Added the X.U.PureX module and generalised type signatures in

`X.U.ExtensibleState`.

CHANGES.md

@@ -12,6 +12,23 @@
 
 ### New Modules
 
+  * `XMonad.Util.Purex`
+
+    Unlike the opaque `IO` actions that `X` actions can wrap, regular reads from
+    the `XConf` and modifications to the `XState` are fundamentally pure --
+    contrary to the current treatment of such actions in most xmonad code. Pure
+    modifications to the `WindowSet` can be readily composed, but due to the
+    need for those modifications to be properly handled by `windows`, other pure
+    changes to the `XState` cannot be interleaved with those changes to the
+    `WindowSet` without superfluous refreshes, hence breaking composability.
+
+    This module aims to rectify that situation by drawing attention to it and
+    providing `PureX`: a pure type with the same monadic interface to state as
+    `X`. The `XLike` typeclass enables writing actions generic over the two
+    monads; if pure, existing `X` actions can be generalised with only a change
+    to the type signature. Various other utilities are provided, in particular
+    the `defile` function which is needed by end-users.
+
 ### Bug Fixes and Minor Changes
 
   * `XMonad.Actions.MessageHandling`

XMonad/Util/ExtensibleState.hs

@@ -27,6 +27,7 @@ module XMonad.Util.ExtensibleState (
 import Data.Typeable (typeOf,cast)
 import qualified Data.Map as M
 import XMonad.Core
+import XMonad.Util.PureX
 import qualified Control.Monad.State as State
 import Data.Maybe (fromMaybe)
 
@@ -75,27 +76,29 @@ import Data.Maybe (fromMaybe)
 --
 
 -- | Modify the map of state extensions by applying the given function.
-modifyStateExts :: (M.Map String (Either String StateExtension)
+modifyStateExts
-                   -> M.Map String (Either String StateExtension))
+  :: XLike m
-                -> X ()
+  => (M.Map String (Either String StateExtension)
+  -> M.Map String (Either String StateExtension))
+  -> m ()
 modifyStateExts f = State.modify $ \st -> st { extensibleState = f (extensibleState st) }
 
 -- | Apply a function to a stored value of the matching type or the initial value if there
 -- is none.
-modify :: ExtensionClass a => (a -> a) -> X ()
+modify :: (ExtensionClass a, XLike m) => (a -> a) -> m ()
 modify f = put . f =<< get
 
 -- | Add a value to the extensible state field. A previously stored value with the same
 -- type will be overwritten. (More precisely: A value whose string representation of its type
 -- is equal to the new one's)
-put :: ExtensionClass a => a -> X ()
+put :: (ExtensionClass a, XLike m) => a -> m ()
 put v = modifyStateExts . M.insert (show . typeOf $ v) . Right . extensionType $ v
 
 -- | Try to retrieve a value of the requested type, return an initial value if there is no such value.
-get :: ExtensionClass a => X a
+get :: (ExtensionClass a, XLike m) => m a
 get = getState' undefined -- `trick' to avoid needing -XScopedTypeVariables
   where toValue val = maybe initialValue id $ cast val
-        getState' :: ExtensionClass a => a -> X a
+        getState' :: (ExtensionClass a, XLike m) => a -> m a
         getState' k = do
           v <- State.gets $ M.lookup (show . typeOf $ k) . extensibleState
           case v of
@@ -110,14 +113,14 @@ get = getState' undefined -- `trick' to avoid needing -XScopedTypeVariables
                          [(x,"")] -> Just x
                          _ -> Nothing
 
-gets :: ExtensionClass a => (a -> b) -> X b
+gets :: (ExtensionClass a, XLike m) => (a -> b) -> m b
 gets = flip fmap get
 
 -- | Remove the value from the extensible state field that has the same type as the supplied argument
-remove :: ExtensionClass a => a -> X ()
+remove :: (ExtensionClass a, XLike m) => a -> m ()
 remove wit = modifyStateExts $ M.delete (show . typeOf $ wit)
 
-modified :: (ExtensionClass a, Eq a) => (a -> a) -> X Bool
+modified :: (ExtensionClass a, Eq a, XLike m) => (a -> a) -> m Bool
 modified f = do
     v <- get
     case f v of

