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