Hello, after succeeding in implementing my first monad (Counter, it increments a counter every time a computation is performed) I though I'd try another one and went on to implement "Tracker". "Tracker" is a monad where a list consisting of the result of every computation is kept alongside the final value, kind of a computation history. It really just serves me as an exercise to implement monads. However, the following source code fails: {-------------------------------------------------} data Tracker a b = Tracker [a] b deriving Show instance Monad (Tracker a) where m >>= f = let Tracker l x = m in let Tracker l' x' = f x in Tracker (x':l) x' return x = Tracker [] x bar = do x <- Tracker [] 12 y <- return (x*2) z <- return (y*3) return (z+3) {-------------------------------------------------} Of course, style recommendations and the like are always appreciated. (by the way, I don't really understand why I have to type instance Monad (Tracker a) instead of instance Monad Tracker which may very well be the problem. If it's not, can someone tell me anyway?) Trying to load this piece of code leads to the following error message: Hugs.Base> :load Test.hs ERROR "Test.hs":30 - Inferred type is not general enough *** Expression : (>>=) *** Expected type : Monad (Tracker a) => Tracker a b -> (b -> Tracker a c) -> Tracker a c *** Inferred type : Monad (Tracker a) => Tracker a b -> (b -> Tracker a a) -> Tracker a a Why does the interpreter infer Tracker a a instead of the more general Tracker a c? Thanks, Julien