License	BSD-style (see the LICENSE file in the distribution)
Maintainer	libraries@haskell.org
Stability	experimental
Portability	portable
Safe Haskell	Trustworthy
Language	Haskell2010

Data.Proxy

Description

Definition of a Proxy type (poly-kinded in GHC)

Since: 4.7.0.0

Synopsis

dataProxy t =Proxy
asProxyTypeOf :: a -> proxy a -> a
dataKProxy (t ::Type) =KProxy

Documentation

dataProxy tSource #

Proxy is a type that holds no data, but has a phantom parameter of arbitrary type (or even kind). Its use is to provide type information, even though there is no value available of that type (or it may be too costly to create one).

Historically,Proxy ::Proxy a is a safer alternative to the'undefined :: a' idiom.

>>>Proxy :: Proxy (Void, Int -> Int)Proxy

Proxy can even hold types of higher kinds,

>>>Proxy :: Proxy EitherProxy

>>>Proxy :: Proxy FunctorProxy

>>>Proxy :: Proxy complicatedStructureProxy

Constructors

Proxy

Instances

Generic1 (Proxy :: k ->Type)Source #
Instance details Defined inGHC.Generics Associated Types typeRep1 Proxy :: k ->Type Source # Methods from1 ::Proxy a ->Rep1 Proxy aSource # to1 ::Rep1 Proxy a ->Proxy aSource #
Monad (Proxy ::Type ->Type)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods (>>=) ::Proxy a -> (a ->Proxy b) ->Proxy bSource # (>>) ::Proxy a ->Proxy b ->Proxy bSource # return :: a ->Proxy aSource # fail ::String ->Proxy aSource #
Functor (Proxy ::Type ->Type)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods fmap :: (a -> b) ->Proxy a ->Proxy bSource # (<$) :: a ->Proxy b ->Proxy aSource #
Applicative (Proxy ::Type ->Type)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods pure :: a ->Proxy aSource # (<>) ::Proxy (a -> b) ->Proxy a ->Proxy bSource # liftA2 :: (a -> b -> c) ->Proxy a ->Proxy b ->Proxy cSource # (>) ::Proxy a ->Proxy b ->Proxy bSource # (<*) ::Proxy a ->Proxy b ->Proxy aSource #
Foldable (Proxy ::Type ->Type)Source #	Since: 4.7.0.0
Instance details Defined inData.Foldable Methods fold ::Monoid m =>Proxy m -> mSource # foldMap ::Monoid m => (a -> m) ->Proxy a -> mSource # foldr :: (a -> b -> b) -> b ->Proxy a -> bSource # foldr' :: (a -> b -> b) -> b ->Proxy a -> bSource # foldl :: (b -> a -> b) -> b ->Proxy a -> bSource # foldl' :: (b -> a -> b) -> b ->Proxy a -> bSource # foldr1 :: (a -> a -> a) ->Proxy a -> aSource # foldl1 :: (a -> a -> a) ->Proxy a -> aSource # toList ::Proxy a -> [a]Source # null ::Proxy a ->Bool Source # length ::Proxy a ->Int Source # elem ::Eq a => a ->Proxy a ->Bool Source # maximum ::Ord a =>Proxy a -> aSource # minimum ::Ord a =>Proxy a -> aSource # sum ::Num a =>Proxy a -> aSource # product ::Num a =>Proxy a -> aSource #
Traversable (Proxy ::Type ->Type)Source #	Since: 4.7.0.0
Instance details Defined inData.Traversable Methods traverse ::Applicative f => (a -> f b) ->Proxy a -> f (Proxy b)Source # sequenceA ::Applicative f =>Proxy (f a) -> f (Proxy a)Source # mapM ::Monad m => (a -> m b) ->Proxy a -> m (Proxy b)Source # sequence ::Monad m =>Proxy (m a) -> m (Proxy a)Source #
MonadPlus (Proxy ::Type ->Type)Source #	Since: 4.9.0.0
Instance details Defined inData.Proxy Methods mzero ::Proxy aSource # mplus ::Proxy a ->Proxy a ->Proxy aSource #
Alternative (Proxy ::Type ->Type)Source #	Since: 4.9.0.0
