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

Data.Proxy

Description

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

Since: 4.7.0.0

Synopsis

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

Generic1 k (Proxy k) #
Associated Types type Rep1 (Proxy k) (f :: Proxy k -> ) :: k -> Source # Methods from1 :: f a -> Rep1 (Proxy k) f a Source # to1 :: Rep1 (Proxy k) f a -> f a Source #
Monad (Proxy *) #	Since: 4.7.0.0
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 *) #	Since: 4.7.0.0
Methods fmap :: (a -> b) -> Proxy * a -> Proxy * b Source # (<$) :: a -> Proxy * b -> Proxy * a Source #
Applicative (Proxy *) #	Since: 4.7.0.0
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 *) #	Since: 4.7.0.0
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 *) #	Since: 4.7.0.0
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 *) #	Since: 4.9.0.0
Methods mzero :: Proxy * a Source # mplus :: Proxy * a -> Proxy * a -> Proxy * a Source #
Alternative (Proxy *) #	Since: 4.9.0.0
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 *) #	Since: 4.9.0.0
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 *) #	Since: 4.9.0.0
Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Proxy * a -> ShowS Source # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [Proxy * a] -> ShowS Source #
Read1 (Proxy *) #	Since: 4.9.0.0
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 *) #	Since: 4.9.0.0
Methods liftCompare :: (a -> b -> Ordering) -> Proxy * a -> Proxy * b -> Ordering Source #
Eq1 (Proxy *) #	Since: 4.9.0.0
Methods liftEq :: (a -> b -> Bool) -> Proxy * a -> Proxy * b -> Bool Source #
Bounded (Proxy k t) #
Methods minBound :: Proxy k t Source # maxBound :: Proxy k t Source #
Enum (Proxy k s) #	Since: 4.7.0.0
Methods succ :: Proxy k s -> Proxy k s Source # pred :: Proxy k s -> Proxy k s Source # toEnum :: Int -> Proxy k s Source # fromEnum :: Proxy k s -> Int Source # enumFrom :: Proxy k s -> [Proxy k s] Source # enumFromThen :: Proxy k s -> Proxy k s -> [Proxy k s] Source # enumFromTo :: Proxy k s -> Proxy k s -> [Proxy k s] Source # enumFromThenTo :: Proxy k s -> Proxy k s -> Proxy k s -> [Proxy k s] Source #
Eq (Proxy k s) #	Since: 4.7.0.0
Methods (==) :: Proxy k s -> Proxy k s -> Bool Source # (/=) :: Proxy k s -> Proxy k s -> Bool Source #
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) 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 k s) #	Since: 4.7.0.0
Methods compare :: Proxy k s -> Proxy k s -> Ordering Source # (<) :: Proxy k s -> Proxy k s -> Bool Source # (<=) :: Proxy k s -> Proxy k s -> Bool Source # (>) :: Proxy k s -> Proxy k s -> Bool Source # (>=) :: Proxy k s -> Proxy k s -> Bool Source # max :: Proxy k s -> Proxy k s -> Proxy k s Source # min :: Proxy k s -> Proxy k s -> Proxy k s Source #
Read (Proxy k t) #	Since: 4.7.0.0
Methods readsPrec :: Int -> ReadS (Proxy k t) Source # readList :: ReadS [Proxy k t] Source # readPrec :: ReadPrec (Proxy k t) Source # readListPrec :: ReadPrec [Proxy k t] Source #
Show (Proxy k s) #	Since: 4.7.0.0
Methods showsPrec :: Int -> Proxy k s -> ShowS Source # show :: Proxy k s -> String Source # showList :: [Proxy k s] -> ShowS Source #
Ix (Proxy k s) #	Since: 4.7.0.0
Methods range :: (Proxy k s, Proxy k s) -> [Proxy k s] Source # index :: (Proxy k s, Proxy k s) -> Proxy k s -> Int Source # unsafeIndex :: (Proxy k s, Proxy k s) -> Proxy k s -> Int inRange :: (Proxy k s, Proxy k s) -> Proxy k s -> Bool Source # rangeSize :: (Proxy k s, Proxy k s) -> Int Source # unsafeRangeSize :: (Proxy k s, Proxy k s) -> Int
Generic (Proxy k t) #
Associated Types type Rep (Proxy k t) :: * -> * Source # Methods from :: Proxy k t -> Rep (Proxy k t) x Source # to :: Rep (Proxy k t) x -> Proxy k t Source #
Semigroup (Proxy k s) #	Since: 4.9.0.0
Methods (<>) :: Proxy k s -> Proxy k s -> Proxy k s Source # sconcat :: NonEmpty (Proxy k s) -> Proxy k s Source # stimes :: Integral b => b -> Proxy k s -> Proxy k s Source #
Monoid (Proxy k s) #	Since: 4.7.0.0
Methods mempty :: Proxy k s Source # mappend :: Proxy k s -> Proxy k s -> Proxy k s Source # mconcat :: [Proxy k s] -> Proxy k s Source #
type Rep1 k (Proxy k) #
type Rep1 k (Proxy k) = D1 k (MetaData "Proxy" "Data.Proxy" "base" False) (C1 k (MetaCons "Proxy" PrefixI False) (U1 k))
type Rep (Proxy k t) #
type Rep (Proxy k t) = D1 * (MetaData "Proxy" "Data.Proxy" "base" False) (C1 * (MetaCons "Proxy" PrefixI False) (U1 *))

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 :: *) 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