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

[Damien Mattei]

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 <whosekiteneverfly at gmail.com>
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 <damien.mattei at gmail.com>:
>
>> 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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>
>
>
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