recursive group context bug?

[Keean Schupke]

You cannot sequence two operations from different monads...

p has type: m (IO ())
id has type, IO () (in this case because this is what p returns)...

You can do:

    p :: (Monad m) => m (IO ())
    p = q >>= (\a -> return a)

Or

    p :: (Monad m) => m (IO ())
    p = run q >>= id -- provided an overloaded definition of run is 
provided for 'm'


    Keean.



Ashley Yakeley wrote:

>I suspect someone's come across this before, so maybe there's an 
>explanation for it.
>
>This does not compile:
>
>module Bug where
>{
>    p :: IO ();
>    p = q >>= id;
>
>    q :: (Monad m) => m (IO ());
>    q = return p;
>}
>
>Bug.hs:3:
>    Mismatched contexts
>    When matching the contexts of the signatures for
>      p :: IO ()
>      q :: forall m. (Monad m) => m (IO ())
>    The signature contexts in a mutually recursive group should all be 
>identical
>    When generalising the type(s) for p, q
>
>
>The code looks correct to me. Why must the signature contexts be 
>identical in this case?
>
>  
>