Add X.U.ExtensibleState

XMonad/Util/ExtensibleState.hs

@@ -0,0 +1,109 @@
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  XMonad.Util.ExtensibleState
+-- Copyright   :  (c) Daniel Schoepe 2009
+-- License     :  BSD3-style (see LICENSE)
+--
+-- Maintainer  :  daniel.schoepe@gmail.com
+-- Stability   :  unstable
+-- Portability :  not portable
+--
+-- Module for storing custom mutable state in xmonad.
+--
+-----------------------------------------------------------------------------
+
+module XMonad.Util.ExtensibleState (
+                              -- * Usage
+                              -- $usage
+                              putState
+                              , modifyState
+                              , removeState
+                              , getState
+                              ) where
+
+import Control.Applicative
+import Data.Typeable (typeOf,Typeable,cast)
+import qualified Data.Map as M
+import XMonad.Core
+import Control.Monad.State
+
+-- ---------------------------------------------------------------------
+-- $usage
+--
+-- To utilize this feature in a contrib module create a data type,
+-- and make it an instance of ExtensionClass. You can then use
+-- the functions from this module for storing your data:
+--
+-- > {-# LANGUAGE DeriveDataTypeable #-}
+-- >
+-- > data ListStorage = ListStorage [Integer] deriving Typeable
+-- > instance ExtensionClass ListStorage where
+-- >   initialValue = ListStorage []
+-- >
+-- > .. putState (ListStorage [23,42])
+--
+-- To retrieve the stored data call:
+--
+-- > .. getState
+--
+-- If the type can't be infered from the usage of the retrieved data, you
+-- might need to add an explicit type signature:
+--
+-- > .. getState :: X ListStorage
+--
+-- To make your data persistent between restarts, the data type needs to be
+-- an instance of Read and Show and the instance declaration has to be changed:
+--
+-- > data ListStorage = ListStorage [Integer] deriving (Typeable,Read,Show)
+-- >
+-- > instance ExtensionClass ListStorage where
+-- >   initialValue = ListStorage []
+-- >   extensionType = PersistentExtension
+--
+-- One should take care that the string representation of the chosen type
+-- is unique among the stored values, otherwise it will be overwritten.
+-- Normally these values contain fully qualified module names when deriving Typeable, so
+-- name collisions should not be a problem in most cases.
+-- A module should not try to store common datatypes(e.g. a list of Integers)
+-- without a custom data type as a wrapper to avoid those collisions.
+--
+
+-- | Modify the map of state extensions by applying the given function.
+modifyStateExts :: (M.Map String (Either String StateExtension)
+                   -> M.Map String (Either String StateExtension))
+                -> X ()
+modifyStateExts f = 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.
+modifyState :: ExtensionClass a => (a -> a) -> X ()
+modifyState f = putState =<< f <$> getState
+
+-- | 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)
+putState :: ExtensionClass a => a -> X ()
+putState 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.
+getState :: ExtensionClass a => X a
+getState = getState' undefined -- `trick' to avoid needing -XScopedTypeVariables
+  where toValue val = maybe initialValue id $ cast val
+        getState' :: ExtensionClass a => a -> X a
+        getState' k = do
+          v <- gets $ M.lookup (show . typeOf $ k) . extensibleState
+          return $ case v of
+                     Just (Right (StateExtension val)) -> toValue val
+                     Just (Right (PersistentExtension val)) -> toValue val
+                     Just (Left str) -> case extensionType (undefined `asTypeOf` k) of
+                                         PersistentExtension x -> maybe initialValue id $
+                                             cast =<< safeRead str `asTypeOf` (Just x)
+                                         _ -> initialValue
+                     _ -> initialValue
+        safeRead str = case reads str of
+                         [(x,"")] -> Just x
+                         _ -> Nothing
+
+-- | Remove the value from the extensible state field that has the same type as the supplied argument
+removeState :: ExtensionClass a => a -> X ()
+removeState wit = modifyStateExts $ M.delete (show . typeOf $ wit)