View source for Xmonad/xmonad development tutorial

{{xmonad}}
[[Category:XMonad]]

So, you've got an idea for a great extension to xmonad, and you think
other people will enjoy using it, too.  The only problem is, you've
never written an xmonad extension before, and
you're not sure what the proper procedures are.  Well, never fear ---
this tutorial will have you writing xmonad extensions in no time!

As a running example, let's suppose you want to develop an extension
to pop up a "Hello world" message when the user presses a certain key combination.  We'll walk through the entire process of creating,
testing, and submitting such an extension.  I encourage you to follow along, either by copying and pasting the code shown here, or you can just use it as a guideline for making your own extension.

Before we begin, here are some other resources which may be useful to
you during this process:

* [https://github.com/bitemyapp/learnhaskell See here for recommended resources for learning Haskell.]
* [http://haskell.org/haskellwiki/Xmonad/Guided_tour_of_the_xmonad_source Guided tour of the xmonad source code].
* Automatically-generated [http://www.xmonad.org/xmonad-docs/ documentation for xmonad and xmonad-contrib].
* [http://xmonad.org/xmonad-docs/xmonad-contrib/XMonad-Doc-Developing.html#8 xmonad coding style guidelines].
* [http://www.haskell.org/haddock/doc/html/index.html Haddock documentation].

If you have any questions or run into any problems while following this tutorial, feel free to ask on the [http://www.haskell.org/mailman/listinfo/xmonad xmonad mailing list], or on the <code>#xmonad</code> channel on irc.freenode.net.

== Getting started ==

=== Getting the latest sources ===

The first step in creating an xmonad extension is to get the latest
development sources from github.  If you
already have the xmonad sources, you can skip this section.

First, create an account on [https://github.com github] if you don't have one already.  Then visit the [https://github.com/xmonad/xmonad xmonad repository on github] and click the "Fork" button to clone it into your personal account.  Do the same for the [https://github.com/xmonad/xmonad-contrib xmonad-contrib repository].
 
Next, create a local directory where you will store the xmonad repositories.

 [brent@xenophon:~/haskell]$ mkdir xmonad-dev-tut
 [brent@xenophon:~/haskell]$ cd xmonad-dev-tut/
 [brent@xenophon:~/haskell/xmonad-dev-tut]$ 

Now, use the <code>git</code> command to clone the forked repos locally:

 [brent@xenophon:~/haskell/xmonad-dev-tut]$ git clone git@github.com:byorgey/xmonad-contrib.git
 Cloning into 'xmonad-contrib'...
 remote: Counting objects: 13578, done.
 remote: Total 13578 (delta 0), reused 0 (delta 0), pack-reused 13578
 Receiving objects: 100% (13578/13578), 9.21 MiB | 2.00 MiB/s, done.
 Resolving deltas: 100% (9879/9879), done.
 Checking connectivity... done.
 [brent@xenophon:~/haskell/xmonad-dev-tut]$ git clone git@github.com:byorgey/xmonad.git
 ...
 [brent@xenophon:~/haskell/xmonad-dev-tut]$ ls
 xmonad  xmonad-contrib

(Of course, you should replace <code>byorgey</code> with your own username.)
Congratulations, you now have the latest xmonad sources!

=== Building ===

To build the latest development versions of <code>xmonad</code> and <code>xmonad-contrib</code>, navigate to the parent directory and issue the command:

 cabal install xmonad/ xmonad-contrib/

Now restart xmonad (run <code>touch ~/.xmonad/xmonad.hs</code>, then hit mod-q).  Note, if you are upgrading from a previous version of xmonad, you may need to first unregister the old packages with ghc.  Type <code>ghc-pkg list xmonad</code> and <code>ghc-pkg list xmonad-contrib</code> at a prompt; if multiple versions are listed you need to unregister all but the newest.  For example:

 [brent@xenophon:~/haskell/xmonad-dev-tut]$ ghc-pkg unregister xmonad-0.5
 Saving old package config file... done.
 Writing new package config file... done.
 [brent@xenophon:~/haskell/xmonad-dev-tut]$ ghc-pkg unregister xmonad-contrib-0.5
 Saving old package config file... done.
 Writing new package config file... done.

