No, I've not tested against head. I'd not heard anything new about that! That sounds exciting. Sorry about the noise if it's all finished already. David On Thu, Oct 22, 2015 at 9:57 AM, Richard Eisenberg wrote:

The Coercible solver has evolved steadily. It should know that (Coercible a b <=> Coercible b a). Do you have a concrete example of where it's not doing this? Have you tested against HEAD?

Thanks, Richard

On Oct 22, 2015, at 9:56 AM, David Feuer wrote:

...

At present, any time we write a function with a `Coercible` constraint, we must take great care to choose `Coercible a b` or `Coercible b a` depending on which will ultimately lead to fewer silly conversions. This is particularly sad because the whole Coercible mechanism guarantees that these have exactly the same run-time representation, and because People Wiser Than Me believe Coercible should *always* remain symmetric. My (admittedly reptilian) brain wonders what it would take to tell the type checker that

forall a b . Coercible a b ~ Coercible b a

and have it over with.

David Feuer _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs