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Data.Proxy

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

Description

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

Since: base-4.7.0.0

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

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/9.4.2/docs/libraries/base-4.17.0.0/Data-Proxy.html