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{-# LANGUAGE AutoDeriveTypeable #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE EmptyCase #-}
{-# LANGUAGE Safe #-}
{-# LANGUAGE StandaloneDeriving #-}
-----------------------------------------------------------------------------
-- |
-- Copyright : (C) 2008-2014 Edward Kmett
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : Edward Kmett <ekmett@gmail.com>
-- Stability : provisional
-- Portability : portable
--
-- A logically uninhabited data type, used to indicate that a given
-- term should not exist.
--
-- @since 4.8.0.0
----------------------------------------------------------------------------
module Data.Void
( Void
, absurd
, vacuous
) where
import Control.Exception
import Data.Data
import Data.Ix
import GHC.Generics
-- | Uninhabited data type
--
-- @since 4.8.0.0
data Void deriving (Generic)
deriving instance Data Void
instance Eq Void where
_ == _ = True
instance Ord Void where
compare _ _ = EQ
-- | Reading a 'Void' value is always a parse error, considering
-- 'Void' as a data type with no constructors.
instance Read Void where
readsPrec _ _ = []
instance Show Void where
showsPrec _ = absurd
instance Ix Void where
range _ = []
index _ = absurd
inRange _ = absurd
rangeSize _ = 0
instance Exception Void
-- | Since 'Void' values logically don't exist, this witnesses the
-- logical reasoning tool of \"ex falso quodlibet\".
--
-- @since 4.8.0.0
absurd :: Void -> a
absurd a = case a of {}
-- | If 'Void' is uninhabited then any 'Functor' that holds only
-- values of type 'Void' is holding no values.
--
-- @since 4.8.0.0
vacuous :: Functor f => f Void -> f a
vacuous = fmap absurd
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