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
Safe Haskell	Safe
Language	Haskell2010

Data.Void

Description

A logically uninhabited data type, used to indicate that a given term should not exist.

Since: base-4.8.0.0

Synopsis

data Void
absurd :: Void -> a
vacuous :: Functor f => f Void -> f a

Documentation

data Void Source #

Uninhabited data type

Since: base-4.8.0.0

Instances

Instances details

Eq Void #	Since: base-4.8.0.0
Instance details Defined in Data.Void Methods (==) :: Void -> Void -> Bool Source # (/=) :: Void -> Void -> Bool Source #
Data Void #	Since: base-4.8.0.0
Instance details Defined in Data.Void Methods gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Void -> c Void Source # gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c Void Source # toConstr :: Void -> Constr Source # dataTypeOf :: Void -> DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c Void) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c Void) Source # gmapT :: (forall b. Data b => b -> b) -> Void -> Void Source # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Void -> r Source # gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Void -> r Source # gmapQ :: (forall d. Data d => d -> u) -> Void -> [u] Source # gmapQi :: Int -> (forall d. Data d => d -> u) -> Void -> u Source # gmapM :: Monad m => (forall d. Data d => d -> m d) -> Void -> m Void Source # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Void -> m Void Source # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Void -> m Void Source #
Ord Void #	Since: base-4.8.0.0
Instance details Defined in Data.Void Methods compare :: Void -> Void -> Ordering Source # (<) :: Void -> Void -> Bool Source # (<=) :: Void -> Void -> Bool Source # (>) :: Void -> Void -> Bool Source # (>=) :: Void -> Void -> Bool Source # max :: Void -> Void -> Void Source # min :: Void -> Void -> Void Source #
Read Void #	Reading a `Void` value is always a parse error, considering `Void` as a data type with no constructors. Since: base-4.8.0.0
Instance details Defined in Data.Void Methods readsPrec :: Int -> ReadS Void Source # readList :: ReadS [Void] Source # readPrec :: ReadPrec Void Source # readListPrec :: ReadPrec [Void] Source #
Show Void #	Since: base-4.8.0.0
Instance details Defined in Data.Void Methods showsPrec :: Int -> Void -> ShowS Source # show :: Void -> String Source # showList :: [Void] -> ShowS Source #
Ix Void #	Since: base-4.8.0.0
Instance details Defined in Data.Void Methods range :: (Void, Void) -> [Void] Source # index :: (Void, Void) -> Void -> Int Source # unsafeIndex :: (Void, Void) -> Void -> Int Source # inRange :: (Void, Void) -> Void -> Bool Source # rangeSize :: (Void, Void) -> Int Source # unsafeRangeSize :: (Void, Void) -> Int Source #
Generic Void #
Instance details Defined in Data.Void Associated Types type Rep Void :: Type -> Type Source # Methods from :: Void -> Rep Void x Source # to :: Rep Void x -> Void Source #
Semigroup Void #	Since: base-4.9.0.0
Instance details Defined in Data.Void Methods (<>) :: Void -> Void -> Void Source # sconcat :: NonEmpty Void -> Void Source # stimes :: Integral b => b -> Void -> Void Source #
Exception Void #	Since: base-4.8.0.0
Instance details Defined in Data.Void Methods toException :: Void -> SomeException Source # fromException :: SomeException -> Maybe Void Source # displayException :: Void -> String Source #
type Rep Void #	Since: base-4.8.0.0
Instance details Defined in Data.Void type Rep Void = D1 ('MetaData "Void" "Data.Void" "base" 'False) (V1 :: Type -> Type)

absurd :: Void -> a Source #

Since Void values logically don't exist, this witnesses the logical reasoning tool of "ex falso quodlibet".

>>> let x :: Either Void Int; x = Right 5
>>> :{
case x of
    Right r -> r
    Left l  -> absurd l
:}
5

Since: base-4.8.0.0

vacuous :: Functor f => f Void -> f a Source #

If Void is uninhabited then any Functor that holds only values of type Void is holding no values.

Using ApplicativeDo: 'vacuous theVoid' can be understood as the do expression

do void <- theVoid
   pure (absurd void)

with an inferred Functor constraint.

Since: base-4.8.0.0