just for tracing the monad i have this : import Control.Monad import Data.Ratio import Data.List (all) import Debug.Trace newtype Prob a = Prob { getProb :: [(a,Rational)] } deriving Show instance Functor Prob where fmap f (Prob xs) = trace " Functor Prob " Prob $ map (\(x,p) -> (f x,p)) xs t flatten :: Prob (Prob a) -> Prob a flatten (Prob xs) = trace (" flatten " ++ show xs) Prob $ concat $ map multAll xs where multAll (Prob innerxs,p) = trace " multAll " map (\(x,r) -> (x,p*r)) innerxs instance Applicative Prob where pure = trace " Applicative Prob return " return (<*>) = trace " Applicative Prob ap " ap instance Monad Prob where return x = trace " Monad Prob return " Prob [(x,1%1)] m >>= f = trace " Monad Prob >>= " flatten (fmap f m) fail _ = trace " Monad Prob fail " Prob [] {- instance Applicative Prob where pure a = Prob [(a,1%1)] Prob fs <*> Prob as = Prob [(f a,x*y) | (f,x) <- fs, (a,y) <- as] instance Monad Prob where Prob as >>= f = Prob [(b,x*y) | (a,x) <- as, let Prob bs = f a, (b,y) <- bs] -} in this : flatten :: Prob (Prob a) -> Prob a flatten (Prob xs) = trace (" flatten " ++ show xs) Prob $ concat $ map multAll xs where multAll (Prob innerxs,p) = trace " multAll " map (\(x,r) -> (x,p*r)) innerxs i have this error: [1 of 1] Compiling Main ( monade.hs, interpreted ) monade.hs:22:43: error: • No instance for (Show a) arising from a use of ‘show’ Possible fix: add (Show a) to the context of the type signature for: flatten :: forall a. Prob (Prob a) -> Prob a • In the second argument of ‘(++)’, namely ‘show xs’ In the first argument of ‘trace’, namely ‘(" flatten " ++ show xs)’ In the expression: trace (" flatten " ++ show xs) Prob | 22 | flatten (Prob xs) = trace (" flatten " ++ show xs) | ^^^^^^^ Failed, no modules loaded. how can i implement a show for xs ? regards, damien