X.L.ConditionalLayoutModifier: Init

CHANGES.md

@@ -130,6 +130,12 @@
 
 ### New Modules
 
+  * `XMonad.Layout.ConditionModifier`
+
+	This module provides a LayoutModifier that modifies an existing
+    LayoutModifier so that its modifications are only applied when a particular
+    condition is met.
+
   * `XMonad.Hooks.TaffybarPagerHints`
 
     Add a module that exports information about XMonads internal state that is

XMonad/Layout/ConditionalModifier.hs

@@ -0,0 +1,100 @@
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE TupleSections #-}
+
+-----------------------------------------------------------------------------
+-- |
+-- Module       : XMonad.Layout.ConditionModifier
+-- Copyright    : (c) Ivan Malison <[email protected]>
+-- License      : BSD
+--
+-- Maintainer   : none
+-- Stability    : unstable
+-- Portability  : portable
+--
+-- This module provides a LayoutModifier that modifies an existing
+-- ModifiedLayout so that its modifications are only applied when a particular
+-- condition is met.
+-----------------------------------------------------------------------------
+
+module XMonad.Layout.ConditionalModifier where
+
+import XMonad
+import XMonad.Layout.LayoutModifier
+
+class (Read c, Show c) => ModifierCondition c where
+  shouldApply :: c -> X Bool
+
+data ConditionalLayoutModifier m c a = (Read (m a), Show (m a), ModifierCondition c) =>
+  ConditionalLayoutModifier c (m a)
+
+deriving instance (Read (m a), Show (m a), ModifierCondition c) =>
+         Show (ConditionalLayoutModifier m c a)
+deriving instance (Read (m a), Show (m a), ModifierCondition c) =>
+         Read (ConditionalLayoutModifier m c a)
+
+data NoOpModifier a = NoOpModifier deriving (Read,Show)
+
+instance LayoutModifier NoOpModifier a
+
+runModifierIfCondition ::
+  (ModifierCondition c, LayoutModifier m a) =>
+  m a -> c -> (forall m1. LayoutModifier m1 a => m1 a -> X b) -> X b
+runModifierIfCondition modifier condition action = do
+  applyModifier <- shouldApply condition
+  if applyModifier
+    then action modifier
+    else action NoOpModifier
+
+instance (ModifierCondition c, LayoutModifier m Window) =>
+  LayoutModifier (ConditionalLayoutModifier m c) Window where
+
+  modifyLayout (ConditionalLayoutModifier condition originalModifier) w r =
+    runModifierIfCondition originalModifier condition
+                             (\modifier -> modifyLayout modifier w r)
+
+  modifyLayoutWithUpdate (ConditionalLayoutModifier condition originalModifier) w r = do
+    applyModifier <- shouldApply condition
+    if applyModifier
+      then do
+        (res, updatedModifier) <- modifyLayoutWithUpdate originalModifier w r
+        let updatedModifiedModifier =
+              ConditionalLayoutModifier condition <$> updatedModifier
+        return (res, updatedModifiedModifier)
+      else (, Nothing) . fst <$> modifyLayoutWithUpdate NoOpModifier w r
+
+  -- This function is not allowed to have any downstream effect, so it seems
+  -- more reasonable to simply allow the message to pass than to make it depend
+  -- on the condition.
+  handleMess (ConditionalLayoutModifier condition originalModifier) mess = do
+    fmap (ConditionalLayoutModifier condition) <$> handleMess originalModifier mess
+
+  handleMessOrMaybeModifyIt (ConditionalLayoutModifier condition originalModifier) mess = do
+    applyModifier <- shouldApply condition
+    if applyModifier
+       then do
+         result <- handleMessOrMaybeModifyIt originalModifier mess
+         return $ case result of
+           Nothing -> Nothing
+           Just (Left updated) -> Just $ Left $ ConditionalLayoutModifier condition updated
+           Just (Right message) -> Just $ Right message
+       else return Nothing
+
+  redoLayout (ConditionalLayoutModifier condition originalModifier) r ms wrs = do
+    applyModifier <- shouldApply condition
+    if applyModifier
+      then do
+        (res, updatedModifier) <- redoLayout originalModifier r ms wrs
+        let updatedModifiedModifier =
+              ConditionalLayoutModifier condition <$> updatedModifier
+        return (res, updatedModifiedModifier)
+      else (, Nothing) . fst <$> redoLayout NoOpModifier r ms wrs
+
+  modifyDescription (ConditionalLayoutModifier _ originalModifier) l =
+    modifyDescription originalModifier l
+
+

xmonad-contrib.cabal

@@ -213,6 +213,7 @@ library
                         XMonad.Layout.Column
                         XMonad.Layout.Combo
                         XMonad.Layout.ComboP
+                        XMonad.Layout.ConditionalModifier
                         XMonad.Layout.Cross
                         XMonad.Layout.Decoration
                         XMonad.Layout.DecorationAddons