On Wed, Jan 2, 2013 at 11:20 AM, Dan Doel <dan.doel@gmail.com> wrote:

Note that even left-to-right behavior covers all cases, as you might have:

f x y

such that y requires x to be checked polymorphically in the same way. There are algorithms that can get this right in general, but it's a little tricky, and they're rather different than GHC's algorithm, so I don't know whether it's possible to make GHC behave correctly.

Oops, that should have been, "note that not even left-to-right behavior covers all cases."

Also, I don't mean to imply that GHC is behaving wrongly. Situations like these are usually the result of necessary trade-offs in the algorithms. GHC does a lot of things that algorithms that successfully check this type of examples don't have to deal with at all. You have to pick your poison.

-- Dan