On Wed, Nov 26, 2008 at 4:38 PM, Ross Paterson
On Wed, Nov 26, 2008 at 04:02:36PM -0500, David Menendez wrote:
I also think trying to shoe-horn a new, incompatible definition for Arrow into a base-3.0.* release was a mistake. The whole point of having a versioning policy is lost if you don't follow it. Given how isolated Arrow is in the standard libraries, they could have just created a new class and deprecated the old one without causing much fuss.
It is isolated in base, but it's also wired into GHC. The whole point of base-compat is to present a different view of the same entities, so that packages using the base-3 interface and those using the base-4 interface can be combined. There was no way that could work with a changed Arrow class.
But now base-3 isn't compatible with *itself*.
I see no reason why GHC couldn't use a mechanism like rebindable
syntax to support two Arrow classes. I'm free to define my own version
of Monad and use the do-synax with it. Besides, the arrow syntax
already requires a language extension; we could just provide a
different pragma for neo-Arrows. The old and new Arrow classes could
coexist perfectly fine.
Of course, Arrow libraries would end up needing a bunch of boilerplate
to provide instances for both classes. Whether that's preferable to
conditional compilation may be a matter of taste.
--
Dave Menendez