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{-# LANGUAGE FlexibleInstances, FunctionalDependencies, MultiParamTypeClasses #-}
module T9612 where
import Data.Monoid
import Control.Monad.Trans.Writer.Lazy( Writer, WriterT )
import Data.Functor.Identity( Identity )
class (Monoid w, Monad m) => MonadWriter w m | m -> w where
writer :: (a,w) -> m a
tell :: w -> m ()
listen :: m a -> m (a, w)
pass :: m (a, w -> w) -> m a
f ::(Eq a) => a -> (Int, a) -> Writer [(Int, a)] (Int, a)
f y (n,x) {- | y == x = return (n+1, x)
| otherwise = -}
= do tell (n,x)
return (1,y)
instance (Monoid w, Monad m) => MonadWriter w (WriterT w m) where
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