That's solid advice in general, but it's still not going to work here if any of the functions needed for the instance of Graph require the type class constraint.

The same problem comes up some times with the Monad class.  For example, if you wanted to make a monad instance for Set a, you need "(Ord a) => " in the instance functions.

I think associated types can work around this, if Graph used them for the a and b types, but I've never tried that so I don't actually know how it works.

A solution to the monad problem I just mentioned is outlined here as 'restricted monads':

Perhaps you can try either associated types or the restricted monad approach?  Unfortunately, I think both of them require you to change FGL instead of just your code, although maybe not with the restricted monad stuff.  I don't recall how invasive that approach is.

Jason