Proxy

Documentation

data Proxy (t :: k) :: forall k. k -> *

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 -> *)

Defined in GHC.Generics

Associated Types

type Rep1 Proxy :: k -> *

Methods

from1 :: Proxy a -> Rep1 Proxy a

to1 :: Rep1 Proxy a -> Proxy a

Monad (Proxy :: * -> *)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

(>>=) :: Proxy a -> (a -> Proxy b) -> Proxy b

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

return :: a -> Proxy a

fail :: String -> Proxy a

Functor (Proxy :: * -> *)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

fmap :: (a -> b) -> Proxy a -> Proxy b

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

Applicative (Proxy :: * -> *)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

pure :: a -> Proxy a

(<*>) :: Proxy (a -> b) -> Proxy a -> Proxy b

liftA2 :: (a -> b -> c) -> Proxy a -> Proxy b -> Proxy c

(*>) :: Proxy a -> Proxy b -> Proxy b

(<*) :: Proxy a -> Proxy b -> Proxy a

Foldable (Proxy :: * -> *)

Since: base-4.7.0.0

Defined in Data.Foldable

Methods

fold :: Monoid m => Proxy m -> m

foldMap :: Monoid m => (a -> m) -> Proxy a -> m

foldr :: (a -> b -> b) -> b -> Proxy a -> b

foldr' :: (a -> b -> b) -> b -> Proxy a -> b

foldl :: (b -> a -> b) -> b -> Proxy a -> b

foldl' :: (b -> a -> b) -> b -> Proxy a -> b

foldr1 :: (a -> a -> a) -> Proxy a -> a

foldl1 :: (a -> a -> a) -> Proxy a -> a

toList :: Proxy a -> [a]

null :: Proxy a -> Bool

length :: Proxy a -> Int

elem :: Eq a => a -> Proxy a -> Bool

maximum :: Ord a => Proxy a -> a

minimum :: Ord a => Proxy a -> a

sum :: Num a => Proxy a -> a

product :: Num a => Proxy a -> a

Traversable (Proxy :: * -> *)

Since: base-4.7.0.0

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a -> f b) -> Proxy a -> f (Proxy b)

sequenceA :: Applicative f => Proxy (f a) -> f (Proxy a)

mapM :: Monad m => (a -> m b) -> Proxy a -> m (Proxy b)

sequence :: Monad m => Proxy (m a) -> m (Proxy a)

Alternative (Proxy :: * -> *)

Since: base-4.9.0.0

Defined in Data.Proxy

Methods

empty :: Proxy a

(<|>) :: Proxy a -> Proxy a -> Proxy a

some :: Proxy a -> Proxy [a]

many :: Proxy a -> Proxy [a]

Distributive (Proxy :: * -> *)

Defined in Data.Distributive

Methods

distribute :: Functor f => f (Proxy a) -> Proxy (f a)

collect :: Functor f => (a -> Proxy b) -> f a -> Proxy (f b)

distributeM :: Monad m => m (Proxy a) -> Proxy (m a)

collectM :: Monad m => (a -> Proxy b) -> m a -> Proxy (m b)

Contravariant (Proxy :: * -> *)

Defined in Data.Functor.Contravariant

Methods

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

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

Representable (Proxy :: * -> *)

Defined in Data.Functor.Rep

Associated Types

type Rep Proxy :: *

Methods

tabulate :: (Rep Proxy -> a) -> Proxy a

index :: Proxy a -> Rep Proxy -> a

ToJSON1 (Proxy :: * -> *)

Defined in Data.Aeson.Types.ToJSON

Methods

liftToJSON :: (a -> Value) -> ([a] -> Value) -> Proxy a -> Value

liftToJSONList :: (a -> Value) -> ([a] -> Value) -> [Proxy a] -> Value

liftToEncoding :: (a -> Encoding) -> ([a] -> Encoding) -> Proxy a -> Encoding

liftToEncodingList :: (a -> Encoding) -> ([a] -> Encoding) -> [Proxy a] -> Encoding

FromJSON1 (Proxy :: * -> *)

Defined in Data.Aeson.Types.FromJSON

Methods

liftParseJSON :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (Proxy a)

liftParseJSONList :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser [Proxy a]

