-- | -- 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)