Re: [Haskell-cafe] example of monad from http://learnyouahaskell.com not working

can you give me a complete solution please? i suppose i can set pure = return but have some diffculties with <*> , ap does not works On Wed, Feb 27, 2019 at 11:10 AM Yuji Yamamoto wrote:

It's the one of the biggest changes of Haskell since LYHG was released. As you guess, now any instance of Monad must be an instance of Applicative.

So you have to declare Prob as an instance of Applicative:

instance Applicative Prob where pure = ... f <*> x = ...

2019年2月27日(水) 18:56 Damien Mattei :

...

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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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