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

[Sylvain Henry]

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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