On Wed, Jun 22, 2011 at 1:25 PM, pipoca
Is there any reason why we don't have either anonymous disjoint union types, or why some of the proposals here (e.g. type (:|:) a b = Either a b ) haven't been implemented, or put into the standard libraries (and publicised in beginner texts)?
I know that {-# LANGUAGE TypeOperators #-} has been in GHC for a while, but I'm not sure how widely accepted among other Haskell implementations it is. It seems to me that you'd need an additional function:
either' :: (a -> c) -> (b -> d) -> Either a b -> Either c d
import Control.Arrow (+++, |||) The '+++' operator from ArrowChoice already does what you need here. And the '|||' operator is a more generic form of the '???' operator I mentioned earlier.
if we want to map a and d over ad using either' to get bcef :: [B :|: C :|: E :|: F] it wouldn't work, we'd get bcef :: [(B :|: C) :|: (E :|: F)] instead, which is presumably not what we wanted...
Actually, we do want [(B :|: C) :|: (E :|: F)] in this case. It's important for generic programming. However, these types are associative, so we could develop a standard set of re-association operators.