XMonad/Util/PureX.hs

@@ -0,0 +1,276 @@
+{-# LANGUAGE GeneralizedNewtypeDeriving, FlexibleContexts #-}
+
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  XMonad.Util.PureX
+-- Copyright   :  L. S. Leary 2018
+-- License     :  BSD3-style (see LICENSE)
+--
+-- Maintainer  :  L. S. Leary
+-- Stability   :  unstable
+-- Portability :  not portable
+--
+-- Unlike the opaque @IO@ actions that @X@ actions can wrap, regular reads from
+-- the 'XConf' and modifications to the 'XState' are fundamentally pure—contrary
+-- to the current treatment of such actions in most xmonad code. Pure
+-- modifications to the 'WindowSet' can be readily composed, but due to the need
+-- for those modifications to be properly handled by 'windows', other pure
+-- changes to the @XState@ cannot be interleaved with those changes to the
+-- @WindowSet@ without superfluous refreshes, hence breaking composability.
+--
+-- This module aims to rectify that situation by drawing attention to it and
+-- providing 'PureX': a pure type with the same monadic interface to state as
+-- @X@. The 'XLike' typeclass enables writing actions generic over the two
+-- monads; if pure, existing @X@ actions can be generalised with only a change
+-- to the type signature. Various other utilities are provided, in particular
+-- the 'defile' function which is needed by end-users.
+--
+-----------------------------------------------------------------------------
+
+-- --< Imports & Exports >-- {{{
+
+module XMonad.Util.PureX (
+  -- * Usage
+  -- $Usage
+  PureX, XLike(..), defile,
+  windowBracket', handlingRefresh,
+  runPureX, toXLike,
+  -- * Utility
+  -- ** Generalised when* functions
+  when', whenM', whenJust',
+  -- ** Infix operators
+  (<?), (&>),
+  -- ** @WindowSet@ operations
+  withWindowSet', withFocii,
+  modify'', modifyWindowSet',
+  getStack, putStack, peek,
+  view, greedyView, invisiView,
+  shift, curScreen, curWorkspace,
+  curTag, curScreenId,
+) where
+
+-- xmonad
+import XMonad
+import qualified XMonad.StackSet as W
+
+-- mtl
+import Control.Monad.State
+import Control.Monad.Reader
+
+-- base
+import Data.Semigroup (Semigroup(..), Any(..))
+import Control.Applicative (liftA2)
+
+-- }}}
+
+-- --< Usage >-- {{{
+
+-- $Usage
+--
+-- The suggested pattern of usage for this module is to write composable, pure
+-- actions as @XLike m => m Any@ or @PureX Any@ values, where the encapsulated
+-- @Any@ value encodes whether or not a refresh is needed to properly institute
+-- changes. These values can then be combined monoidally (i.e. with '<>' AKA
+-- '<+>') or with operators such as '<*', '*>', '<?' and '&>' to build seamless
+-- new actions. The end user can run and handle the effects of the pure actions
+-- in the @X@ monad by applying the @defile@ function, which you may want to
+-- re-export. Alternatively, if an action does not make stackset changes that
+-- need to be handled by @windows@, it can be written with as an
+-- @XLike m => m ()@ and used directly.
+--
+-- Unfortunately since layouts must handle messages in the @X@ monad, this
+-- approach does not quite apply to actions involving them. However a relatively
+-- direct translation to impure actions is possible: you can write composable,
+-- refresh-tracking actions as @X Any@ values, making sure to eschew
+-- refresh-inducing functions like @windows@ and @sendMessage@ in favour of
+-- 'modifyWindowSet' and utilities provided by "XMonad.Actions.MessageFeedback".
+-- The 'windowBracket_' function recently added to "XMonad.Operations" is the
+-- impure analogue of @defile@. Note that @PureX Any@ actions can be composed
