License | BSD-style (see the LICENSE file in the distribution) |
---|---|
Maintainer | libraries@haskell.org |
Stability | experimental |
Portability | portable |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
Definition of a Proxy type (poly-kinded in GHC)
Since: 4.7.0.0
Documentation
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,
is a safer alternative to the
Proxy
:: Proxy
a
idiom.undefined
:: a
>>>
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
Instances
Generic1 (Proxy :: k -> Type) Source # | Since: 4.6.0.0 |
Monad (Proxy :: Type -> Type) Source # | Since: 4.7.0.0 |
Functor (Proxy :: Type -> Type) Source # | Since: 4.7.0.0 |
Applicative (Proxy :: Type -> Type) Source # | Since: 4.7.0.0 |
Foldable (Proxy :: Type -> Type) Source # | Since: 4.7.0.0 |
Defined in Data.Foldable 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 # | |
Traversable (Proxy :: Type -> Type) Source # | Since: 4.7.0.0 |
MonadPlus (Proxy :: Type -> Type) Source # | Since: 4.9.0.0 |
Alternative (Proxy :: Type -> Type) Source # | Since: 4.9.0.0 |
MonadZip (Proxy :: Type -> Type) Source # | Since: 4.9.0.0 |
Show1 (Proxy :: Type -> Type) Source # | Since: 4.9.0.0 |
Read1 (Proxy :: Type -> Type) Source # | Since: 4.9.0.0 |
Defined in Data.Functor.Classes 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) Source # | Since: 4.9.0.0 |
Defined in Data.Functor.Classes | |
Eq1 (Proxy :: Type -> Type) Source # | Since: 4.9.0.0 |
Contravariant (Proxy :: Type -> Type) Source # | |
Bounded (Proxy t) Source # | Since: 4.7.0.0 |
Enum (Proxy s) Source # | Since: 4.7.0.0 |
Defined in Data.Proxy 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) Source # | Since: 4.7.0.0 |
Data t => Data (Proxy t) Source # | Since: 4.7.0.0 |
Defined in Data.Data 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 # | |
Ord (Proxy s) Source # | Since: 4.7.0.0 |
Read (Proxy t) Source # | Since: 4.7.0.0 |
Show (Proxy s) Source # | Since: 4.7.0.0 |
Ix (Proxy s) Source # | Since: 4.7.0.0 |
Generic (Proxy t) Source # | Since: 4.6.0.0 |
Semigroup (Proxy s) Source # | Since: 4.9.0.0 |
Monoid (Proxy s) Source # | Since: 4.7.0.0 |
type Rep1 (Proxy :: k -> Type) Source # | |
type Rep (Proxy t) Source # | |
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