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|
{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE PolyKinds #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Proxy
-- License : BSD-style (see the LICENSE file in the distribution)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : portable
--
-- Definition of a Proxy type (poly-kinded in GHC)
--
-- @since 4.7.0.0
-----------------------------------------------------------------------------
module Data.Proxy
(
Proxy(..), asProxyTypeOf
, KProxy(..)
) where
import GHC.Base
import GHC.Show
import GHC.Read
import GHC.Enum
import GHC.Arr
-- | A concrete, poly-kinded proxy type
data Proxy t = Proxy
-- | 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
data KProxy (t :: *) = KProxy
-- It's common to use (undefined :: Proxy t) and (Proxy :: Proxy t)
-- interchangeably, so all of these instances are hand-written to be
-- lazy in Proxy arguments.
instance Eq (Proxy s) where
_ == _ = True
instance Ord (Proxy s) where
compare _ _ = EQ
instance Show (Proxy s) where
showsPrec _ _ = showString "Proxy"
instance Read (Proxy s) where
readsPrec d = readParen (d > 10) (\r -> [(Proxy, s) | ("Proxy",s) <- lex r ])
instance Enum (Proxy s) where
succ _ = error "Proxy.succ"
pred _ = error "Proxy.pred"
fromEnum _ = 0
toEnum 0 = Proxy
toEnum _ = error "Proxy.toEnum: 0 expected"
enumFrom _ = [Proxy]
enumFromThen _ _ = [Proxy]
enumFromThenTo _ _ _ = [Proxy]
enumFromTo _ _ = [Proxy]
instance Ix (Proxy s) where
range _ = [Proxy]
index _ _ = 0
inRange _ _ = True
rangeSize _ = 1
unsafeIndex _ _ = 0
unsafeRangeSize _ = 1
instance Bounded (Proxy s) where
minBound = Proxy
maxBound = Proxy
instance Monoid (Proxy s) where
mempty = Proxy
mappend _ _ = Proxy
mconcat _ = Proxy
instance Functor Proxy where
fmap _ _ = Proxy
{-# INLINE fmap #-}
instance Applicative Proxy where
pure _ = Proxy
{-# INLINE pure #-}
_ <*> _ = Proxy
{-# INLINE (<*>) #-}
instance Monad Proxy where
return _ = Proxy
{-# INLINE return #-}
_ >>= _ = Proxy
{-# INLINE (>>=) #-}
-- | '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.
asProxyTypeOf :: a -> Proxy a -> a
asProxyTypeOf = const
{-# INLINE asProxyTypeOf #-}
|