=== Important note on rebuilding ===

If you pull new patches into the xmonad repository or make changes to it, and rebuild the xmonad library, you should do a clean build of the xmonad-contrib library afterwards (<code>cabal clean && cabal install</code>).  Otherwise, the Cabal build process for xmonad-contrib may not notice that the xmonad library (which xmonad-contrib depends on) has changed, and xmonad-contrib will not get rebuilt properly, leading to possible crashes at runtime.  Annoying, but necessary.

== Creating an extension ==

=== Adding the module file ===

So, let's add that module!  We'll call it <code>XMonad.Actions.HelloWorld</code>, so it needs to go in the <code>XMonad/Actions/</code> subdirectory of the <code>xmonad-contrib</code> repository:

 [brent@xenophon:~/haskell/xmonad-dev-tut/xmonad-contrib]$ $EDITOR XMonad/Actions/HelloWorld.hs &

To avoid flame wars, I won't reveal my favorite editor here. =)  

Now we've got... a nice, blank module!  What now?  Well, let's begin by adding some standard header stuff.  In practice you can just open up some other module and copy and paste, changing the stuff that's appropriate to change.

<haskell>
-----------------------------------------------------------------------------
-- |
-- Module      :  XMonad.Actions.HelloWorld
-- Copyright   :  (c) 2008 Brent Yorgey
-- License     :  BSD3-style (see LICENSE)
--
-- Maintainer  :  Brent Yorgey <byorgey@gmail.com>
-- Stability   :  unstable
-- Portability :  unportable
--
-- Provides an action to pop up a \"hello, world\" window using xmessage.
--
-----------------------------------------------------------------------------

module XMonad.Actions.HelloWorld where
</haskell>

OK, that's a good start.  Now let's go to a prompt and see what we've done so far:

 [brent@xenophon:~/haskell/xmonad-dev-tut/xmonad-contrib]$ git status
 On branch master
 Your branch is up-to-date with 'origin/master'.
 Untracked files:
   (use "git add <file>..." to include in what will be committed)
 
         HelloWorld.hs
 
 nothing added to commit but untracked files present (use "git add" to track)

<code>HelloWorld.hs</code> is so far not being tracked by <code>git</code>; we can follow the given suggestion to add it:

 [brent@xenophon:~/haskell/xmonad-dev-tut/XMonadContrib]$ git add XMonad/Actions/HelloWorld.hs 

To finish things off for now, we add the <hask>helloWorld</hask> function, and a required import (<hask>XMonad.Core</hask> is where we get the <hask>spawn</hask> function).

<haskell>
import XMonad.Core

helloWorld :: X ()
helloWorld = spawn "xmessage 'Hello, world!'"
</haskell>

=== Adding our new module to the xmonad-contrib library ===

Our new module is great, but it won't get compiled as part of the xmonad-contrib library unless we add a reference to it in <code>xmonad-contrib.cabal</code>.

 [brent@xenophon:~/haskell/xmonad-dev-tut/xmonad-contrib]$ $EDITOR xmonad-contrib.cabal
 [brent@xenophon:~/haskell/xmonad-dev-tut/xmonad-contrib]$ git diff
 index 490cec2..743f6d4 100644
 --- a/XMonad/Actions/HelloWorld.hs
 +++ b/XMonad/Actions/HelloWorld.hs
 @@ -13,3 +13,8 @@
  -----------------------------------------------------------------------------
  
  module XMonad.Actions.HelloWorld where
 +
 +import           XMonad.Core
 +
 +helloWorld :: X ()
 +helloWorld = spawn "xmessage 'Hello, world!'"
 diff --git a/xmonad-contrib.cabal b/xmonad-contrib.cabal
 index 12721ce..4dc9fda 100644
 --- a/xmonad-contrib.cabal
 +++ b/xmonad-contrib.cabal
 @@ -109,6 +109,7 @@ library
                          XMonad.Actions.FocusNth
                          XMonad.Actions.GridSelect
                          XMonad.Actions.GroupNavigation
 +                        XMonad.Actions.HelloWorld
                          XMonad.Actions.Launcher
                          XMonad.Actions.LinkWorkspaces
                          XMonad.Actions.MessageFeedback

The + lines indicate additions that we have made. Note in particular the line we added to the .cabal file. (The module list is generally kept in alphabetical order.) Be sure to indent with spaces, not tabs!  Since indentation is important in .cabal files (just as in Python or Haskell), tabs are not allowed.

