fix

gistfile1.hs

import Control.Applicative
import Control.Monad.Fix

data CounterRep = CounterRep { x :: Int }

data Counter = Counter { get :: Int,
                         set :: Int -> Counter,
                         inc :: Counter }

counterClass = \rep ->
                 \this ->  
                   Counter { get = x rep,
                             set = newCounter,
                             inc = newCounter $ (get this) + 1 } 

newCounter x = fix (counterClass $ CounterRep x)