[GHC] #974: Add unzipEithers, lefts, rights to Data.Either

[kahl at cas.mcmaster.ca]

Aaron Denney wrote:
 > 
 > On 2006-11-13, roconnor at theorem.ca <roconnor at theorem.ca> wrote:
 > > On Mon, 13 Nov 2006, Bulat Ziganshin wrote:
 > >
 > >> i prefer name splitEithers. [...]
 > >
 > > [...]
 > 
 > Unzip is reversible though.  The order is preserved, and
 > (uncurry zip) . unzip = id = unzip . (uncurry zip)
 > There is no "zipEithers" that is any sort of reverse.

Only one-sided, of course:

splitEithers . (\ (xs, ys) -> map Left xs ++ map Right ys) = id


 > 
 > Hmm.  (,) is the generic product, and Either is the generic coproduct.
 > counzip?

Or, better even, ``cozip'' ;-)       (I couldn't resist...)

(uncurry zip) :: ([a],[b]) -> [a * b]
                                /
                               /
                              /
                             /
                            /
                           /
                          /
                         /
                        /
                       /
cozip         ::   [a + b] -> ([a],[b])


;-)


Wolfram



P.S.:  I currently have:

eitherList :: [Either a b] -> ([a], [b])

eitherPair2 :: (a, Either b c) -> Either (a,b) (a,c)

eitherMap :: (a -> c) -> (b -> d) -> Either a b -> Either c d