Data/Functor/Constant.hs

    {-# LANGUAGE CPP #-}
    #if __GLASGOW_HASKELL__ >= 709
    {-# LANGUAGE AutoDeriveTypeable #-}
    #endif
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Functor.Constant
    -- Copyright   :  (c) Ross Paterson 2010
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  ross@soi.city.ac.uk
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- The constant functor.
    -----------------------------------------------------------------------------

    module Data.Functor.Constant (
        Constant(..),
      ) where

    import Data.Functor.Classes

    import Control.Applicative
    import Data.Foldable (Foldable(foldMap))
    import Data.Monoid (Monoid(..))
    import Data.Traversable (Traversable(traverse))

    -- | Constant functor.
    newtype Constant a b = Constant { getConstant :: a }
        deriving (Eq, Ord)

    -- These instances would be equivalent to the derived instances of the
    -- newtype if the field were removed.

    instance (Read a) => Read (Constant a b) where
        readsPrec = readsData $ readsUnary "Constant" Constant

    instance (Show a) => Show (Constant a b) where
        showsPrec d (Constant x) = showsUnary "Constant" d x

    instance (Eq a) => Eq1 (Constant a) where eq1 = (==)
    instance (Ord a) => Ord1 (Constant a) where compare1 = compare
    instance (Read a) => Read1 (Constant a) where readsPrec1 = readsPrec
    instance (Show a) => Show1 (Constant a) where showsPrec1 = showsPrec

    instance Functor (Constant a) where
        fmap _ (Constant x) = Constant x

    instance Foldable (Constant a) where
        foldMap _ (Constant _) = mempty

    instance Traversable (Constant a) where
        traverse _ (Constant x) = pure (Constant x)

    instance (Monoid a) => Applicative (Constant a) where
        pure _ = Constant mempty
        Constant x <*> Constant y = Constant (x `mappend` y)