+-- into impure ones after applying 'toX'; don't use @defile@ for this. E.g.
+--
+-- > windowBracket_ (composableImpureAction <> toX composablePureAction)
+--
+-- Although both @X@ and @PureX@ have Monoid instances over monoidal values,
+-- @(XLike m, Monoid a)@ is not enough to infer @Monoid (m a)@ (due to the
+-- open-world assumption). Hence a @Monoid (m Any)@ constraint may need to be
+-- used when working with @XLike m => m Any@ where no context is forcing @m@ to
+-- unify with @X@ or @PureX@. This can also be avoided by working with
+-- @PureX Any@ values and generalising them with 'toXLike' where necessary.
+--
+-- @PureX@ also enables a more monadic style when writing windowset operations;
+-- see the implementation of the utilities in this module for examples.
+-- For an example of a whole module written in terms of this one, see
+-- "XMonad.Hooks.RefocusLast".
+--
+
+-- }}}
+
+-- --< Core >-- {{{
+
+-- | The @PureX@ newtype over @ReaderT XConf (State XState) a@.
+newtype PureX a = PureX (ReaderT XConf (State XState) a)
+  deriving (Functor, Applicative, Monad, MonadReader XConf, MonadState XState)
+
+instance Semigroup a => Semigroup (PureX a) where
+  (<>) = liftA2 (<>)
+
+instance Monoid a => Monoid (PureX a) where
+  mappend = liftA2 mappend
+  mempty  = return mempty
+
+-- | The @XLike@ typeclass over monads reading @XConf@ values and tracking
+--   @XState@ state.
+class (MonadReader XConf m, MonadState XState m) => XLike m where
+  toX :: m a -> X a
+
+instance XLike X where
+  toX = id
+
+instance XLike PureX where
+  toX = toXLike
+
+-- | Consume a @PureX a@.
+runPureX :: PureX a -> XConf -> XState -> (a, XState)
+runPureX (PureX m) = runState . runReaderT m
+
+-- | Despite appearing less general, @PureX a@ is actually isomorphic to
+--   @XLike m => m a@.
+toXLike :: XLike m => PureX a -> m a
+toXLike pa = state =<< runPureX pa <$> ask
+
+-- | A generalisation of 'windowBracket'. Handles refreshing for an action that
+--   __performs no refresh of its own__ but can indicate that it needs one
+--   through a return value that's tested against the supplied predicate. The
+--   action can interleave changes to the @WindowSet@ with @IO@ or changes to
+--   the @XState@.
+windowBracket' :: XLike m => (a -> Bool) -> m a -> X a
+windowBracket' p = windowBracket p . toX
+
+-- | A version of @windowBracket'@ specialised to take a @PureX Any@ action and
+--   handle windowset changes with a refresh when the @Any@ holds @True@.
+--   Analogous to 'windowBracket_'. Don't bake this into your action; it's for
+--   the end-user.
+defile :: PureX Any -> X ()
+defile = void . windowBracket' getAny
+
+-- | A version of @windowBracket@ specialised to take an @X ()@ action and
+--   perform a refresh handling any changes it makes.
+handlingRefresh :: X () -> X ()
+handlingRefresh = windowBracket (\_ -> True)
+
+-- }}}
+
+-- --< Utility >-- {{{
+
+-- | A 'when' that accepts a monoidal return value.
+when' :: (Monad m, Monoid a) => Bool -> m a -> m a
+when' b ma = if b then ma else return mempty
+
+-- | A @whenX@/@whenM@ that accepts a monoidal return value.
+whenM' :: (Monad m, Monoid a) => m Bool -> m a -> m a
+whenM' mb m = when' <$> mb >>= ($ m)
+
+-- | A 'whenJust' that accepts a monoidal return value.
+whenJust' :: (Monad m, Monoid b) => Maybe a -> (a -> m b) -> m b
+whenJust' = flip $ maybe (return mempty)
+
+-- | Akin to @<*@. Discarding the wrapped value in the second argument either
+--   way, keep its effects iff the first argument returns @Any True@.
+(<?) :: Monad m => m Any -> m a -> m Any
+ifthis <? thenthis = do
+  Any b <- ifthis
+  when' b (Any b <$ thenthis)
