    {-# OPTIONS_GHC -fglasgow-exts #-} -- For deriving Data/Typeable
    {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, PatternGuards #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module       : XMonad.Layout.LayoutModifier
    -- Copyright    : (c) David Roundy <droundy@darcs.net>
    -- License      : BSD
    --
    -- Maintainer   : none
    -- Stability    : unstable
    -- Portability  : portable
    --
    -- A module for writing easy layout modifiers, which do not define a
    -- layout in and of themselves, but modify the behavior of or add new
    -- functionality to other layouts.  If you ever find yourself writing
    -- a layout which takes another layout as a parameter, chances are you
    -- should be writing a LayoutModifier instead!
    --
    -- In case it is not clear, this module is not intended to help you
    -- configure xmonad, it is to help you write other extension modules.
    -- So get hacking!
    -----------------------------------------------------------------------------

    module XMonad.Layout.LayoutModifier (
        -- * Usage
        -- $usage

        -- * The 'LayoutModifier' class
        LayoutModifier(..), ModifiedLayout(..)
        ) where

    import XMonad
    import XMonad.StackSet ( Stack, Workspace (..) )

    -- $usage
    --
    -- The 'LayoutModifier' class is provided to help extension developers
    -- write easy layout modifiers.  End users won't find much of interest
    -- here. =)
    --
    -- To write a layout modifier using the 'LayoutModifier' class, define
    -- a data type to represent the layout modification (storing any
    -- necessary state), define an instance of 'LayoutModifier', and
    -- export an appropriate function for applying the modifier.  For example:
    --
    -- > data MyModifier a = MyModifier MyState
    -- >   deriving (Show, Read)
    -- >
    -- > instance LayoutModifier MyModifier a where
    -- >   -- override whatever methods from LayoutModifier you like
    -- >
    -- > modify :: l a -> ModifiedLayout MyModifier l a
    -- > modify = ModifiedLayout (MyModifier initialState)
    --
    -- When defining an instance of 'LayoutModifier', you are free to
    -- override as many or as few of the methods as you see fit.  See the
    -- documentation below for specific information about the effect of
    -- overriding each method.  Every method has a default implementation;
    -- an instance of 'LayoutModifier' which did not provide a non-default
    -- implementation of any of the methods would simply act as the
    -- identity on any layouts to which it is applied.
    --
    -- For more specific usage examples, see
    --
    -- * "XMonad.Layout.WorkspaceDir"
    --
    -- * "XMonad.Layout.Magnifier"
    --
    -- * "XMonad.Layout.NoBorders"
    --
    -- * "XMonad.Layout.Reflect"
    --
    -- * "XMonad.Layout.Named"
    --
    -- * "XMonad.Layout.WindowNavigation"
    --
    -- and several others.  You probably want to start by looking at some
    -- of the above examples; the documentation below is detailed but
    -- possibly confusing, and in many cases the creation of a
    -- 'LayoutModifier' is actually quite simple.
    --
    -- /Important note/: because of the way the 'LayoutModifier' class is
    -- intended to be used, by overriding any of its methods and keeping
    -- default implementations for all the others, 'LayoutModifier'
    -- methods should never be called explicitly.  It is likely that such
    -- explicit calls will not have the intended effect.  Rather, the
    -- 'LayoutModifier' methods should only be called indirectly through
    -- the 'LayoutClass' instance for 'ModifiedLayout', since it is this
    -- instance that defines the semantics of overriding the various
    -- 'LayoutModifier' methods.

    class (Show (m a), Read (m a)) => LayoutModifier m a where

        -- | 'modifyLayout' allows you to intercept a call to 'runLayout'
        --   /before/ it is called on the underlying layout, in order to
        --   perform some effect in the X monad, and\/or modify some of
        --   the parameters before passing them on to the 'runLayout'
        --   method of the underlying layout.
        --
        --   The default implementation of 'modifyLayout' simply calls
        --   'runLayout' on the underlying layout.
        modifyLayout :: (LayoutClass l a) =>
                        m a                             -- ^ the layout modifier
                     -> Workspace WorkspaceId (l a) a   -- ^ current workspace
                     -> Rectangle                       -- ^ screen rectangle
                     -> X ([(a, Rectangle)], Maybe (l a))
        modifyLayout _ w r = runLayout w r

