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

[Damien Mattei]

thanks for your code, pretty example, i understand well flatten and all the
function but at last i cannot figure out how the result come....
in the example:
flipThree :: Prob Bool
flipThree = do
    a <- coin
    b <- coin
    c <- loadedCoin
    return (all (==Tails) [a,b,c])

the strange thing is if i add more
variables as :
d <- coin
it oes not change the probability but the output yes!
example:
Prob {getProb = [(True,1 % 80),(True,1 % 80),(True,9 % 80),(True,9 %
80),(True,1 % 80),(True,1 % 80),(True,9 % 80),(True,9 % 80),(True,1 %
80),(True,1 % 80),(True,9 % 80),(True,9 % 80),(True,1 % 80),(True,1 %
80),(True,9 % 80),(True,9 % 80)]}

On Wed, Feb 27, 2019 at 12:01 PM Jos Kusiek <jos.kusiek at tu-dortmund.de>
wrote:

> That is most likely, because ap is not in Prelude. You need to import
> Control.Monad.
>
>
>
> *Von: *Damien Mattei <damien.mattei at gmail.com>
> *Gesendet: *Mittwoch, 27. Februar 2019 11:54
> *An: *Yuji Yamamoto <whosekiteneverfly at gmail.com>
> *Cc: *haskell-cafe <haskell-cafe at haskell.org>
> *Betreff: *Re: [Haskell-cafe] example of monad fromhttp://
> 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 <
> 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
>
> _______________________________________________
> 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.
>
>
>
> --
>
> 山本悠滋
> twitter: https://twitter.com/igrep
> GitHub: https://github.com/igrep
>
> GitLab: https://gitlab.com/igrep
> Facebook: http://www.facebook.com/igrep
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20190228/4ed63622/attachment.html>