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

Jos Kusiek jos.kusiek at tu-dortmund.de
Wed Feb 27 11:01:24 UTC 2019

That is most likely, because ap is not in Prelude. You need to import Control.Monad.

Von: Damien Mattei
Gesendet: Mittwoch, 27. Februar 2019 11:54
An: Yuji Yamamoto
Cc: haskell-cafe
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 :

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? 

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