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Xmonad/Guided tour of the xmonad source/Core.hs

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The next source file to examine is Core.hs. It defines several core data types and some of the core functionality of xmonad. If StackSet.hs is the heart of xmonad, Core.hs is its guts.

Contents

1
XState
,
XConf
, and
XConfig

These three record types make up the core of xmonad's state and configuration:

  • A value of type
    XState
    stores xmonad's mutable runtime state, consisting of the list of workspaces, a set of mapped windows, something to do with keeping track of pending UnmapEvents, and something to do with dragging. 
data XState = XState
    { windowset    :: !WindowSet           -- ^ workspace list
    , mapped       :: !(S.Set Window)      -- ^ the Set of mapped windows
    , waitingUnmap :: !(M.Map Window Int)  -- ^ the number of expected UnmapEvents
    , dragging     :: !(Maybe (Position -> Position -> X (), X ())) }
Note that the
WindowSet
type is just a
StackSet
with various concrete types substituted for its type parameters: 
type WindowSet   = StackSet  WorkspaceId (Layout Window) Window ScreenId ScreenDetail
type WindowSpace = Workspace WorkspaceId (Layout Window) Window
 
-- | Virtual workspace indicies
type WorkspaceId = String
 
-- | Physical screen indicies
newtype ScreenId    = S Int deriving (Eq,Ord,Show,Read,Enum,Num,Integral,Real)
 
-- | The 'Rectangle' with screen dimensions and the list of gaps
data ScreenDetail   = SD { screenRect :: !Rectangle
                         , statusGap  :: !(Int,Int,Int,Int) -- ^ width of status bar on the screen
                         } deriving (Eq,Show, Read)
The type of workspace tags,
WorkspaceId
, is just
String
; a
ScreenDetail
stores information about the screen dimension as well as any gaps which should be left at the edges of the screen for status bars and other such things. Now, you may wonder why we have 
newtype ScreenId = S Int deriving (...)
rather than just
type ScreenId = Int
? The reason is that if
type ScreenId = Int
, it would be possible to accidentally do arithmetic with
ScreenId
s mixed with other
Int
values, which clearly doesn't make sense. Making
ScreenId
a separate type means that the type system will enforce non-mixing of
ScreenId
s with other types, while using
newtype
with automatic instance deriving means none of the convenience is lost -- we can write code just as if
ScreenId
s are normal
Int
values, but be sure that we can't accidentally get mixed up and do something silly like add a
ScreenId
to the width of a window. This is the power of a rich static type system like Haskell's -- we can encode certain invariants and constraints in the type system, and have them automatically checked at compile time.
  • An
    XConf
    record stores xmonad's (immutable) configuration data, such as window border colors, the keymap, information about the X11 display and root window, and other user-specified configuration information. The reason this record is separated from
    XState
    is that, as we'll see later, xmonad's code provides a static guarantee that the data stored in this record is truly read-only, and cannot be changed while xmonad is running.
  • XConfig
    provides a way for the user to customize xmonad's configuration, by defining an
    XConfig
    record in their
    xmonad.hs
    file. You're probably already familiar with this record type.

2 The
X
monad

And now, what you've all been waiting for: the X monad! 

newtype X a = X (ReaderT XConf (StateT XState IO) a)
    deriving (Functor, Monad, MonadIO, MonadState XState, MonadReader XConf)
The X monad represents a common pattern for building custom monad instances: using monad transformers, one can simply 'layer' the capabilities and effects of several monads into one, and then use GHC's newtype deriving capabilities to automatically derive instances of the relevant type classes. In this case, the base monad out of which the X monad is built is IO; this is necessary since communicating with the X server involves IO operations. On top of that is
StateT XState
, which automatically threads a mutable
XState
record through computations in the X monad; finally there is a
ReaderT XConf
which also threads a read-only
XConf
record through. As noted in the comments in the source, the
XState
record can be accessed with any functions in the <div class="inline-code">
MonadState
type class, such as
get
,
put
,
gets
, and
modify
; the
XConf
record can be accessed with
MonadReader
</div> functions, such as
ask
.

For more information on monad transformers in general, I recommend reading Martin Grabmüller's excellent tutorial paper, Monad Transformers Step-by-Step; for more information on this particular style of composing monad transformers and using automatic newtype deriving, read Cale Gibbard's tutorial, How To Use Monad Transformers.

Along with the X monad, several utility functions are provided, such as
runX
, which turns an action in the X monad into an IO action, and
catchX
, which provides error handling for X actions. There are also several higher-order functions provided for convenience, including
withDisplay
(apply a function producing an X action to the current display) and
withWindowSet
(apply a function to the current window set).

3
ManageHook

This is a bit more advanced, and not really a central part of the system, so I'm skipping it for now, hopefully coming back to add more later.

