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

Data.Proxy

Description

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

Since: base-4.7.0.0

Synopsis

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

Documentation

data Proxy t Source #

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 Either
Proxy

>>> Proxy :: Proxy Functor
Proxy

>>> Proxy :: Proxy complicatedStructure
Proxy

Constructors

Proxy

Instances

Instances details

Generic1 (Proxy :: k -> Type) Source #
Instance details Defined in GHC.Generics Associated Types type Rep1 Proxy :: k -> Type Source # Methods from1 :: forall (a :: k0). Proxy a -> Rep1 Proxy a Source # to1 :: forall (a :: k0). Rep1 Proxy a -> Proxy a Source #
MonadZip (Proxy :: Type -> Type) Source #	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Zip Methods mzip :: Proxy a -> Proxy b -> Proxy (a, b) Source # mzipWith :: (a -> b -> c) -> Proxy a -> Proxy b -> Proxy c Source # munzip :: Proxy (a, b) -> (Proxy a, Proxy b) Source #
Foldable (Proxy :: Type -> Type) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Foldable Methods fold :: Monoid m => Proxy m -> m Source # foldMap :: Monoid m => (a -> m) -> Proxy a -> m Source # foldMap' :: Monoid m => (a -> m) -> Proxy a -> m Source # foldr :: (a -> b -> b) -> b -> Proxy a -> b Source # foldr' :: (a -> b -> b) -> b -> Proxy a -> b Source # foldl :: (b -> a -> b) -> b -> Proxy a -> b Source # foldl' :: (b -> a -> b) -> b -> Proxy a -> b Source # foldr1 :: (a -> a -> a) -> Proxy a -> a Source # foldl1 :: (a -> a -> a) -> Proxy a -> a Source # 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 -> a Source # minimum :: Ord a => Proxy a -> a Source # sum :: Num a => Proxy a -> a Source # product :: Num a => Proxy a -> a Source #
Eq1 (Proxy :: Type -> Type) Source #	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftEq :: (a -> b -> Bool) -> Proxy a -> Proxy b -> Bool Source #
Ord1 (Proxy :: Type -> Type) Source #	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftCompare :: (a -> b -> Ordering) -> Proxy a -> Proxy b -> Ordering Source #
Read1 (Proxy :: Type -> Type) Source #	Since: base-4.9.0.0
Instance details Defined in Data.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 #
Show1 (Proxy :: Type -> Type) Source #	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Proxy a -> ShowS Source # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [Proxy a] -> ShowS Source #
Contravariant (Proxy :: Type -> Type) Source #
Instance details Defined in Data.Functor.Contravariant Methods contramap :: (a' -> a) -> Proxy a -> Proxy a' Source # (>$) :: b -> Proxy b -> Proxy a Source #
Traversable (Proxy :: Type -> Type) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Traversable Methods traverse :: Applicative f => (a -> f b) -> Proxy a -> f (Proxy b) Source # sequenceA :: Applicative f => Proxy (f a) -> f (Proxy a) Source # mapM :: Monad m => (a -> m b) -> Proxy a -> m (Proxy b) Source # sequence :: Monad m => Proxy (m a) -> m (Proxy a) Source #
Alternative (Proxy :: Type -> Type) Source #	Since: base-4.9.0.0
Instance details Defined in Data.Proxy Methods empty :: Proxy a Source # (<\|>) :: Proxy a -> Proxy a -> Proxy a Source # some :: Proxy a -> Proxy [a] Source # many :: Proxy a -> Proxy [a] Source #
Applicative (Proxy :: Type -> Type) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods pure :: a -> Proxy a Source # (<>) :: Proxy (a -> b) -> Proxy a -> Proxy b Source # liftA2 :: (a -> b -> c) -> Proxy a -> Proxy b -> Proxy c Source # (>) :: Proxy a -> Proxy b -> Proxy b Source # (<*) :: Proxy a -> Proxy b -> Proxy a Source #
Functor (Proxy :: Type -> Type) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods fmap :: (a -> b) -> Proxy a -> Proxy b Source # (<$) :: a -> Proxy b -> Proxy a Source #
