Data/Functor/Constant.hs

-- |
-- 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 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 }

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

instance Foldable (Constant a) where
 foldMap f (Constant x) = mempty

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

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