How to write a Haskell program

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Revision as of 10:13, 27 February 2007 by ArthurVanLeeuwen (talk | contribs)
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A guide to the best practice for creating a new Haskell project or program.

Recommended tools

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

Revision control

Use Darcs unless you have a specific reason not to. It's much more powerful than most competing systems (and it's written in Haskell).

Build system

Built with Cabal

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


For libraries, use Haddock. We recommend using the latest version of haddock (currently 0.8).


Pure code can be tested using QuickCheck or SmallCheck, impure code with HUnit.

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.


The new 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.

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
  • _darcs -- revision control
  • README -- info
  • LICENSE -- license

You can of course 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 on how you'd create a minimal darcs and cabalised Haskell project, for the cool new Haskell program "haq", build it, install it and release.

The new tool 'mkcabal' automates all this for you, but its important to understand all the parts first.

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 -
-- GPL version 2 or later (see
import System.Environment

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

haqify s = "Haq! " ++ s

Stick it in darcs

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

$ darcs init
$ darcs add Haq.hs 
$ darcs record
addfile ./Haq.hs
Shall I record this change? (1/?)  [ynWsfqadjkc], or ? for help: y
hunk ./Haq.hs 1
+-- Copyright (c) 2006 Don Stewart -
+-- GPL version 2 or later (see
+import System.Environment
+-- | 'main' runs the main program
+main :: IO ()
+main = getArgs >>= print . haqify . head
+haqify s = "Haq! " ++ s
Shall I record this change? (2/?)  [ynWsfqadjkc], or ? for help: y
What is the patch name? Import haq source
Do you want to add a long comment? [yn]n
Finished recording patch 'Import haq source'

And we can see that darcs is now running the show:

$ ls
Haq.hs _darcs

Add a build system

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

$ cat > haq.cabal
Name:                haq
Version:             0.0
Description:         Super cool mega lambdas
License:             GPL
License-file:        LICENSE
Author:              Don Stewart
Build-Depends:       base

Executable:          haq
Main-is:             Haq.hs
ghc-options:         -O

(If your package uses other packages, e.g. haskell98, you'll need to add them to the Build-Depends: field.) Add a Setup.lhs that will actually do the building:

$ cat > Setup.lhs
#! /usr/bin/env runhaskell

> import Distribution.Simple
> main = defaultMain

Cabal allows either Setup.hs or Setup.lhs, but we recommend writing the setup file this way so that it can be executed directly by Unix shells.

Now would also be a good time to add a LICENSE file and a README file. Examples are in the tarball for HNop.

Record your changes:

$ darcs add haq.cabal Setup.lhs LICENSE README
$ darcs record --all
What is the patch name? Add a build system
Do you want to add a long comment? [yn]n
Finished recording patch 'Add a build system'

Build your project

Now build it!

$ runhaskell Setup.lhs configure --prefix=$HOME
$ runhaskell Setup.lhs build
$ runhaskell Setup.lhs install

This will install your newly minted haq program in $HOME/bin.

Run it

And now you can run your cool project:

$ haq me
"Haq! me"

You can also run it in-place, even if you skip the install phase:

$ dist/build/haq/haq you
"Haq! you"

Build some haddock documentation

Generate some API documentation into dist/doc/*

$ runhaskell Setup.lhs haddock

which generates files in dist/doc/ including:

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

 main :: IO ()


 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.

Add some automated testing: QuickCheck

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 Char
import 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.


Running the test suite from darcs

We can arrange for darcs to run the test suite on every commit:

$ darcs setpref test "runhaskell Tests.hs"
Changing value of test from '' to 'runhaskell Tests.hs'

will run the full set of QuickChecks. (If your test requires it you may need to ensure other things are built too eg: darcs setpref test "alex Tokens.x;happy Grammar.y;runhaskell Tests.hs").

Let's commit a new patch:

$ darcs add Tests.hs
$ darcs record --all
What is the patch name? Add testsuite
Do you want to add a long comment? [yn]n
Running test...
reverse.reverse/id       : OK, passed 100 tests.
drop.haq/id              : OK, passed 100 tests.
Test ran successfully.
Looks like a good patch.
Finished recording patch 'Add testsuite'

Excellent, now patches must pass the test suite before they can be committed.