MonadPlus (Proxy :: * -> *)

Since: base-4.9.0.0

Defined in Data.Proxy

Methods

mzero :: Proxy a

mplus :: Proxy a -> Proxy a -> Proxy a

Eq1 (Proxy :: * -> *)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

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

Ord1 (Proxy :: * -> *)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

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

Read1 (Proxy :: * -> *)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (Proxy a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [Proxy a]

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (Proxy a)

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [Proxy a]

Show1 (Proxy :: * -> *)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Proxy a -> ShowS

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [Proxy a] -> ShowS

MonadZip (Proxy :: * -> *)

Since: base-4.9.0.0

Defined in Control.Monad.Zip

Methods

mzip :: Proxy a -> Proxy b -> Proxy (a, b)

mzipWith :: (a -> b -> c) -> Proxy a -> Proxy b -> Proxy c

munzip :: Proxy (a, b) -> (Proxy a, Proxy b)

Divisible (Proxy :: * -> *)

Defined in Data.Functor.Contravariant.Divisible

Methods

divide :: (a -> (b, c)) -> Proxy b -> Proxy c -> Proxy a

conquer :: Proxy a

Decidable (Proxy :: * -> *)

Defined in Data.Functor.Contravariant.Divisible

Methods

lose :: (a -> Void) -> Proxy a

choose :: (a -> Either b c) -> Proxy b -> Proxy c -> Proxy a

NFData1 (Proxy :: * -> *)

Since: deepseq-1.4.3.0

Defined in Control.DeepSeq

Methods

liftRnf :: (a -> ()) -> Proxy a -> ()

Hashable1 (Proxy :: * -> *)

Defined in Data.Hashable.Class

Methods

liftHashWithSalt :: (Int -> a -> Int) -> Int -> Proxy a -> Int

Apply (Proxy :: * -> *)

Defined in Data.Functor.Bind.Class

Methods

(<.>) :: Proxy (a -> b) -> Proxy a -> Proxy b

(.>) :: Proxy a -> Proxy b -> Proxy b

(<.) :: Proxy a -> Proxy b -> Proxy a

liftF2 :: (a -> b -> c) -> Proxy a -> Proxy b -> Proxy c

Pointed (Proxy :: * -> *)

Defined in Data.Pointed

Methods

point :: a -> Proxy a

Plus (Proxy :: * -> *)

Defined in Data.Functor.Plus

Methods

zero :: Proxy a

Alt (Proxy :: * -> *)

Defined in Data.Functor.Alt

Methods

(<!>) :: Proxy a -> Proxy a -> Proxy a

some :: Applicative Proxy => Proxy a -> Proxy [a]

many :: Applicative Proxy => Proxy a -> Proxy [a]

Bind (Proxy :: * -> *)

Defined in Data.Functor.Bind.Class

Methods

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

join :: Proxy (Proxy a) -> Proxy a

Extend (Proxy :: * -> *)

Defined in Data.Functor.Extend

Methods

duplicated :: Proxy a -> Proxy (Proxy a)

extended :: (Proxy a -> b) -> Proxy a -> Proxy b

Bounded (Proxy t)

Defined in Data.Proxy

Methods

minBound :: Proxy t

maxBound :: Proxy t

Enum (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

succ :: Proxy s -> Proxy s

pred :: Proxy s -> Proxy s

toEnum :: Int -> Proxy s

fromEnum :: Proxy s -> Int

enumFrom :: Proxy s -> [Proxy s]

enumFromThen :: Proxy s -> Proxy s -> [Proxy s]

enumFromTo :: Proxy s -> Proxy s -> [Proxy s]

enumFromThenTo :: Proxy s -> Proxy s -> Proxy s -> [Proxy s]

Eq (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

(==) :: Proxy s -> Proxy s -> Bool

(/=) :: Proxy s -> Proxy s -> Bool

Data t => Data (Proxy t)

