Data.Proxy

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

Since: 4.7.0.0

Documentation

data Proxy t

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 -> *)

Associated Types

type Rep1 Proxy :: k -> *

Methods

from1 :: Proxy a -> Rep1 Proxy a

to1 :: Rep1 Proxy a -> Proxy a

Monad (Proxy :: * -> *)

Since: 4.7.0.0

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: 4.7.0.0

Methods

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

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

Applicative (Proxy :: * -> *)

Since: 4.7.0.0

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: 4.7.0.0

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: 4.7.0.0

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)

MonadPlus (Proxy :: * -> *)

Since: 4.9.0.0

Methods

mzero :: Proxy a

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

Alternative (Proxy :: * -> *)

Since: 4.9.0.0

Methods

empty :: Proxy a

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

some :: Proxy a -> Proxy [a]

many :: Proxy a -> Proxy [a]

MonadZip (Proxy :: * -> *)

Since: 4.9.0.0

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)

Show1 (Proxy :: * -> *)

Since: 4.9.0.0

Methods

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

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

Read1 (Proxy :: * -> *)

Since: 4.9.0.0

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]

Ord1 (Proxy :: * -> *)

Since: 4.9.0.0

Methods

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

Eq1 (Proxy :: * -> *)

Since: 4.9.0.0

Methods

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

Bounded (Proxy t)

Methods

minBound :: Proxy t

maxBound :: Proxy t

Enum (Proxy s)

Since: 4.7.0.0

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: 4.7.0.0

Methods

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

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

Data t => Data (Proxy t)

Since: 4.7.0.0

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: 4.7.0.0

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: 4.7.0.0

Methods

readsPrec :: Int -> ReadS (Proxy t)

readList :: ReadS [Proxy t]

readPrec :: ReadPrec (Proxy t)

readListPrec :: ReadPrec [Proxy t]

Show (Proxy s)

Since: 4.7.0.0

Methods

showsPrec :: Int -> Proxy s -> ShowS

show :: Proxy s -> String

showList :: [Proxy s] -> ShowS

Ix (Proxy s)

Since: 4.7.0.0

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)

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: 4.9.0.0

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: 4.7.0.0

Methods

mempty :: Proxy s

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

mconcat :: [Proxy s] -> Proxy s

type Rep1 (Proxy :: k -> *)

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

type Rep (Proxy t)

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