Damien Mattei damien.mattei at gmail.com
Wed Feb 27 09:56:15 UTC 2019

```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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