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

[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
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