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|
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.Monoid
-- Copyright : (c) Andy Gill 2001,
-- (c) Oregon Graduate Institute of Science and Technology, 2001
-- License : BSD-style (see the file libraries/core/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable ( requires mulit-parameter type classes )
--
-- Declaration of the Monoid class,and instances for list and functions
--
-- Inspired by the paper
-- /Functional Programming with Overloading and
-- Higher-Order Polymorphism/,
-- Mark P Jones (<http://www.cse.ogi.edu/~mpj>)
-- Advanced School of Functional Programming, 1995.
-----------------------------------------------------------------------------
module Control.Monad.Monoid (
Monoid(..)
) where
import Prelude
-- ---------------------------------------------------------------------------
-- The Monoid class
class Monoid a where
mempty :: a
mappend :: a -> a -> a
mconcat :: [a] -> a
-- Now the default for mconcat. For most types, this
-- default will be used, but is included in the class definition so
-- that optimized version of mconcat can be provided
-- for specific types.
mconcat = foldr mappend mempty
-- Monoid instances.
instance Monoid [a] where
mempty = []
mappend = (++)
instance Monoid (a -> a) where
mempty = id
mappend = (.)
instance Monoid () where
-- Should it be strict?
mempty = ()
_ `mappend` _ = ()
mconcat _ = ()
|