+infixl 4 <?
+
+-- | Akin to a low precedence @<>@. Combines applicative effects left-to-right
+--   and wrapped @Bool@s with @&&@ (instead of @||@).
+(&>) :: Applicative f => f Any -> f Any -> f Any
+(&>) = liftA2 $ \(Any b1) (Any b2) -> Any (b1 && b2)
+infixl 1 &>
+
+-- | A generalisation of 'withWindowSet'.
+withWindowSet' :: XLike m => (WindowSet -> m a) -> m a
+withWindowSet' = (=<< gets windowset)
+
+-- | If there is a current tag and a focused window, perform an operation with
+--   them, otherwise return mempty.
+withFocii :: (XLike m, Monoid a) => (WorkspaceId -> Window -> m a) -> m a
+withFocii f = join $ (whenJust' <$> peek) <*> (f <$> curTag)
+
+-- | A generalisation of 'modifyWindowSet'.
+modifyWindowSet' :: XLike m => (WindowSet -> WindowSet) -> m ()
+modifyWindowSet' f = modify $ \xs -> xs { windowset = f (windowset xs) }
+
+-- | A variant of @W.modify@ and @W.modify'@ handling the @Nothing@ and @Just@
+--   cases uniformly.
+modify''
+  :: (Maybe (W.Stack a) -> Maybe (W.Stack a))
+  -> (W.StackSet i l a s sd -> W.StackSet i l a s sd)
+modify'' f = W.modify (f Nothing) (f . Just)
+
+-- | Get the stack from the current workspace.
+getStack :: XLike m => m (Maybe (W.Stack Window))
+getStack = W.stack <$> curWorkspace
+
+-- | Set the stack on the current workspace.
+putStack :: XLike m => Maybe (W.Stack Window) -> m ()
+putStack mst = modifyWindowSet' . modify'' $ \_ -> mst
+
+-- | Get the focused window if there is one.
+peek :: XLike m => m (Maybe Window)
+peek = withWindowSet' (return . W.peek)
+
+-- | Get the current screen.
+curScreen
+  :: XLike m
+  => m (W.Screen WorkspaceId (Layout Window) Window ScreenId ScreenDetail)
+curScreen = withWindowSet' (return . W.current)
+
+-- | Get the current workspace.
+curWorkspace :: XLike m => m WindowSpace
+curWorkspace = W.workspace <$> curScreen
+
+-- | Get the current tag.
+curTag :: XLike m => m WorkspaceId
+curTag = W.tag <$> curWorkspace
+
+-- | Get the current @ScreenId@.
+curScreenId :: XLike m => m ScreenId
+curScreenId = W.screen <$> curScreen
+
+-- | Internal. Refresh-tracking logic of view operations.
+viewWith
+  :: XLike m => (WorkspaceId -> WindowSet -> WindowSet) -> WorkspaceId -> m Any
+viewWith viewer tag = do
+  itag <- curTag
+  when' (tag /= itag) $ do
+    modifyWindowSet' (viewer tag)
+    Any . (tag ==) <$> curTag
+
+-- | A version of @W.view@ that tracks the need to refresh.
+view :: XLike m => WorkspaceId -> m Any
+view = viewWith W.view
+
+-- | A version of @W.greedyView@ that tracks the need to refresh.
+greedyView :: XLike m => WorkspaceId -> m Any
+greedyView = viewWith W.greedyView
+
+-- | View a workspace if it's not visible. An alternative to @view@ and
+--   @greedyView@ that—rather than changing the current screen or affecting
+--   another—opts not to act.
+invisiView :: XLike m => WorkspaceId -> m Any
+invisiView = viewWith $ \tag ws ->
+  if   tag `elem` (W.tag . W.workspace <$> W.current ws : W.visible ws)
+  then W.view tag ws
+  else ws
+
+-- | A refresh-tracking version of @W.Shift@.
+shift :: XLike m => WorkspaceId -> m Any
+shift tag = withFocii $ \ctag fw ->
+  when' (tag /= ctag) $ do
+    modifyWindowSet' (W.shiftWin tag fw)
+    mfw' <- peek
+    return (Any $ Just fw /= mfw')
+
+-- }}}
+

xmonad-contrib.cabal

@@ -329,6 +329,7 @@ library
                         XMonad.Util.NoTaskbar
                         XMonad.Util.Paste
                         XMonad.Util.PositionStore
+                        XMonad.Util.PureX
                         XMonad.Util.Rectangle
                         XMonad.Util.RemoteWindows
                         XMonad.Util.Replace