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-- |
-- 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)
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