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Data.Proxy

License BSD-style (see the LICENSE file in the distribution)
Maintainer [email protected]
Stability experimental
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
Generic1 (Proxy :: k -> Type)
Instance details

Defined in GHC.Generics

Associated Types

type Rep1 Proxy :: k -> Type Source

Methods

from1 :: Proxy a -> Rep1 Proxy a Source

to1 :: Rep1 Proxy a -> Proxy a Source

Monad (Proxy :: Type -> Type)

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

fail :: String -> Proxy a Source

Functor (Proxy :: Type -> Type)

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

Applicative (Proxy :: Type -> Type)

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

Foldable (Proxy :: Type -> Type)

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

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

Traversable (Proxy :: Type -> Type)

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

MonadPlus (Proxy :: Type -> Type)

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

Alternative (Proxy :: Type -> Type)

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

MonadZip (Proxy :: Type -> Type)

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

Show1 (Proxy :: Type -> Type)

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

Read1 (Proxy :: Type -> Type)

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

Ord1 (Proxy :: Type -> Type)

Since: base-4.9.0.0
Instance details

Defined in Data.Functor.Classes

Methods

liftCompare :: (a -> b -> Ordering) -> Proxy a -> Proxy b -> Ordering Source

Eq1 (Proxy :: Type -> Type)

Since: base-4.9.0.0
Instance details

Defined in Data.Functor.Classes

Methods

liftEq :: (a -> b -> Bool) -> Proxy a -> Proxy b -> Bool Source

Contravariant (Proxy :: Type -> Type)
Instance details

Defined in Data.Functor.Contravariant

Methods

contramap :: (a -> b) -> Proxy b -> Proxy a Source

(>$) :: b -> Proxy b -> Proxy a Source

Bounded (Proxy t)

Since: base-4.7.0.0
Instance details

Defined in Data.Proxy

Methods

minBound :: Proxy t Source

maxBound :: Proxy t Source

Enum (Proxy s)

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

Eq (Proxy s)

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

Data t => Data (Proxy t)

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 :: (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

Ord (Proxy s)

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

Read (Proxy t)

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)

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

Ix (Proxy s)

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

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)
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

Semigroup (Proxy s)

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

Monoid (Proxy s)

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

type Rep1 (Proxy :: k -> Type)

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)

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/8.6.1/docs/html/libraries/base-4.12.0.0/Data-Proxy.html