        -- | 'handleMess' allows you to spy on messages to the underlying
        --   layout, in order to have an effect in the X monad, or alter
        --   the layout modifier state in some way (by returning @Just
        --   nm@, where @nm@ is a new modifier).  In all cases, the
        --   underlying layout will also receive the message as usual,
        --   after the message has been processed by 'handleMess'.
        --
        --   If you wish to possibly modify a message before it reaches
        --   the underlying layout, you should use
        --   'handleMessOrMaybeModifyIt' instead.  If you do not need to
        --   modify messages or have access to the X monad, you should use
        --   'pureMess' instead.
        --
        --   The default implementation of 'handleMess' calls 'unhook'
        --   when receiving a 'Hide' or 'ReleaseResources' method (after
        --   which it returns @Nothing@), and otherwise passes the message
        --   on to 'pureMess'.
        handleMess :: m a -> SomeMessage -> X (Maybe (m a))
        handleMess m mess | Just Hide <- fromMessage mess             = doUnhook
                          | Just ReleaseResources <- fromMessage mess = doUnhook
                          | otherwise = return $ pureMess m mess
         where doUnhook = do unhook m; return Nothing

        -- | 'handleMessOrMaybeModifyIt' allows you to intercept messages
        --   sent to the underlying layout, in order to have an effect in
        --   the X monad, alter the layout modifier state, or produce a
        --   modified message to be passed on to the underlying layout.
        --
        --   The default implementation of 'handleMessOrMaybeModifyIt'
        --   simply passes on the message to 'handleMess'.
        handleMessOrMaybeModifyIt :: m a -> SomeMessage -> X (Maybe (Either (m a) SomeMessage))
        handleMessOrMaybeModifyIt m mess = do mm' <- handleMess m mess
                                              return (Left `fmap` mm')

        -- | 'pureMess' allows you to spy on messages sent to the
        --   underlying layout, in order to possibly change the layout
        --   modifier state.
        --
        --   The default implementation of 'pureMess' ignores messages
        --   sent to it, and returns @Nothing@ (causing the layout
        --   modifier to remain unchanged).
        pureMess :: m a -> SomeMessage -> Maybe (m a)
        pureMess _ _ = Nothing

        -- | 'redoLayout' allows you to intercept a call to 'runLayout' on
        --   workspaces with at least one window, /after/ it is called on
        --   the underlying layout, in order to perform some effect in the
        --   X monad, possibly return a new layout modifier, and\/or
        --   modify the results of 'runLayout' before returning them.
        --
        --   If you don't need access to the X monad, use 'pureModifier'
        --   instead.  Also, if the behavior you need can be cleanly
        --   separated into an effect in the X monad, followed by a pure
        --   transformation of the results of 'runLayout', you should
        --   consider implementing 'hook' and 'pureModifier' instead of
        --   'redoLayout'.
        --
        --   If you also need to perform some action when 'runLayout' is
        --   called on an empty workspace, see 'emptyLayoutMod'.
        --
        --   The default implementation of 'redoLayout' calls 'hook' and
        --   then 'pureModifier'.
        redoLayout :: m a          -- ^ the layout modifier
                   -> Rectangle    -- ^ screen rectangle
                   -> Stack a      -- ^ current window stack
                   -> [(a, Rectangle)]  -- ^ (window,rectangle) pairs returned
                                        -- by the underlying layout
                   -> X ([(a, Rectangle)], Maybe (m a))
        redoLayout m r s wrs = do hook m; return $ pureModifier m r s wrs

        -- | 'pureModifier' allows you to intercept a call to 'runLayout'
        --   /after/ it is called on the underlying layout, in order to
        --   modify the list of window\/rectangle pairings it has returned,
        --   and\/or return a new layout modifier.
        --
        --   The default implementation of 'pureModifier' returns the
        --   window rectangles unmodified.
        pureModifier :: m a        -- ^ the layout modifier
                     -> Rectangle  -- ^ screen rectangle
                     -> Stack a    -- ^ current window stack
                     -> [(a, Rectangle)]  -- ^ (window, rectangle) pairs returned
                                          -- by the underlying layout
                     -> ([(a, Rectangle)], Maybe (m a))
        pureModifier _ _ _ wrs = (wrs, Nothing)

