Portability | portable |
---|---|
Stability | unstable |
Maintainer | andrea.rossato@unibz.it |
This module gives a brief overview of the xmonad internals. It is intended for advanced users who are curious about the xmonad source code and want an brief overview. This document may also be helpful for the beginner/intermediate Haskell programmer who is motivated to write an xmonad extension as a way to deepen her understanding of this powerful functional language; however, there is not space here to go into much detail. For a more comprehensive document covering some of the same material in more depth, see the guided tour of the xmonad source on the xmonad wiki: http://haskell.org/haskellwiki/Xmonad/Guided_tour_of_the_xmonad_source.
If you write an extension module and think it may be useful for others, consider releasing it. Coding guidelines and licensing policies are covered at the end of this document, and must be followed if you want your code to be included in the official repositories. For a basic tutorial on the nuts and bolts of developing a new extension for xmonad, see the tutorial on the wiki: http://haskell.org/haskellwiki/Xmonad/xmonad_development_tutorial.
Writing new extensions
Libraries for writing window managers
Starting with version 0.5, xmonad and xmonad-contrib are packaged and distributed as libraries, instead of components which must be compiled by the user into a binary (as they were prior to version 0.5). This way of distributing xmonad has many advantages, since it allows packaging by GNU/Linux distributions while still allowing the user to customize the window manager to fit her needs.
Basically, xmonad and the xmonad-contrib libraries let users write
their own window manager in just a few lines of code. While
~/.xmonad/xmonad.hs
at first seems to be simply a configuration
file, it is actually a complete Haskell program which uses the xmonad
and xmonad-contrib libraries to create a custom window manager.
This makes it possible not only to edit the default xmonad configuration, as we have seen in the XMonad.Doc.Extending document, but to use the Haskell programming language to extend the window manager you are writing in any way you see fit.
xmonad internals
The main
entry point
xmonad installs a binary, xmonad
, which must be executed by the
Xsession starting script. This binary, whose code can be read in
Main.hs
of the xmonad source tree, will use XMonad.Core.recompile
to run ghc
in order to build a binary from ~/.xmonad/xmonad.hs
.
If this compilation process fails, for any reason, a default main
entry point will be used, which calls the XMonad.Main.xmonad
function with a default configuration.
Thus, the real main
entry point, the one that even the users' custom
window manager application in ~/.xmonad/xmonad.hs
must call, is
the XMonad.Main.xmonad
function. This function takes a configuration
as its only argument, whose type (XMonad.Core.XConfig
)
is defined in XMonad.Core.
XMonad.Main.xmonad
takes care of opening the connection with the X
server, initializing the state (or deserializing it when restarted)
and the configuration, and calling the event handler
(XMonad.Main.handle
) that goes into an infinite loop (using
Prelude.forever
) waiting for events and acting accordingly.
The X monad and the internal state
The event loop which calls XMonad.Main.handle
to react to events is
run within the XMonad.Core.X
monad, which is a
Control.Monad.State.StateT
transformer over IO
, encapsulated
within a Control.Monad.Reader.ReaderT
transformer. The
Control.Monad.State.StateT
transformer encapsulates the
(read/writable) state of the window manager (of type
XMonad.Core.XState
), whereas the Control.Monad.Reader.ReaderT
transformer encapsulates the (read-only) configuration (of type
XMonad.Core.XConf
).
Thanks to GHC's newtype deriving feature, the instance of the
Control.Monad.State.MonadState
class parametrized over
XMonad.Core.XState
and the instance of the
Control.Monad.Reader.MonadReader
class parametrized over
XMonad.Core.XConf
are automatically derived for the XMonad.Core.X
monad. This way we can use Control.Monad.State.get
,
Control.Monad.State.gets
and Control.Monad.State.modify
for the
XMonad.Core.XState
, and Control.Monad.Reader.ask
and
Control.Monad.Reader.asks
for reading the XMonad.Core.XConf
.
XMonad.Core.XState
is where all the sensitive information about
window management is stored. The most important field of the
XMonad.Core.XState
is the XMonad.Core.windowset
, whose type
(XMonad.Core.WindowSet
) is a synonym for a
XMonad.StackSet.StackSet
parametrized over a
XMonad.Core.WorkspaceID
(a String
), a layout type wrapped inside
the XMonad.Layout.Layout
existential data type, the
Graphics.X11.Types.Window
type, the XMonad.Core.ScreenID
and the
XMonad.Core.ScreenDetail
s.
What a XMonad.StackSet.StackSet
is and how it can be manipulated
with pure functions is described in the Haddock documentation of the
XMonad.StackSet module.