=== Building and testing ===

We first rebuild and reinstall the xmonad-contrib library.  

 [brent@xenophon:~/haskell/xmonad-dev-tut]$ cabal install xmonad-contrib/
 Configuring xmonad-contrib-0.9.1...
 Preprocessing library xmonad-contrib-0.9.1...
 Building xmonad-contrib-0.9.1...
 [ 91 of 112] Compiling XMonad.Actions.HelloWorld ( XMonad/Actions/HelloWorld.hs, dist/build/XMonad/Actions/HelloWorld.o )
 /usr/bin/ar: creating dist/build/libHSxmonad-contrib-0.9.1.a
 ...

Now, to test out our new module, we can just import it into xmonad.hs, add a keybinding for the <hask>helloWorld</hask> action, and restart with mod-q. Now (assuming you have the xmessage utility installed) hitting the selected keybinding should pop up a little "Hello, world" window in the upper-left corner of the screen.  Neat!

=== Clean-up and coding standards ===

Well, our module works, but it's in no state to be distributed to the wider world yet!  We'll have to clean it up to conform to a few standards for xmonad-contrib source code.  In particular:

* All exported functions should have Haddock documentation and explicit type signatures.  It's a good idea to give documentation and explicit type signatures for unexported functions, too, to make it easier for others to understand or modify your code.
* A "Usage" section should be added in the comments, explaining the purpose of the module and giving examples of its use.
* It's usual to explicitly list the module's exports, rather than implicitly exporting everything.

There are [http://xmonad.org/xmonad-docs/xmonad-contrib/XMonad-Doc-Developing.html#8 other standards, too], but these are the most relevant to us at the moment. 

Here's our module after making these updates (not including the header comments):

<haskell>
module XMonad.Actions.HelloWorld (
                                  -- * Usage
                                  -- $usage

                                  helloWorld

                                 ) where

import XMonad.Core

-- $usage
-- You can use this module with the following in your @~\/.xmonad\/xmonad.hs@:
--
-- > import XMonad.Actions.HelloWorld
--
-- Then add a keybinding for 'helloWorld':
--
-- >   , ((modMask x .|. controlMask, xK_h), helloWorld)
--
-- For detailed instructions on editing your key bindings, see
-- "XMonad.Doc.Extending#Editing_key_bindings".

-- | Pop up a \"hello, world\" window using @xmessage@.  You /must/ have
--   the @xmessage@ utility installed for this to work.
helloWorld :: X ()
helloWorld = spawn "xmessage 'Hello, world!'"
</haskell>

Note how we added a type signature and Haddock documentation for <hask>helloWorld</hask> (the pipe character | indicates a Haddock comment), and some usage information which will get included in the generated Haddock documentation (Haddock generates documentation for exported functions in the order they are listed in the export list between parentheses following the module declaration, along with any extra comments included there).  Some quick notes about Haddock documentation format:

* Use @ symbols to surround code which should be printed in a <code>verbatim style</code>.
* Use 'single quotes' around function names to generate links to their documentation.  For example, see how 'helloWorld' has single quotes in the usage information shown above.
* Use "double quotes" around module names to generate links to their documentation.
* Use /front slashes/ to italicize.
* Escape literal quotes and frontslashes with a backslash.
* Literal code blocks can be included with "bird tracks", i.e. greater-than symbols preceding the code.  Be sure to put a blank line on either side of such code blocks.

For more information, see the [http://www.haskell.org/haddock/doc/html/index.html Haddock documentation].

