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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:<br>
<br>
instance Show a => Show (Prob a) where ...<br>
<br>
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:<br>
<br>
flatten :: Show a => Prob (Prob a) -> Prob a<br>
<br>
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:<br>
<br>
(>>=) :: Prob a -> (a -> Prob b) -> Prob b<br>
<br>
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.<br>
<br>
<div class="moz-cite-prefix">On 28.02.19 11:00, Damien Mattei wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CADEOadczGwo-kw0NaGg1kNY9B6wOb+N8HL2Nw7=7C9LEC1Ts+g@mail.gmail.com">
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<div class="gmail_default" style="font-size:large">just for
tracing the monad i have this :</div>
<div class="gmail_default" style="font-size:large"><br>
</div>
<div class="gmail_default" style="font-size:large">import
Control.Monad<br>
<br>
import Data.Ratio<br>
import Data.List (all)<br>
import Debug.Trace<br>
<br>
newtype Prob a = Prob { getProb :: [(a,Rational)] }
deriving Show<br>
<br>
instance Functor Prob where<br>
fmap f (Prob xs) = trace " Functor Prob "<br>
Prob $ map (\(x,p) -> (f x,p))
xs<br>
<br>
<br>
t<br>
<br>
<br>
flatten :: Prob (Prob a) -> Prob a<br>
flatten (Prob xs) = trace (" flatten " ++ show xs)<br>
Prob $ concat $ map multAll xs<br>
where multAll (Prob innerxs,p) = trace " multAll "<br>
map (\(x,r) ->
(x,p*r)) innerxs <br>
<br>
<br>
instance Applicative Prob where<br>
pure = trace " Applicative Prob return " return<br>
(<*>) = trace " Applicative Prob ap " ap<br>
<br>
instance Monad Prob where<br>
return x = trace " Monad Prob return "<br>
Prob [(x,1%1)]<br>
m >>= f = trace " Monad Prob >>= "<br>
flatten (fmap f m)<br>
fail _ = trace " Monad Prob fail "<br>
Prob []<br>
<br>
<br>
{-<br>
instance Applicative Prob where<br>
<br>
pure a = Prob [(a,1%1)]<br>
<br>
Prob fs <*> Prob as = Prob [(f a,x*y) | (f,x)
<- fs, (a,y) <- as]<br>
<br>
<br>
instance Monad Prob where<br>
<br>
Prob as >>= f = Prob [(b,x*y) | (a,x) <- as,
let Prob bs = f a, (b,y) <- bs]<br>
<br>
-}</div>
<div class="gmail_default" style="font-size:large"><br>
</div>
<div class="gmail_default" style="font-size:large"><br>
</div>
<div class="gmail_default" style="font-size:large"><br>
</div>
<div class="gmail_default" style="font-size:large">in this :</div>
<div class="gmail_default" style="font-size:large"><br>
</div>
<div class="gmail_default" style="font-size:large">flatten
:: Prob (Prob a) -> Prob a<br>
flatten (Prob xs) = trace (" flatten " ++ show xs)<br>
Prob $ concat $ map multAll xs<br>
where multAll (Prob innerxs,p) = trace " multAll "<br>
map (\(x,r) ->
(x,p*r)) innerxs <br>
</div>
<div class="gmail_default" style="font-size:large"><br>
</div>
<div class="gmail_default" style="font-size:large"><br>
</div>
<div class="gmail_default" style="font-size:large">i have
this error:</div>
<div class="gmail_default" style="font-size:large"><br>
</div>
<div class="gmail_default" style="font-size:large">[1 of 1]
Compiling Main ( monade.hs, interpreted )<br>
<br>
monade.hs:22:43: error:<br>
• No instance for (Show a) arising from a use of
‘show’<br>
Possible fix:<br>
add (Show a) to the context of<br>
the type signature for:<br>
flatten :: forall a. Prob (Prob a) -> Prob
a<br>
• In the second argument of ‘(++)’, namely ‘show xs’<br>
In the first argument of ‘trace’, namely ‘(" flatten
" ++ show xs)’<br>
In the expression: trace (" flatten " ++ show xs)
Prob<br>
|<br>
22 | flatten (Prob xs) = trace (" flatten " ++ show xs)<br>
| ^^^^^^^<br>
Failed, no modules loaded.<br>
</div>
<div class="gmail_default" style="font-size:large"><br>
</div>
<div class="gmail_default" style="font-size:large">how can i
implement a show for xs ?</div>
<div class="gmail_default" style="font-size:large">regards,</div>
<div class="gmail_default" style="font-size:large">damien<br>
</div>
</div>
</div>
</div>
<br>
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</blockquote>
<br>
<pre class="moz-signature" cols="72">--
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</pre>
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