Instance details Defined inData.Proxy Methods empty ::Proxy aSource # (<\|>) ::Proxy a ->Proxy a ->Proxy aSource # some ::Proxy a ->Proxy [a]Source # many ::Proxy a ->Proxy [a]Source #
MonadZip (Proxy ::Type ->Type)Source #	Since: 4.9.0.0
Instance details Defined inControl.Monad.Zip Methods mzip ::Proxy a ->Proxy b ->Proxy (a, b)Source # mzipWith :: (a -> b -> c) ->Proxy a ->Proxy b ->Proxy cSource # munzip ::Proxy (a, b) -> (Proxy a,Proxy b)Source #
Show1 (Proxy ::Type ->Type)Source #	Since: 4.9.0.0
Instance details Defined inData.Functor.Classes Methods liftShowsPrec :: (Int -> a ->ShowS) -> ([a] ->ShowS) ->Int ->Proxy a ->ShowS Source # liftShowList :: (Int -> a ->ShowS) -> ([a] ->ShowS) -> [Proxy a] ->ShowS Source #
Read1 (Proxy ::Type ->Type)Source #	Since: 4.9.0.0
Instance details Defined inData.Functor.Classes Methods liftReadsPrec :: (Int ->ReadS a) ->ReadS [a] ->Int ->ReadS (Proxy a)Source # liftReadList :: (Int ->ReadS a) ->ReadS [a] ->ReadS [Proxy a]Source # liftReadPrec ::ReadPrec a ->ReadPrec [a] ->ReadPrec (Proxy a)Source # liftReadListPrec ::ReadPrec a ->ReadPrec [a] ->ReadPrec [Proxy a]Source #
Ord1 (Proxy ::Type ->Type)Source #	Since: 4.9.0.0
Instance details Defined inData.Functor.Classes Methods liftCompare :: (a -> b ->Ordering) ->Proxy a ->Proxy b ->Ordering Source #
Eq1 (Proxy ::Type ->Type)Source #	Since: 4.9.0.0
Instance details Defined inData.Functor.Classes Methods liftEq :: (a -> b ->Bool) ->Proxy a ->Proxy b ->Bool Source #
Contravariant (Proxy ::Type ->Type)Source #
Instance details Defined inData.Functor.Contravariant Methods contramap :: (a -> b) ->Proxy b ->Proxy aSource # (>$) :: b ->Proxy b ->Proxy aSource #
Bounded (Proxy t)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods minBound ::Proxy tSource # maxBound ::Proxy tSource #
Enum (Proxy s)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods succ ::Proxy s ->Proxy sSource # pred ::Proxy s ->Proxy sSource # toEnum ::Int ->Proxy sSource # fromEnum ::Proxy s ->Int Source # enumFrom ::Proxy s -> [Proxy s]Source # enumFromThen ::Proxy s ->Proxy s -> [Proxy s]Source # enumFromTo ::Proxy s ->Proxy s -> [Proxy s]Source # enumFromThenTo ::Proxy s ->Proxy s ->Proxy s -> [Proxy s]Source #
Eq (Proxy s)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods (==) ::Proxy s ->Proxy s ->Bool # (/=) ::Proxy s ->Proxy s ->Bool #
Data t =>Data (Proxy t)Source #	Since: 4.7.0.0
Instance details Defined inData.Data Methods gfoldl :: (forall d b.Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) ->Proxy t -> c (Proxy t)Source # gunfold :: (forall b r.Data b => c (b -> r) -> c r) -> (forall r. r -> c r) ->Constr -> c (Proxy t)Source # toConstr ::Proxy t ->Constr Source # dataTypeOf ::Proxy t ->DataType Source # dataCast1 ::Typeable t0 => (forall d.Data d => c (t0 d)) ->Maybe (c (Proxy t))Source # dataCast2 ::Typeable t0 => (forall d e. (Data d,Data e) => c (t0 d e)) ->Maybe (c (Proxy t))Source # gmapT :: (forall b.Data b => b -> b) ->Proxy t ->Proxy tSource # gmapQl :: (r -> r' -> r) -> r -> (forall d.Data d => d -> r') ->Proxy t -> rSource # gmapQr :: (r' -> r -> r) -> r -> (forall d.Data d => d -> r') ->Proxy t -> rSource # gmapQ :: (forall d.Data d => d -> u) ->Proxy t -> [u]Source # gmapQi ::Int -> (forall d.Data d => d -> u) ->Proxy t -> uSource # gmapM ::Monad m => (forall d.Data d => d -> m d) ->Proxy t -> m (Proxy t)Source # gmapMp ::MonadPlus m => (forall d.Data d => d -> m d) ->Proxy t -> m (Proxy t)Source # gmapMo ::MonadPlus m => (forall d.Data d => d -> m d) ->Proxy t -> m (Proxy t)Source #
