Data/Proxy.hs

    {-# 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 #-}