[Haskell-cafe] show in monad

[Damien Mattei]

even with a definition of show i can not use it in flatten:
import Control.Monad

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

newtype Prob a = Prob { getProb :: [(a,Rational)] }-- deriving Show



instance Show a => Show (Prob a) where
  show (Prob [(x,r)]) = ((show x) ++ " _ " ++ (show r))


instance Functor Prob where
    fmap f (Prob xs) = trace " Functor Prob "
                       Prob $ map (\(x,p) -> (f x,p)) xs




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 p= " ++ (show p) ++ " ")
                                   map (\(x,r) -> (x,p*r)) innerxs

monade.hs:23:44: 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
   |
23 | flatten (Prob xs) = trace (" flatten " ++ (show xs))
   |                                            ^^^^^^^
Failed, no modules loaded.

it seems show i defined is not in the context of flatten???

damien



On Thu, Feb 28, 2019 at 12:57 PM Jos Kusiek <jos.kusiek at tu-dortmund.de>
wrote:

> 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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>
> --
> Dipl.-Inf. Jos Kusiek
>
> Technische Universität Dortmund
> Fakultät 4 - Informatik / Lehrstuhl 1 - Logik in der Informatik
> Otto-Hahn-Straße 12, Raum 3.020
> 44227 Dortmund
>
> Tel.: +49 231-755 7523
>
>
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