{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances, TypeOperators #-} ----------------------------------------------------------------------------- -- | -- Module : Data.Monoid.Multiplicative -- Copyright : (c) Edward Kmett 2009 -- License : BSD-style -- Maintainer : ekmett@gmail.com -- Stability : experimental -- Portability : portable (but instances use MPTCs) -- -- When dealing with a 'Ring' or other structure, you often need a pair of -- 'Monoid' instances that are closely related. Making a @newtype@ for one -- is unsatisfying and yields an unnatural programming style. -- -- A 'Multiplicative' is a 'Monoid' that is intended for use in a scenario -- that can be extended to have another 'Monoid' slot in for addition. This -- enables one to use common notation. -- -- Any 'Multiplicative' can be turned into a 'Monoid' using the 'Log' wrapper. -- -- Any 'Monoid' can be turned into a 'Multiplicative' using the 'Exp' wrapper. -- -- Instances are supplied for common Monads of Monoids, in a fashion -- which can be extended if the 'Monad' is a 'MonadPlus' to yield a 'RightSemiNearRing' -- -- Instances are also supplied for common Applicatives of Monoids, in a -- fashion which can be extended if the 'Applicative' is 'Alternative' to -- yield a 'RightSemiNearRing' ----------------------------------------------------------------------------- module Data.Monoid.Multiplicative ( Multiplicative , one, times -- * Multiplicative to Monoid , Log(Log, getLog) -- * Monoid to Multiplicative , Exp(Exp, getExp) ) where import Control.Applicative import Control.Monad (liftM2) import Data.Monoid (Monoid, mappend, mempty, Dual(..)) import Data.Generator import Data.Monoid.Self import Data.Ratio import Data.FingerTree import Data.Sequence (Seq) class Multiplicative m where one :: m times :: m -> m -> m instance Multiplicative m => Multiplicative (Dual m) where one = Dual one Dual x `times` Dual y = Dual (y `times` x) -- | Convert a 'Multiplicative' into a 'Monoid'. Mnemonic: @Log a + Log b = Log (a * b)@ data Log m = Log { getLog :: m } instance Multiplicative m => Monoid (Log m) where mempty = Log one Log a `mappend` Log b = Log (a `times` b) -- | Convert a 'Monoid' into a 'Multiplicative'. Mnemonic: @Exp a * Exp b = Exp (a + b)@ data Exp m = Exp { getExp :: m } instance Monoid m => Multiplicative (Exp m) where one = Exp mempty Exp a `times` Exp b = Exp (a `mappend` b) instance Multiplicative m => Multiplicative (Self m) where one = Self one Self a `times` Self b = Self (a `times` b) instance Monoid m => Multiplicative [m] where one = return mempty times = liftM2 mappend instance Monoid m => Multiplicative (Maybe m) where one = return mempty times = liftM2 mappend instance Monoid n => Multiplicative (IO n) where one = return mempty times = liftM2 mappend -- Applicative instances instance Monoid n => Multiplicative (ZipList n) where one = pure mempty times = liftA2 mappend instance Monoid m => Multiplicative (Const m a) where one = pure undefined times = liftA2 undefined -- Numeric instances instance Multiplicative Int where one = 1 times = (*) instance Multiplicative Integer where one = 1 times = (*) instance Integral m => Multiplicative (Ratio m) where one = 1 times = (*) instance Monoid m => Multiplicative (Seq m) where one = return mempty times = liftM2 mappend -- not quite be a Monad in Haskell instance (Measured v m, Monoid m) => Multiplicative (FingerTree v m) where one = singleton mempty xss `times` yss = getSelf $ mapReduce (flip fmap' yss . mappend) xss