink-source --html-location='http://hackage.haskell.org/package/haq/docs' --contents-location='http://hackage.haskell.org/package/haq'

which generates files in dist/doc/ including:

$ w3m -dump dist/doc/html/haq/Main.html

haq Contents Index
Main

Synopsis
main :: IO ()

Documentation

main :: IO ()
main runs the main program

Produced by Haddock version 0.7

No output? Make sure you have actually installed haddock. It is a separate program, not something that comes with Cabal. Note that the stylized comment in the source gets picked up by Haddock.

(Optional) Improve your code: HLint

HLint can be a valuable tool for improving your coding style, particularly if you're new to Haskell. Let's run it now.

$ hlint .
./Haq.hs:11:1: Warning: Eta reduce
Found:
  haqify s = "Haq! " ++ s
Why not:
  haqify = ("Haq! " ++)

The existing code will work, but let's follow that suggestion. Open Haq.hs in your favourite editor and change the line:

    where haqify s = "Haq! " ++ s

to:

    where haqify = ("Haq! " ++)

Add some automated testing: QuickCheck

QuickCheck v1

We'll use QuickCheck to specify a simple property of our Haq.hs code. Create a tests module, Tests.hs, with some QuickCheck boilerplate:

$ cat > Tests.hs
import Data.Char
import Data.List
import Test.QuickCheck
import Text.Printf

main  = mapM_ (\(s,a) -> printf "%-25s: " s >> a) tests

instance Arbitrary Char where
    arbitrary     = choose ('\0', '\128')
    coarbitrary c = variant (ord c `rem` 4)

Now let's write a simple property:

$ cat >> Tests.hs 
-- reversing twice a finite list, is the same as identity
prop_reversereverse s = (reverse . reverse) s == id s
    where _ = s :: [Int]

-- and add this to the tests list
tests  = [("reverse.reverse/id", test prop_reversereverse)]

We can now run this test, and have QuickCheck generate the test data:

$ runhaskell Tests.hs
reverse.reverse/id       : OK, passed 100 tests.

Let's add a test for the 'haqify' function:

-- Dropping the "Haq! " string is the same as identity
prop_haq s = drop (length "Haq! ") (haqify s) == id s
    where haqify s = "Haq! " ++ s

tests  = [("reverse.reverse/id", test prop_reversereverse)
        ,("drop.haq/id",        test prop_haq)]

and let's test that:

$ runhaskell Tests.hs
reverse.reverse/id       : OK, passed 100 tests.
drop.haq/id              : OK, passed 100 tests.

Great!

QuickCheck v2

If you're using version 2 of QuickCheck, the code in the previous section needs some minor modifications:

$ cat > Tests.hs
import Data.Char
import Data.List
import Test.QuickCheck
import Text.Printf
 
main  = mapM_ (\(s,a) -> printf "%-25s: " s >> a) tests

-- reversing twice a finite list, is the same as identity
prop_reversereverse s = (reverse . reverse) s == id s
    where _ = s :: [Int]

-- Dropping the "Haq! " string is the same as identity
prop_haq s = drop (length "Haq! ") (haqify s) == id s
    where haqify s = "Haq! " ++ s
 
tests  = [("reverse.reverse/id", quickCheck prop_reversereverse)
        ,("drop.haq/id",        quickCheck prop_haq)]

To run the test:

$ runhaskell Tests.hs
reverse.reverse/id       : +++ OK, passed 100 tests.
drop.haq/id              : +++ OK, passed 100 tests.

Success!

Tag the stable version, create a tarball, and sell it!

Tag the stable version:

$ git tag 0.0

Create a tarball

You can do this using either Cabal or an explicit tar command.