Since: base-4.7.0.0

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)

gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Proxy t)

toConstr :: Proxy t -> Constr

dataTypeOf :: Proxy t -> DataType

dataCast1 :: Typeable t0 => (forall d. Data d => c (t0 d)) -> Maybe (c (Proxy t))

dataCast2 :: Typeable t0 => (forall d e. (Data d, Data e) => c (t0 d e)) -> Maybe (c (Proxy t))

gmapT :: (forall b. Data b => b -> b) -> Proxy t -> Proxy t

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Proxy t -> r

gmapQr :: (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Proxy t -> r

gmapQ :: (forall d. Data d => d -> u) -> Proxy t -> [u]

gmapQi :: Int -> (forall d. Data d => d -> u) -> Proxy t -> u

gmapM :: Monad m => (forall d. Data d => d -> m d) -> Proxy t -> m (Proxy t)

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Proxy t -> m (Proxy t)

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Proxy t -> m (Proxy t)

Ord (Proxy s)

Since: base-4.7.0.0

Defined in Data.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)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

readsPrec :: Int -> ReadS (Proxy t)

readList :: ReadS [Proxy t]

readPrec :: ReadPrec (Proxy t)

readListPrec :: ReadPrec [Proxy t]

Show (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

showsPrec :: Int -> Proxy s -> ShowS

show :: Proxy s -> String

showList :: [Proxy s] -> ShowS

Ix (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

range :: (Proxy s, Proxy s) -> [Proxy s]

index :: (Proxy s, Proxy s) -> Proxy s -> Int

unsafeIndex :: (Proxy s, Proxy s) -> Proxy s -> Int

inRange :: (Proxy s, Proxy s) -> Proxy s -> Bool

rangeSize :: (Proxy s, Proxy s) -> Int

unsafeRangeSize :: (Proxy s, Proxy s) -> Int

Generic (Proxy t)

Defined in GHC.Generics

Associated Types

type Rep (Proxy t) :: * -> *

Methods

from :: Proxy t -> Rep (Proxy t) x

to :: Rep (Proxy t) x -> Proxy t

Semigroup (Proxy s)

Since: base-4.9.0.0

Defined in Data.Proxy

Methods

(<>) :: Proxy s -> Proxy s -> Proxy s

sconcat :: NonEmpty (Proxy s) -> Proxy s

stimes :: Integral b => b -> Proxy s -> Proxy s

Monoid (Proxy s)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

mempty :: Proxy s

mappend :: Proxy s -> Proxy s -> Proxy s

mconcat :: [Proxy s] -> Proxy s

Hashable (Proxy a)

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> Proxy a -> Int

hash :: Proxy a -> Int

ToJSON (Proxy a)

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Proxy a -> Value

toEncoding :: Proxy a -> Encoding

toJSONList :: [Proxy a] -> Value

toEncodingList :: [Proxy a] -> Encoding

FromJSON (Proxy a)

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Proxy a)

parseJSONList :: Value -> Parser [Proxy a]

NFData (Proxy a)

Since: deepseq-1.4.0.0

Defined in Control.DeepSeq

Methods

rnf :: Proxy a -> ()

Upper (Proxy a)

Defined in Data.Semilattice.Upper

Methods

upperBound :: Proxy a

Lower (Proxy a)

Defined in Data.Semilattice.Lower

Methods

lowerBound :: Proxy a

Serialise (Proxy a)

Since: serialise-0.2.0.0

Defined in Codec.Serialise.Class

Methods

encode :: Proxy a -> Encoding

decode :: Decoder s (Proxy a)

encodeList :: [Proxy a] -> Encoding

decodeList :: Decoder s [Proxy a]

type Rep1 (Proxy :: k -> *)

Defined in GHC.Generics

type Rep1 (Proxy :: k -> *) = D1 (MetaData "Proxy" "Data.Proxy" "base" False) (C1 (MetaCons "Proxy" PrefixI False) (U1 :: k -> *))

type Rep (Proxy :: * -> *)

Defined in Data.Functor.Rep

type Rep (Proxy :: * -> *) = Void

type Rep (Proxy t)

Defined in GHC.Generics

type Rep (Proxy t) = D1 (MetaData "Proxy" "Data.Proxy" "base" False) (C1 (MetaCons "Proxy" PrefixI False) (U1 :: * -> *))

asProxyTypeOf :: a -> proxy a -> a

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

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