Damien Mattei damien.mattei at gmail.com
Wed Feb 27 23:05:44 UTC 2019

```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
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
>
>
>
> *Von: *Damien Mattei <damien.mattei at gmail.com>
> *Gesendet: *Mittwoch, 27. Februar 2019 11:54
> *An: *Yuji Yamamoto <whosekiteneverfly at gmail.com>
>
>
>
> 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 :
>
>
>
>
> 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
>
>
>   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
> [1 of 1] Compiling Main             ( monade.hs, interpreted )
>
>     • 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
>    |          ^^^^^^^^^^
>
>
>
> it fails when i add the last part of the example:
>
>
>
>   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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