Using Cabal

Since the code is cabalised, we can create a tarball with cabal-install directly (you can also use runhaskell Setup.hs sdist, but you need tar on your system [1]):

$ cabal sdist
Building source dist for haq-0.0...
Source tarball created: dist/haq-0.0.tar.gz

This has the advantage that Cabal will do a bit more checking, and ensure that the tarball has the structure that HackageDB expects. Note that it does require the LICENSE file to exist. It packages up the files needed to build the project; to include other files (such as Test.hs in the above example, and our README), we need to add:

extra-source-files: Tests.hs README

to the .cabal file to have everything included.

Check that your source package is complete

Just to make sure everything works, try building the source package in some temporary directory:

$ tar xzf haq-0.0.tar.gz
$ cd haq-0.0
$ cabal configure
$ cabal build

and for packages containing libraries,

$ cabal haddock

Upload your package to Hackage

Whichever of the above methods you've used to create your package, you can upload it to the Hackage package collection via a web interface. You may wish to use the package checking interface there first, and fix things it warns about, before uploading your package.

= Upload your package documentation to Hackage

Assuming you built your documentation like this:

$ cabal haddock --hyperlink-source --html-location='http://hackage.haskell.org/package/haq/docs' --contents-location='http://hackage.haskell.org/package/haq'

You can force upload (so you don't have to wait on the server to build it for you) your documentation for your package like so:

$ cp -R ./dist/doc/html/haq/ haq-0.0-docs $ tar cvzf --format=ustar -f haq-0.0-docs.tar.gz haq-0.0-docs $ curl -X PUT -H 'Content-Type: application/x-tar' -H 'Content-Encoding: gzip' --data-binary '@haq-0.0-docs.tar.gz' 'https://$USERNAME:$PASSWORD@hackage.haskell.org/package/haq-0.0/docs'

Summary

The following files were created:

   $ ls
   Haq.hs           Tests.hs         dist             haq.cabal
   Setup.hs         .git             haq-0.0.tar.gz

Libraries

The process for creating a Haskell library is almost identical. The differences are as follows, for the hypothetical "ltree" library:

Hierarchical source

The source should live under a directory path that fits into the existing module layout guide. So we would create the following directory structure, for the module Data.LTree:

   $ mkdir Data
   $ cat > Data/LTree.hs 
   module Data.LTree where

So our Data.LTree module lives in Data/LTree.hs

The Cabal file

Cabal files for libraries list the publically visible modules, and have no executable section:

   $ cat > ltree.cabal 
   Name:                ltree
   Version:             0.1
   Description:         Lambda tree implementation
   License:             BSD3
   License-file:        LICENSE
   Author:              Don Stewart
   Maintainer:          dons@cse.unsw.edu.au
   Build-Type:          Simple
   Cabal-Version:       >=1.2
   
   Library
     Build-Depends:     base >= 3 && < 5
     Exposed-modules:   Data.LTree
     ghc-options:       -Wall

We can thus build our library:

   $ cabal configure --prefix=$HOME --user
   $ cabal build    
   Preprocessing library ltree-0.1...
   Building ltree-0.1...
   [1 of 1] Compiling Data.LTree       ( Data/LTree.hs, dist/build/Data/LTree.o )
   /usr/bin/ar: creating dist/build/libHSltree-0.1.a

and our library has been created as a object archive. Now install it:

   $ cabal install
   Installing: /home/dons/lib/ltree-0.1/ghc-6.6 & /home/dons/bin ltree-0.1...
   Registering ltree-0.1...
   Reading package info from ".installed-pkg-config" ... done.
   Saving old package config file... done.
   Writing new package config file... done.

And we're done! To try it out, first make sure that your working directory is anything but the source directory of your library:

   $ cd ..

And then use your new library from, for example, ghci:

   $ ghci -package ltree
   Prelude> :m + Data.LTree
   Prelude Data.LTree> 

The new library is in scope, and ready to go.

More complex build systems

For larger projects, you may want to store source trees in subdirectories. This can be done simply by creating a directory -- for example, "src" -- into which you will put your src tree.

To have Cabal find this code, you add the following line to your Cabal file:

   hs-source-dirs: src

You can also set up Cabal to run configure scripts, among other features. For more information consult the Cabal user guide.