Ord (Proxy s)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods compare ::Proxy s ->Proxy s ->Ordering # (<) ::Proxy s ->Proxy s ->Bool # (<=) ::Proxy s ->Proxy s ->Bool # (>) ::Proxy s ->Proxy s ->Bool # (>=) ::Proxy s ->Proxy s ->Bool # max ::Proxy s ->Proxy s ->Proxy s# min ::Proxy s ->Proxy s ->Proxy s#
Read (Proxy t)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods readsPrec ::Int ->ReadS (Proxy t)Source # readList ::ReadS [Proxy t]Source # readPrec ::ReadPrec (Proxy t)Source # readListPrec ::ReadPrec [Proxy t]Source #
Show (Proxy s)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods showsPrec ::Int ->Proxy s ->ShowS Source # show ::Proxy s ->String Source # showList :: [Proxy s] ->ShowS Source #
Ix (Proxy s)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods range :: (Proxy s,Proxy s) -> [Proxy s]Source # index :: (Proxy s,Proxy s) ->Proxy s ->Int Source # unsafeIndex :: (Proxy s,Proxy s) ->Proxy s ->Int inRange :: (Proxy s,Proxy s) ->Proxy s ->Bool Source # rangeSize :: (Proxy s,Proxy s) ->Int Source # unsafeRangeSize :: (Proxy s,Proxy s) ->Int
Generic (Proxy t)Source #
Instance details Defined inGHC.Generics Associated Types typeRep (Proxy t) ::Type ->Type Source # Methods from ::Proxy t ->Rep (Proxy t) xSource # to ::Rep (Proxy t) x ->Proxy tSource #
Semigroup (Proxy s)Source #	Since: 4.9.0.0
Instance details Defined inData.Proxy Methods (<>) ::Proxy s ->Proxy s ->Proxy sSource # sconcat ::NonEmpty (Proxy s) ->Proxy sSource # stimes ::Integral b => b ->Proxy s ->Proxy sSource #
Monoid (Proxy s)Source #	Since: 4.7.0.0
Instance details Defined inData.Proxy Methods mempty ::Proxy sSource # mappend ::Proxy s ->Proxy s ->Proxy sSource # mconcat :: [Proxy s] ->Proxy sSource #
typeRep1 (Proxy :: k ->Type)Source #	Since: 4.6.0.0
Instance details Defined inGHC.Generics typeRep1 (Proxy :: k ->Type) =D1 (MetaData "Proxy" "Data.Proxy" "base"False) (C1 (MetaCons "Proxy"PrefixI False) (U1 :: k ->Type))
typeRep (Proxy t)Source #	Since: 4.6.0.0
Instance details Defined inGHC.Generics typeRep (Proxy t) =D1 (MetaData "Proxy" "Data.Proxy" "base"False) (C1 (MetaCons "Proxy"PrefixI False) (U1 ::Type ->Type))

asProxyTypeOf :: a -> proxy a -> aSource #

asProxyTypeOf is a type-restricted version ofconst. It is usually used as an infix operator, and its typing forces its first argument (which is usually overloaded) to have the same type as the tag of the second.

>>>import Data.Word>>>:type asProxyTypeOf 123 (Proxy :: Proxy Word8)asProxyTypeOf 123 (Proxy :: Proxy Word8) :: Word8

Note the lower-caseproxy in the definition. This allows any type constructor with just one argument to be passed to the function, for example we could also write

>>>import Data.Word>>>:type asProxyTypeOf 123 (Just (undefined :: Word8))asProxyTypeOf 123 (Just (undefined :: Word8)) :: Word8

dataKProxy (t ::Type)Source #

A concrete, promotable proxy type, for use at the kind level There are no instances for this because it is intended at the kind level only

Constructors

KProxy

Movatterモバイル変換

Documentation