Re: Coercion logic

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