That's because Applicative is now a superclass of Monad. See the rationale here: https://wiki.haskell.org/Functor-Applicative-Monad_Proposal On 27/02/2019 10:56, Damien Mattei wrote:
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
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