[Haskell-beginners] several questions: multi-param typeclasses,
mpm at alumni.caltech.edu
Sun Nov 1 11:32:34 EST 2009
Brent Yorgey wrote:
> The m -> e thing isn't a constraint that needs to be satisfied; it
> gives some extra information to help the compiler with inferring which
> instance to use. In particular, "m -> e" says "the type chosen for m
> DETERMINES the type chosen for e"; put another way, "there cannot be
> two instances with the same type for m but different types for e". So
> in this case you could not also make an instance
> MonadError String (Either e).
Thanks, Brent. Now what I'm a bit confused about:
if you wrote
instance (Error e) => MonadError e (Either e)
and no other instance with Either e, then the compiler would have only one
choice. So why would it need the extra information in the functional
On the other hand, if you added
instance (Error e) => MonadError String (Either e)
and didn't include the functional dependency, the compiler would still run
into a problem with overlapping instances and have no way to decide, which
I presume is still an error.
So it looks to me (no doubt because I don't understand correctly) that the
functional dependency doesn't add any information or clarify any situation.
More information about the Beginners