4
LayoutClass

Next, let's take a look at the
LayoutClass
. This is one of the places that Haskell's type classes really shine.
LayoutClass
is a type class of which every layout must be an instance. It defines the basic functions which define what a layout is and how it behaves. The comments in the source code explain very clearly what each of these functions is supposed to do, but here are some highlights:
  • Note that all the
    LayoutClass
    functions provide default implementations, so that
    LayoutClass
    instances do not have to provide implementations of those functions where the default behavior is desired. For example, by default,
    doLayout
    simply calls
    pureLayout
    , so a layout that does not require access to the X monad need only implement the
    pureLayout
    function. (For example, the Accordion, Square, and Grid layouts from the contrib library all use this approach.)
  • Layouts can have their own private state, by storing this state in the
    LayoutClass
    instance and returning a modified structue (via
    doLayout
    ) when the state changes.
  • Both
    doLayout
    and
    handleMessage
    have corresponding "pure" versions, which do not give results in the X monad. These functions are never called directly by the xmonad core, which only calls
    doLayout
    and
    handleMessage
    , but a layout may choose to implement one (or both) of these "pure" functions, which will be called by the default implementation of the "impure" versions. Layouts which implement
    pureLayout
    or
    pureMessage
    are guaranteed to only make decisions about layout or messages (respectively) based on the internal layout state, and not on the state of the system or the window manager in general.
Now, every distinct layout will have a distinct type, although of course they all must be instances of
LayoutClass
. Given Haskell's strong typing, how can we store different layouts with different workspaces, or even change layouts on the fly? The solution is to wrap layouts using an existential type which hides the particular layout type and only exposes the fact that it is an instance of
LayoutClass
. Not only does this solve the problems caused by separate types for each layout, but it also guarantees that the xmonad core can only ever interact with a layout by calling functions from its
LayoutClass
instance. (Actually, this is a lie, since
doLayout
and
handleMessage
give results in the
X
monad, meaning they have access to the window manager state...) 
-- | An existential type that can hold any object that is in Read and LayoutClass.
data Layout a = forall l. (LayoutClass l a, Read (l a)) => Layout (l a)
The
forall l.
abstracts over all layout types (types which are an instance of
LayoutClass
); this is what makes it an existential type. Note that
l
does not appear on the left-hand side of the type declaration. A variable
lay
may have the highly specific type of some particular layout (for example, the types of layouts formed by stacking various layout combinators and transformers can get rather long and hairy!), but no matter what the type of
lay
,
Layout lay
will simply have the type
Layout a
for some window type
a
(usually
Window
). Note that
(l a)
is also required to be an instance of the
Read
type class -- this is so the state of all the layouts can be serialized and then read back in during dynamic restarts.

5 Messages

The xmonad core uses messages to communicate with layouts. The obvious, simple way to define messages would be by defining a new data type, something like 

-- WARNING: not real xmonad code!
 
data Message = Hide | ReleaseResources | ShowMonkey

thus defining three messages types, which tell a layout to hide itself, release any resources, and display a monkey, respectively.

The problem with such an approach should be obvious: it is completely inflexible and inextensible; adding new message types later would be a pain, and it would be practically impossible for extension layouts to define their own message types without modifying the core. So, xmonad uses a more sophisticated system (at the cost of making things slightly harder to read and understand).

Instead of defining a
Message
data type, xmonad defines a
Message
type class: 
class Typeable a => Message a
 
data SomeMessage = forall a. Message a => SomeMessage a
The
Message
class comes along with an existential wrapper
SomeMessage
, just like
LayoutClass
comes along with the
Layout
wrapper. Note also that the
Message
type class doesn't declare any methods; it simply serves as a marker for types whose values we wish to use as messages. The
Typeable
constraint ensures that the types of values used as messages can be carried around as values at runtime, so dynamic type checks can be performed on messages extracted from a
SomeMessage
wrapper: 
-- | And now, unwrap a given, unknown Message type, performing a (dynamic)
-- type check on the result.
--
fromMessage :: Message m => SomeMessage -> Maybe m
fromMessage (SomeMessage m) = cast m

Complicated? A bit, perhaps, but the good news is that you probably don't have to worry too much about it. =)

Finally, raw X events (like key presses, mouse movements, and so on) count as
Message
s, and two core message values are also defined as members of the
LayoutMessages
type. 
-- | X Events are valid Messages
instance Message Event
 
-- | LayoutMessages are core messages that all layouts (especially stateful
-- layouts) should consider handling.
data LayoutMessages = Hide              -- ^ sent when a layout becomes non-visible
                    | ReleaseResources  -- ^ sent when xmonad is exiting or restarting
    deriving (Typeable, Eq)
 
instance Message LayoutMessages

6 Utility functions

The end of 
Core.hs
includes a number of utility functions which are used throughout the rest of the xmonad codebase; you should take some time to familiarize yourself with the functions here. Notable examples include
spawn
and
doubleFork
, which allow for creation of new threads;
io
, a short synonym for
liftIO
(which can be used to turn
IO ()
actions into
X ()
actions); and
runOnWorkspaces
, which maps
(WindowSpace -> X WindowSpace)
functions over all workspaces.

7 On-the-fly recompilation

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