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

Tag the stable version:

$ darcs tag
What is the version name? 0.0
Finished tagging patch 'TAG 0.0'

Tarballs via Cabal

Since the code is cabalised, we can create a tarball with Cabal directly:

$ runhaskell Setup.lhs 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 expected by HackageDB. 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.

Tarballs via darcs

Alternatively, you can use darcs:

$ darcs dist -d haq-0.0
Created dist as haq-0.0.tar.gz

And you're all set up!

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.


The following files were created:

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


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
   Build-Depends:       base
   Exposed-modules:     Data.LTree
   ghc-options:         -Wall -O

We can thus build our library:

   $ runhaskell Setup.lhs configure --prefix=$HOME
   $ runhaskell Setup.lhs 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:

   $ runhaskell Setup.lhs 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! You can 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 it is useful to have source trees stored 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

Cabal can set up to also run configure scripts, along with a range of other features. For more information consult the Cabal documentation.


A tool to automatically populate a new cabal project is available (beta!):

   darcs get

Usage is:

$ mkcabal
Project name: haq
What license ["GPL","LGPL","BSD3","BSD4","PublicDomain","AllRightsReserved"] ["BSD3"]: 
What kind of project [Executable,Library] [Executable]: 
Is this your name? - "Don Stewart " [Y/n]: 
Is this your email address? - "<>" [Y/n]: 
Created Setup.lhs and haq.cabal
$ ls
Haq.hs    LICENSE   Setup.lhs _darcs    dist      haq.cabal

which will fill out some stub Cabal files for the project 'haq'.

To create an entirely new project tree:

$ mkcabal --init-project
Project name: haq
What license ["GPL","LGPL","BSD3","BSD4","PublicDomain","AllRightsReserved"] ["BSD3"]: 
What kind of project [Executable,Library] [Executable]: 
Is this your name? - "Don Stewart " [Y/n]: 
Is this your email address? - "<>" [Y/n]: 
Created new project directory: haq
$ cd haq
$ ls
Haq.hs    LICENSE   README    Setup.lhs haq.cabal


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 thing.


It's important to release your code as stable, tagged tarballs. Don't just rely on darcs for distribution.

  • darcs dist generates tarballs directly from a darcs repository

For example:

$ cd fps
$ ls       
Data      LICENSE   README    Setup.hs  TODO      _darcs    cbits dist      fps.cabal tests
$ darcs dist -d fps-0.8
Created dist as fps-0.8.tar.gz

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

You can also have darcs do the equivalent of 'daily snapshots' for you by using a post-hook.

put the following in _darcs/prefs/defaults:

 apply posthook darcs dist
 apply run-posthook


  • Tag each release using darcs tag. For example:
$ darcs tag 0.8
Finished tagging patch 'TAG 0.8'

Then people can darcs pull --partial -t 0.8, to get just the tagged version (and not the entire history).


A Darcs repository can be published simply by making it available from a web page. If you don't have an account online, or prefer not to do this yourself, source can be hosted on (you will need to email Simon Marlow to do this). itself has some user accounts available.

There are also many free hosting places for open source, such as

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 keep in mind the user's expectations for how the library is to be built and used.

Introductory information and build guide

The intended approach for a user of a library is roughly:

  1. Visit
  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;
    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. Even if steps 1..2 are successful, and ensuring step 5 is the main concern of code authors and users, it is often steps 3..4 that 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 with a README file, distributed with the library or application, and often accompanied with similar text on a more extensive web page.


Generated haddock documentation is not enough, usually, for new programmers to learn how to use a library. It is critical to also provide accompanying examples, and even tutorials on the use of the library.

Please consider providing example, type correct code, with commentary, on the use of your library (or application).

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.


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 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 (Haskell's CPAN wanna-be).

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


A complete example of writing, packaging and releasing a new Haskell library under this process has been documented.