Licenses

Code for the common base library package must be BSD licensed. Otherwise, it is entirely up to you as the author. Choose a licence (inspired by this). Check the licences of things you use (both other Haskell packages and C libraries), since these may impose conditions you must follow. Use the same licence as related projects, where possible. The Haskell community is split into 2 camps, roughly: those who release everything under BSD, and (L)GPLers. Some Haskellers recommend avoiding LGPL, due to cross-module optimisation issues. Like many licensing questions, this advice is controversial. Several Haskell projects (wxHaskell, HaXml, etc) use the LGPL with an extra permissive clause which gets round the cross-module optimisation problem.

Releases

It's important to release your code as stable, tagged tarballs. Don't just rely on your commit hashes for tracking history, tag your released versions and milestones!

• git archive generates tarballs directly from a git repository based on the provided tag.

For example:

$ cd fps
$ ls       
Data      LICENSE   README    Setup.hs  TODO      .git    cbits dist      fps.cabal tests
$ git tag v0.8
$ git archive --format=tar.gz v0.8 > fps-0.8.tar.gz

You can now just post your fps-0.8.tar.gz

• Tag each release using git tag. For example:

Hosting

Hosting for repos is available from Github and the Haskell community server

There is also a (minimal) Github equivalent for Darcs at hub.darcs.net.

Web page

Create a web page documenting your project! An easy way to do this is to add a project specific page to the Haskell wiki

The user experience

When developing a new Haskell library, it is important to remember how the user expects to be able to build and use a library.

Introductory information and build guide

A typical library user expects to:

1. Visit Haskell.org
2. Find the library/program they are looking for:
   1. if not found, try mailing list;
   2. if it is hidden, try improving the documentation on haskell.org;
   3. if it does not exist, try contributing code and documentation)
3. Download
4. Build and install
5. Enjoy

Each of these steps can pose potential road blocks, and code authors can do a lot to help code users avoid such blocks. Steps 1..2 may be easy enough, and many coders and users are mainly concerned with step 5. Steps 3..4 are the ones that often get in the way. In particular, the following questions should have clear answers:

• Which is the latest version?
• What state is it in?
• What are its aims?
• Where is the documentation?
• Which is the right version for given OS and Haskell implementation?
• How is it packaged, and what tools are needed to get and unpack it?
• How is it installed, and what tools are needed to install it?
• How do we handle dependencies?
• How do we provide/acquire the knowledge and tool-chains needed?

The best place to answer these questions is a README file, distributed with the library or application, and often accompanied with similar text on a more extensive web page.

Tutorials

Generated haddock documentation is usually not enough to help new programmers learn how to use a library. You must also provide accompanying examples, and even tutorials about the library.

Please consider providing example code for your library or application. The code should be type-correct and well-commented.

Program structure

Monad transformers are very useful for programming in the large, encapsulating state, and controlling side effects. To learn more about this approach, try Monad Transformers Step by Step.

Publicity

The best code in the world is meaningless if nobody knows about it. The process to follow once you've tagged and released your code is:

Join the community

If you haven't already, join the community. The best way to do this is to subscribe to at least haskell-cafe@ and haskell@ mailing lists. Joining the #haskell IRC channel is also an excellent idea.

Announce your project on haskell@

Most important: announce your project releases to the haskell@haskell.org mailing list. Tag your email subject line with "ANNOUNCE: ...". This ensure it will then make it into the Haskell Weekly News. To be doubly sure, you can email the release text to the HWN editor.

Add your code to the public collections

• Add your library or application to the Libraries and tools page, under the relevant category, so people can find it.

• If your release is a Cabal package, add it to the Hackage database

• If your library stays up to date with its dependencies, add it to the vetted packages on Stackage.

Blog about it

Blog about it! Blog about your new code on Planet Haskell. Write about your project in your blog, then email the Planet Haskell maintainer (ibid on #haskell) the RSS feed url for your blog