[Haskell-cafe] example of monad from http://learnyouahaskell.comnot working

Jos Kusiek jos.kusiek at tu-dortmund.de
Wed Feb 27 10:55:13 UTC 2019

The Monad class has changed, since a few GHC versions. The old class dependency chain was Functor <- Monad, but it is now Functor <- Applicative <- Monad. You can just ignore it and always instanciate it with:

instance Applicative M where
    pure  = return
    (<*>) = ap

Where M is a Monad. So in your case replace "M" with "Prob".

If you are interested in properly instanciating the Applicative class, then the operator (<*>) is the new thing. The function "pure" should always do exactly the same as "return". The function "ap" should also always do the same as (<*>). The type of both is just a bit stricter and limited on Monads and not just on Applictive functors. If you look at the type it is a bit like fmap but with the function to lift "boxed" in a Functor/Applicative/Monad:

(<*>) :: Applicative f => f (a -> b) -> f a -> f b

So what you need to do is unwrap the function the same way as you unwrap the parameter, apply the function to the parameter and then wrap it again:

instance Applicative Prob where
  pure a = Prob [(a,1%1)]
  Prob fs <*> Prob as = Prob [(f a,x*y) | (f,x) <- fs, (a,y) <- as]

If you do the Applicative class first you can skip defining return, since it is defaulted with "return = pure":

instance Monad Prob where
    m >>= f = flatten (fmap f m)


instance Monad Prob where
  Prob as >>= f = Prob [(b,x*y) | (a,x) <- as, let Prob bs = f a, (b,y) <- bs]
Also fail from the Monad class is no longer used. It has been moved to the class MonadFail from the Control.Monad.Fail module:

import Control.Monad.Fail

instance MonadFail Prob where
    fail _ = Prob []

Von: Damien Mattei
Gesendet: Mittwoch, 27. Februar 2019 10:57
An: haskell-cafe
Betreff: [Haskell-cafe] example of monad from http://learnyouahaskell.comnot working

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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