You simply cannot do that. To be more precise, you cannot use show inside the bind operator on Prob (but you could use it in flatten). Deriving Show creates a Show instance which looks something like that:

instance Show a => Show (Prob a) where ...

This instance needs "a" to instanciate Show, so you can only use show with Prob types, where "a" is an instance of Show itself, e. g. Prob Int. Your flatten function does not guarantee that "a" is an instance of Show. The type says, any type for "a" will do it. You can easily restrict that with a class constraint:

flatten :: Show a => Prob (Prob a) -> Prob a

But now you have a problem with the bind operator. You can no longer use flatten here. The bind operator for Prob has the following type:

(>>=) :: Prob a -> (a -> Prob b) -> Prob b

There are no constraints here and you cannot add any constraints. The type is predefined by the Monad class. So it is not guaranteed, that this Prob type has a show function and you cannot guarantee it in any way. So you cannot use show on your first parameter type (Prob a) or your result type (Prob b) inside the bind or any function that is called by bind.

On 28.02.19 11:00, Damien Mattei wrote:

just for tracing the monad i have this :

import Control.Monad

import Data.Ratio
import Data.List (all)
import Debug.Trace

newtype Prob a = Prob { getProb :: [(a,Rational)] } deriving Show
       
instance Functor Prob where
    fmap f (Prob xs) = trace " Functor Prob "
                       Prob $ map (\(x,p) -> (f x,p)) xs

   
t


flatten :: Prob (Prob a) -> Prob a
flatten (Prob xs) = trace (" flatten " ++ show xs)
                    Prob $ concat $ map multAll xs
  where multAll (Prob innerxs,p) = trace " multAll "
                                   map (\(x,r) -> (x,p*r)) innerxs  

 
instance Applicative Prob where
   pure = trace " Applicative Prob return " return
   (<*>) = trace " Applicative Prob ap " ap

instance Monad Prob where
  return x = trace " Monad Prob return "
             Prob [(x,1%1)]
  m >>= f = trace " Monad Prob >>= "
            flatten (fmap f m)
  fail _ = trace " Monad Prob fail "
           Prob []


{-
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]
 

instance Monad Prob where

  Prob as >>= f = Prob [(b,x*y) | (a,x) <- as, let Prob bs = f a, (b,y) <- bs]

-}

in this :

flatten :: Prob (Prob a) -> Prob a
flatten (Prob xs) = trace (" flatten " ++ show xs)
                    Prob $ concat $ map multAll xs
  where multAll (Prob innerxs,p) = trace " multAll "
                                   map (\(x,r) -> (x,p*r)) innerxs  

i have this error:

[1 of 1] Compiling Main             ( monade.hs, interpreted )

monade.hs:22:43: error:
    • No instance for (Show a) arising from a use of ‘show’
      Possible fix:
        add (Show a) to the context of
          the type signature for:
            flatten :: forall a. Prob (Prob a) -> Prob a
    • In the second argument of ‘(++)’, namely ‘show xs’
      In the first argument of ‘trace’, namely ‘(" flatten " ++ show xs)’
      In the expression: trace (" flatten " ++ show xs) Prob
   |
22 | flatten (Prob xs) = trace (" flatten " ++ show xs)
   |                                           ^^^^^^^
Failed, no modules loaded.

how can i implement a show for xs ?

regards,

damien

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