Monad (Proxy :: Type -> Type) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods (>>=) :: Proxy a -> (a -> Proxy b) -> Proxy b Source # (>>) :: Proxy a -> Proxy b -> Proxy b Source # return :: a -> Proxy a Source #
MonadPlus (Proxy :: Type -> Type) Source #	Since: base-4.9.0.0
Instance details Defined in Data.Proxy Methods mzero :: Proxy a Source # mplus :: Proxy a -> Proxy a -> Proxy a Source #
Data t => Data (Proxy t) Source #	Since: base-4.7.0.0
Instance details Defined in Data.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 t Source # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Proxy t -> r Source # gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Proxy t -> r Source # gmapQ :: (forall d. Data d => d -> u) -> Proxy t -> [u] Source # gmapQi :: Int -> (forall d. Data d => d -> u) -> Proxy t -> u Source # 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 #
Monoid (Proxy s) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods mempty :: Proxy s Source # mappend :: Proxy s -> Proxy s -> Proxy s Source # mconcat :: [Proxy s] -> Proxy s Source #
Semigroup (Proxy s) Source #	Since: base-4.9.0.0
Instance details Defined in Data.Proxy Methods (<>) :: Proxy s -> Proxy s -> Proxy s Source # sconcat :: NonEmpty (Proxy s) -> Proxy s Source # stimes :: Integral b => b -> Proxy s -> Proxy s Source #
Bounded (Proxy t) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods minBound :: Proxy t Source # maxBound :: Proxy t Source #
Enum (Proxy s) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods succ :: Proxy s -> Proxy s Source # pred :: Proxy s -> Proxy s Source # toEnum :: Int -> Proxy s Source # 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 #
Generic (Proxy t) Source #
Instance details Defined in GHC.Generics Associated Types type Rep (Proxy t) :: Type -> Type Source # Methods from :: Proxy t -> Rep (Proxy t) x Source # to :: Rep (Proxy t) x -> Proxy t Source #
Ix (Proxy s) Source #	Since: base-4.7.0.0
Instance details Defined in Data.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 Source # inRange :: (Proxy s, Proxy s) -> Proxy s -> Bool Source # rangeSize :: (Proxy s, Proxy s) -> Int Source # unsafeRangeSize :: (Proxy s, Proxy s) -> Int Source #
Read (Proxy t) Source #	Since: base-4.7.0.0
Instance details Defined in Data.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: base-4.7.0.0
Instance details Defined in Data.Proxy Methods showsPrec :: Int -> Proxy s -> ShowS Source # show :: Proxy s -> String Source # showList :: [Proxy s] -> ShowS Source #
Eq (Proxy s) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods (==) :: Proxy s -> Proxy s -> Bool Source # (/=) :: Proxy s -> Proxy s -> Bool Source #
Ord (Proxy s) Source #	Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods compare :: Proxy s -> Proxy s -> Ordering Source # (<) :: Proxy s -> Proxy s -> Bool Source # (<=) :: Proxy s -> Proxy s -> Bool Source # (>) :: Proxy s -> Proxy s -> Bool Source # (>=) :: Proxy s -> Proxy s -> Bool Source # max :: Proxy s -> Proxy s -> Proxy s Source # min :: Proxy s -> Proxy s -> Proxy s Source #
type Rep1 (Proxy :: k -> Type) Source #	Since: base-4.6.0.0
Instance details Defined in GHC.Generics type Rep1 (Proxy :: k -> Type) = D1 ('MetaData "Proxy" "Data.Proxy" "base" 'False) (C1 ('MetaCons "Proxy" 'PrefixI 'False) (U1 :: k -> Type))
type Rep (Proxy t) Source #	Since: base-4.6.0.0
Instance details Defined in GHC.Generics type Rep (Proxy t) = D1 ('MetaData "Proxy" "Data.Proxy" "base" 'False) (C1 ('MetaCons "Proxy" 'PrefixI 'False) (U1 :: Type -> Type))

asProxyTypeOf :: a -> proxy a -> a Source #

asProxyTypeOf is a type-restricted version of const. 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-case proxy 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

data KProxy (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