=== Testing documentation ===

It's really important to test the generated documentation before submitting our new extension.  A module with poor or missing documentation (or great documentation which is not included because of parse errors) will not be very useful to people.  Conversely, a module with excellent documentation and good examples will be a pleasure to use, easier to modify and extend, and a help to others trying to use the code as an example for creating their own extension!

To generate the documentation, first rebuild the xmonad-contrib library, then use <code>cabal haddock</code>:

 [brent@xenophon:~/haskell/xmonad-dev-tut/xmonad-contrib]$ cabal haddock
 Preprocessing library xmonad-contrib-0.9.1...
 Running Haddock for xmonad-contrib-0.9.1...
 ...lots of warnings that can be ignored...
 Documentation created: dist/doc/html/xmonad-contrib/index.html

Generating the Haddock documentation will generate a ton of warnings such as "could not find link destinations for: M.Map"; you can ignore these.  The important line is the last one, which says whether the documentation was successfully created, and where it was generated.

In this case, to view the generated documentation, you should point your browser to something like <code>file:///path/to/XMonadContrib/dist/doc/html/xmonad-contrib/index.html</code>, then navigate to the documentation for your module. Check that the formatting, links, and so on are correct.  For example, when first writing this tutorial, I forgot to escape the double quotes around "hello world" in the comments for the <hask>helloWorld</hask> function, which therefore showed up as a link to the (nonexistent) "hello world" module, instead of showing literal double quote characters.

If you make any changes to your documentation, you can simply rerun <code>cabal haddock</code> to regenerate; there's no need to rebuild everything as long as you only changed documentation. 

Once you are satisfied with your documentation, it's probably a good idea to rebuild the xmonad-contrib library, restart xmonad, and test your extension one more time.

====Source links====
On newer versions of Cabal, install the hscolour package, and build your docs using the following command to include links to source from each function, type, etc.

<code>cabal haddock --hyperlink-source</code>

====Dialing in documentation for a single module====

While it's no huge chore to build all the Haddock documentation, and you should do that before submitting your extension, sometimes you just want to build documentation for one or a few modules that aren't necessarily even finished yet. The following bash alias is a quick and dirty way to build html documentation in a draft document directory for the files specified on the command line.

<code>
alias "hddck."='tmp=~/tmp/haddock; <nowiki>[[</nowiki> -d $tmp/html ]] || mkdir $tmp/html; base=`pwd`; haddock --html --source-module file://'$base'/%F --odir $tmp/html '
</code>

== Submitting your extension ==

=== Recording your commits ===

The first step is to record our changes as one or more git commits.  First, let's see what we've changed:

 [brent@xenophon:~/haskell/xmonad-dev-tut/xmonad-contrib]$ git status
 On branch master
 Your branch is up-to-date with 'origin/master'.
 Changes to be committed:
   (use "git reset HEAD <file>..." to unstage)
 
         new file:   HelloWorld.hs
 
 Changes not staged for commit:
   (use "git add <file>..." to update what will be committed)
   (use "git checkout -- <file>..." to discard changes in working directory)
 
         modified:   HelloWorld.hs
         modified:   ../../xmonad-contrib.cabal

To commit everything, type something like <code>git commit -a -m 'new HelloWorld module'</code> at a prompt.  Then push your changes up to your forked repository on github with <code>git push -u origin master</code> (the <code>-u</code> is only necessary the very first time you push).

=== Making a pull request ===

Now open your browser and navigate to your personal fork of the repository on github.  Click the "Pull Request" button, fill out the resulting form with some details, and click "submit"!  Hopefully someone will be able to take a look at it soon and give you some feedback (though be patient --- xmonad development can be somewhat glacial these days!).

== Postscript ==

Questions? Suggestions? Confusions? Edit this tutorial yourself (it's a wiki, after all!), send an email to the [http://www.haskell.org/mailman/listinfo/xmonad xmonad mailing list], or join the <code>#xmonad</code> channel on irc.freenode.net.  

For further reading, check out the [http://haskell.org/haskellwiki/Xmonad/Guided_tour_of_the_xmonad_source guided tour of the xmonad source code], or browse the [http://www.xmonad.org/xmonad-docs/ xmonad and xmonad-contrib documentation].