        -- | 'emptyLayoutMod' allows you to intercept a call to
        --   'runLayout' on an empty workspace, /after/ it is called on
        --   the underlying layout, in order to perform some effect in the
        --   X monad, possibly return a new layout modifier, and\/or
        --   modify the results of 'runLayout' before returning them.
        --
        --   If you don't need access to the X monad, then tough luck.
        --   There isn't a pure version of 'emptyLayoutMod'.
        --
        --   The default implementation of 'emptyLayoutMod' ignores its
        --   arguments and returns an empty list of window\/rectangle
        --   pairings.
        --
        --   /NOTE/: 'emptyLayoutMod' will likely be combined with
        --   'redoLayout' soon!
        emptyLayoutMod :: m a -> Rectangle -> [(a, Rectangle)]
                       -> X ([(a, Rectangle)], Maybe (m a))
        emptyLayoutMod _ _ _ = return ([], Nothing)

        -- | 'hook' is called by the default implementation of
        --   'redoLayout', and as such represents an X action which is to
        --   be run each time 'runLayout' is called on the underlying
        --   layout, /after/ 'runLayout' has completed.  Of course, if you
        --   override 'redoLayout', then 'hook' will not be called unless
        --   you explicitly call it.
        --
        --   The default implementation of 'hook' is @return ()@ (i.e., it
        --   has no effect).
        hook :: m a -> X ()
        hook _ = return ()

        -- | 'unhook' is called by the default implementation of
        --   'handleMess' upon receiving a 'Hide' or a 'ReleaseResources'
        --   message.
        --
        --   The default implementation, of course, does nothing.
        unhook :: m a -> X ()
        unhook _ = return ()

        -- | 'modifierDescription' is used to give a String description to
        --   this layout modifier.  It is the empty string by default; you
        --   should only override this if it is important that the
        --   presence of the layout modifier be displayed in text
        --   representations of the layout (for example, in the status bar
        --   of a "XMonad.Hooks.DynamicLog" user).
        modifierDescription :: m a -> String
        modifierDescription = const ""

        -- | 'modifyDescription' gives a String description for the entire
        --   layout (modifier + underlying layout).  By default, it is
        --   derived from the concatenation of the 'modifierDescription'
        --   with the 'description' of the underlying layout, with a
        --   \"smart space\" in between (the space is not included if the
        --   'modifierDescription' is empty).
        modifyDescription :: (LayoutClass l a) => m a -> l a -> String
        modifyDescription m l = modifierDescription m <> description l
            where "" <> x = x
                  x <> y = x ++ " " ++ y

    -- | The 'LayoutClass' instance for a 'ModifiedLayout' defines the
    --   semantics of a 'LayoutModifier' applied to an underlying layout.
    instance (LayoutModifier m a, LayoutClass l a) => LayoutClass (ModifiedLayout m l) a where
        runLayout (Workspace i (ModifiedLayout m l) ms) r =
            do (ws, ml')  <- modifyLayout m (Workspace i l ms) r
               (ws', mm') <- case ms of
                               Just s  -> redoLayout m r s ws
                               Nothing -> emptyLayoutMod m r ws
               let ml'' = case mm' of
                            Just m' -> Just $ (ModifiedLayout m') $ maybe l id ml'
                            Nothing -> ModifiedLayout m `fmap` ml'
               return (ws', ml'')

        handleMessage (ModifiedLayout m l) mess =
            do mm' <- handleMessOrMaybeModifyIt m mess
               ml' <- case mm' of
                      Just (Right mess') -> handleMessage l mess'
                      _ -> handleMessage l mess
               return $ case mm' of
                        Just (Left m') -> Just $ (ModifiedLayout m') $ maybe l id ml'
                        _ -> (ModifiedLayout m) `fmap` ml'
        description (ModifiedLayout m l) = modifyDescription m l

    -- | A 'ModifiedLayout' is simply a container for a layout modifier
    --   combined with an underlying layout.  It is, of course, itself a
    --   layout (i.e. an instance of 'LayoutClass').
    data ModifiedLayout m l a = ModifiedLayout (m a) (l a) deriving ( Read, Show )

    -- N.B. I think there is a Haddock bug here; the Haddock output for
    -- the above does not parenthesize (m a) and (l a), which is obviously
    -- incorrect.