The Monad class has changed, since a few GHC versions. The old class dependency chain was Functor <- Monad, but it is now Functor <- Applicative <- Monad. You can just ignore it and always instanciate it with: instance Applicative M where pure = return (<*>) = ap Where M is a Monad. So in your case replace "M" with "Prob". If you are interested in properly instanciating the Applicative class, then the operator (<*>) is the new thing. The function "pure" should always do exactly the same as "return". The function "ap" should also always do the same as (<*>). The type of both is just a bit stricter and limited on Monads and not just on Applictive functors. If you look at the type it is a bit like fmap but with the function to lift "boxed" in a Functor/Applicative/Monad: (<*>) :: Applicative f => f (a -> b) -> f a -> f b So what you need to do is unwrap the function the same way as you unwrap the parameter, apply the function to the parameter and then wrap it again: 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] If you do the Applicative class first you can skip defining return, since it is defaulted with "return = pure": instance Monad Prob where m >>= f = flatten (fmap f m) or instance Monad Prob where Prob as >>= f = Prob [(b,x*y) | (a,x) <- as, let Prob bs = f a, (b,y) <- bs] Also fail from the Monad class is no longer used. It has been moved to the class MonadFail from the Control.Monad.Fail module: import Control.Monad.Fail instance MonadFail Prob where fail _ = Prob [] Von: Damien Mattei Gesendet: Mittwoch, 27. Februar 2019 10:57 An: haskell-cafe Betreff: [Haskell-cafe] example of monad from http://learnyouahaskell.comnot working i'm trying this example (see code below) from : http://learnyouahaskell.com/for-a-few-monads-more#making-monads when trying to compile this: import Data.Ratio newtype Prob a = Prob { getProb :: [(a,Rational)] } deriving Show instance Functor Prob where fmap f (Prob xs) = Prob $ map (\(x,p) -> (f x,p)) xs thisSituation :: Prob (Prob Char) thisSituation = Prob [( Prob [('a',1%2),('b',1%2)] , 1%4 ) ,( Prob [('c',1%2),('d',1%2)] , 3%4) ] flatten :: Prob (Prob a) -> Prob a flatten (Prob xs) = Prob $ concat $ map multAll xs where multAll (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs instance Monad Prob where return x = Prob [(x,1%1)] m >>= f = flatten (fmap f m) fail _ = Prob [] l1 = Prob [('a',2%3),('b',1%3)] multAllExt :: (Prob a, Rational) -> [(a, Rational)] multAllExt (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs --Main> :type multAllExt --multAllExt :: (Prob a, Rational) -> [(a, Rational)] --Main> multAllExt (l1,1 % 4) --[('a',1 % 6),('b',1 % 12)] i get this error: GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> :load monade.hs [1 of 1] Compiling Main ( monade.hs, interpreted ) monade.hs:21:10: error: • No instance for (Applicative Prob) arising from the superclasses of an instance declaration • In the instance declaration for ‘Monad Prob’ | 21 | instance Monad Prob where | ^^^^^^^^^^ Failed, no modules loaded. it fails when i add the last part of the example: instance Monad Prob where return x = Prob [(x,1%1)] m >>= f = flatten (fmap f m) fail _ = Prob [] seems the Monad needs an instance of the Applicative to be instanciated... what is wrong? regards, Damien