The XMonad.StackSet.StackSet
(XMonad.Core.WindowSet
) has four
fields:
-
XMonad.StackSet.current
, for the current, focused workspace. This is aXMonad.StackSet.Screen
, which is composed of aXMonad.StackSet.Workspace
together with the screen information (for Xinerama support). -
XMonad.StackSet.visible
, a list ofXMonad.StackSet.Screen
s for the other visible (with Xinerama) workspaces. For non-Xinerama setups, this list is always empty. -
XMonad.StackSet.hidden
, the list of non-visibleXMonad.StackSet.Workspace
s. -
XMonad.StackSet.floating
, a map from floatingGraphics.X11.Types.Window
s toXMonad.StackSet.RationalRect
s specifying their geometry.
The XMonad.StackSet.Workspace
type is made of a
XMonad.StackSet.tag
, a XMonad.StackSet.layout
and
a (possibly empty) XMonad.StackSet.stack
of windows.
XMonad.StackSet (which should usually be imported qualified, to
avoid name clashes with Prelude functions such as Prelude.delete
and
filter
) provides many pure functions to manipulate the
XMonad.StackSet.StackSet
. These functions are most commonly used as
an argument to XMonad.Operations.windows
, which takes a pure
function to manipulate the XMonad.Core.WindowSet
and does all the
needed operations to refresh the screen and save the modified
XMonad.Core.XState
.
During each XMonad.Operations.windows
call, the
XMonad.StackSet.layout
field of the XMonad.StackSet.current
and
XMonad.StackSet.visible
XMonad.StackSet.Workspace
s are used to
physically arrange the XMonad.StackSet.stack
of windows on each
workspace.
The possibility of manipulating the XMonad.StackSet.StackSet
(XMonad.Core.WindowSet
) with pure functions makes it possible to
test all the properties of those functions with QuickCheck, providing
greater reliability of the core code. Every change to the
XMonad.StackSet module must be accompanied by appropriate QuickCheck
properties before being applied.
Event handling and messages
Event handling is the core activity of xmonad. Events generated by the X server are most important, but there may also be events generated by layouts or the user.
XMonad.Core defines a class that generalizes the concept of events,
XMonad.Core.Message
, constrained to types with a
Data.Typeable.Typeable
instance definition (which can be
automatically derived by GHC). XMonad.Core.Message
s are wrapped
within an existential type XMonad.Core.SomeMessage
. The
Data.Typeable.Typeable
constraint allows for the definition of a
XMonad.Core.fromMessage
function that can unwrap the message with
Data.Typeable.cast
. X Events are instances of this class, along
with any messages used by xmonad itself or by extension modules.
Using the Data.Typeable.Typeable
class for any kind of
XMonad.Core.Message
s and events allows us to define polymorphic functions
for processing messages or unhandled events.
This is precisely what happens with X events: xmonad passes them to
XMonad.Main.handle
. If the main event handling function doesn't have
anything to do with the event, the event is sent to all visible
layouts by XMonad.Operations.broadcastMessage
.
This messaging system allows the user to create new message types,
simply declare an instance of the Data.Typeable.Typeable
and use
XMonad.Operations.sendMessage
to send commands to layouts.
And, finally, layouts may handle X events and other messages within the same function... miracles of polymorphism.
The LayoutClass
Coding style
These are the coding guidelines for contributing to xmonad and the xmonad contributed extensions.
- Comment every top level function (particularly exported funtions), and provide a type signature.
- Use Haddock syntax in the comments (see below).
- Follow the coding style of the other modules.
- Code should be compilable with ghc-options: -Wall -Werror set in the xmonad-contrib.cabal file. There should be no warnings.
- Partial functions should be avoided: the window manager should not
crash, so never call
error
orundefined
. - Tabs are illegal. Use 4 spaces for indenting.
- Any pure function added to the core must have QuickCheck properties precisely defining its behaviour. Tests for everything else are encouraged.
For examples of Haddock documentation syntax, have a look at other extensions. Important points are:
- Every exported function (or even better, every function) should have a Haddock comment explaining what it does, and providing examples.
- Literal chunks of code can be written in comments using "birdtrack" notation (a greater-than symbol at the beginning of each line). Be sure to leave a blank line before and after each birdtrack-quoted section.
- Link to functions by surrounding the names in single quotes, modules in double quotes.
- Literal quote marks and slashes should be escaped with a backslash.
To generate and view the Haddock documentation for your extension, run
runhaskell Setup haddock
and then point your browser to /path/to/XMonadContrib/dist/doc/html/xmonad-contrib/index.html
.
For more information, see the Haddock documentation: http://www.haskell.org/haddock/doc/html/index.html.
For more information on the nuts and bolts of how to develop your own extension, see the tutorial on the wiki: http://haskell.org/haskellwiki/Xmonad/xmonad_development_tutorial.
Licensing policy
New modules should identify the author, and be submitted under the same license as xmonad (BSD3 license or freer).