[Haskellcafe] show in monad
Damien Mattei
damien.mattei at gmail.com
Fri Mar 1 09:57:32 UTC 2019
it's not clear, redaing back the thread it seems that it is not possible in
a monad with the bind operator to have a display with show and now that
adding the constraint and changing the type of flatten will do it, so i
change the definition of flatten but it does not compile:
flatten :: Show a => 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
Prelude> :load monade.hs
[1 of 1] Compiling Main ( monade.hs, interpreted )
monade.hs:37:13: error:
• No instance for (Show b) arising from a use of ‘flatten’
Possible fix:
add (Show b) to the context of
the type signature for:
(>>=) :: forall a b. Prob a > (a > Prob b) > Prob b
• In the second argument of ‘trace’, namely ‘flatten’
In the expression: trace " Monad Prob >>= " flatten (fmap f m)
In an equation for ‘>>=’:
m >>= f = trace " Monad Prob >>= " flatten (fmap f m)

37  flatten (fmap f m)
 ^^^^^^^
Failed, no modules loaded.
Damien
On Thu, Feb 28, 2019 at 4:58 PM Jos Kusiek <jos.kusiek at tudortmund.de>
wrote:
> You do not need to change the Show instance. The one generated by deriving
> Show is fine. As I said, you need to change the type of flatten and add the
> constraint.
>
> flatten :: Show a => Prob (Prob a) > Prob a
>
> On 28.02.19 15:30, Damien Mattei wrote:
>
> 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 tudortmund.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
>> OttoHahnStraße 12, Raum 3.020
>> 44227 Dortmund
>>
>> Tel.: +49 231755 7523
>>
>>
> 
> Dipl.Inf. Jos Kusiek
>
> Technische Universität Dortmund
> Fakultät 4  Informatik / Lehrstuhl 1  Logik in der Informatik
> OttoHahnStraße 12, Raum 3.020
> 44227 Dortmund
>
> Tel.: +49 231755 7523
>
>
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