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

[Ross Paterson]

On Thu, Nov 09, 2006 at 04:29:09PM +0100, Henning Thielemann wrote:
> On Thu, 9 Nov 2006, Ross Paterson wrote:
> > I'd suggest also having the following variant, by analogy with mapMaybe:
> > 
> > -- | Map values and separate the 'Left' and 'Right' results.
> > mapEither :: (a -> Either b c) -> [a] -> ([b], [c])
> > mapEither f = foldr (add . f) ([], [])
> >   where add (Left b)  (bs, cs) = (b:bs, cs)
> >         add (Right c) (bs, cs) = (bs, c:cs)
> 
> After my turnaround
>  http://www.haskell.org/pipermail/libraries/2006-November/006204.html
>   that's what I called partitionEither. From a 'map' function I expect a
> similarity to 'fmap', e.g. that the main input and the output have the
> same type constructor (here []).

As I said before, it's a bit of both map and partition, in the same way
that mapMaybe combines map and filter.  But mapMaybe is already there,
and this naming follows it.