Brent Yorgey wrote:
On Sun, Jun 21, 2009 at 09:16:12PM +0100, Andrew Coppin wrote:
Niklas Broberg wrote:
That's what GADTs are for:
data Flag = HasZoo | NoZoo
data Foobar a where Foo :: Foobar a -> Foobar a Bar :: Foobar a -> Foobar a Zoo :: Foobar a -> Foobar HasZoo
Ouch #1: This appears to instantly disable deriving the Eq, Ord and Show instances I want. :-/
Ah, yes, that is a pain. Maybe try playing around with tools like Data.Derive? I haven't played with them much myself so I don't know if they will help.
Not nearly as annoying as this: data Foobar a where Foo :: X -> Y -> Foobar NoZoo Bar :: X -> Y -> Foobar NoZoo Zoo :: Foobar NoZoo -> Foobar Zoo For some reason, if I do this I get endless type check errors. I have to change the top two back to Foobar a before it will work. *sigh*