Hierarchify

darcs-hash:20071101201059-a5988-fc1f1262bec1b69e13ba18ae7cefeafc8c4471d4.gz

1 files changed, 45 insertions, 0 deletions

diff --git a/XMonad/Util/Invisible.hs b/XMonad/Util/Invisible.hs
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+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  XMonad.Util.Invisible
+-- Copyright   :  (c) 2007 Andrea Rossato, David Roundy
+-- License     :  BSD-style (see xmonad/LICENSE)
+-- 
+-- Maintainer  :  andrea.rossato@unibz.it, droundy@darcs.net
+-- Stability   :  unstable
+-- Portability :  unportable
+--
+-- A data type to store the layout state
+--
+-----------------------------------------------------------------------------
+
+module XMonad.Util.Invisible ( 
+                             -- * Usage:
+                             -- $usage
+                             Invisible (..)
+                            , whenIJust
+                            , fromIMaybe
+                            ) where
+
+-- $usage
+-- A wrapper data type to store layout state that shouldn't be persisted across
+-- restarts. A common wrapped type to use is @Maybe a@.
+-- Invisible derives trivial definitions for Read and Show, so the wrapped data
+-- type need not do so.
+
+newtype Invisible m a = I (m a) deriving (Monad, Functor)
+
+instance (Functor m, Monad m) => Read (Invisible m a) where
+    readsPrec _ s = [(fail "Read Invisible", s)]
+
+instance Monad m => Show (Invisible m a) where
+    show _ = ""
+
+whenIJust :: (Monad m) => Invisible Maybe a -> (a -> m ()) -> m ()
+whenIJust (I (Just x)) f  = f x
+whenIJust (I  Nothing) _  = return ()
+
+fromIMaybe :: a -> Invisible Maybe a -> a
+fromIMaybe _ (I (Just x)) = x
+fromIMaybe a (I  Nothing) = a