Data/Functor/Const.hs

    {-# LANGUAGE DeriveGeneric #-}
    {-# LANGUAGE GeneralizedNewtypeDeriving #-}
    {-# LANGUAGE NoImplicitPrelude #-}
    {-# LANGUAGE PolyKinds #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE Trustworthy #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Functor.Const
    -- Copyright   :  Conor McBride and Ross Paterson 2005
    -- License     :  BSD-style (see the LICENSE file in the distribution)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  portable

    -- The 'Const' functor.
    --
    -- @since 4.9.0.0

    module Data.Functor.Const (Const(..)) where

    import Data.Bits (Bits, FiniteBits)
    import Data.Foldable (Foldable(foldMap))
    import Foreign.Storable (Storable)

    import GHC.Ix (Ix)
    import GHC.Base
    import GHC.Enum (Bounded, Enum)
    import GHC.Float (Floating, RealFloat)
    import GHC.Generics (Generic, Generic1)
    import GHC.Num (Num)
    import GHC.Real (Fractional, Integral, Real, RealFrac)
    import GHC.Read (Read(readsPrec), readParen, lex)
    import GHC.Show (Show(showsPrec), showParen, showString)

    -- | The 'Const' functor.
    newtype Const a b = Const { getConst :: a }
        deriving ( Bits       -- ^ @since 4.9.0.0
                 , Bounded    -- ^ @since 4.9.0.0
                 , Enum       -- ^ @since 4.9.0.0
                 , Eq         -- ^ @since 4.9.0.0
                 , FiniteBits -- ^ @since 4.9.0.0
                 , Floating   -- ^ @since 4.9.0.0
                 , Fractional -- ^ @since 4.9.0.0
                 , Generic    -- ^ @since 4.9.0.0
                 , Generic1   -- ^ @since 4.9.0.0
                 , Integral   -- ^ @since 4.9.0.0
                 , Ix         -- ^ @since 4.9.0.0
                 , Semigroup  -- ^ @since 4.9.0.0
                 , Monoid     -- ^ @since 4.9.0.0
                 , Num        -- ^ @since 4.9.0.0
                 , Ord        -- ^ @since 4.9.0.0
                 , Real       -- ^ @since 4.9.0.0
                 , RealFrac   -- ^ @since 4.9.0.0
                 , RealFloat  -- ^ @since 4.9.0.0
                 , Storable   -- ^ @since 4.9.0.0
                 )

    -- | This instance would be equivalent to the derived instances of the
    -- 'Const' newtype if the 'getConst' field were removed
    --
    -- @since 4.8.0.0
    instance Read a => Read (Const a b) where
        readsPrec d = readParen (d > 10)
            $ \r -> [(Const x,t) | ("Const", s) <- lex r, (x, t) <- readsPrec 11 s]

    -- | This instance would be equivalent to the derived instances of the
    -- 'Const' newtype if the 'getConst' field were removed
    --
    -- @since 4.8.0.0
    instance Show a => Show (Const a b) where
        showsPrec d (Const x) = showParen (d > 10) $
                                showString "Const " . showsPrec 11 x

    -- | @since 4.7.0.0
    instance Foldable (Const m) where
        foldMap _ _ = mempty

    -- | @since 2.01
    instance Functor (Const m) where
        fmap _ (Const v) = Const v

    -- | @since 2.0.1
    instance Monoid m => Applicative (Const m) where
        pure _ = Const mempty
        liftA2 _ (Const x) (Const y) = Const (x `mappend` y)
        (<*>) = coerce (mappend :: m -> m -> m)
    -- This is pretty much the same as
    -- Const f <*> Const v = Const (f `mappend` v)
    -- but guarantees that mappend for Const a b will have the same arity
    -- as the one for a; it won't create a closure to raise the arity
    -- to 2.