Correction by the author:
It seems that ghc doesn't like the fact that I am saying 'foo' must
return a class 'b' of typeclass 'Bar', while providing a function that
returns a concrete data instance of 'Bar' (viz., FU or FI) later on
when I implement 'foo' in each type classes.
Should read:
In your class, you have:
class Cls c where
foo :: (Bar b) = c - b
There's an implicit forall for b, meaning that the caller of the
method gets to choose what it wants for b (as long as it's an instance
of Bar). For you to be able to write such a method you'd need to write
functions that can
