Data/Functor/Const.hs

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

    -----------------------------------------------------------------------------
    -- |
    -- 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.Foldable (Foldable(foldMap))
    import Foreign.Storable (Storable)

    import GHC.Arr (Ix)
    import GHC.Base
    import GHC.Enum (Bounded, Enum)
    import GHC.Generics (Generic, Generic1)
    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 ( Generic, Generic1, Bounded, Enum, Eq, Ix, Ord
                               , Monoid, Storable)

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

    instance Foldable (Const m) where
        foldMap _ _ = mempty

    instance Functor (Const m) where
        fmap _ (Const v) = Const v

    instance Monoid m => Applicative (Const m) where
        pure _ = Const mempty
        (<*>) = 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.