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

[Henning Thielemann]

On Thu, 9 Nov 2006, Ross Paterson wrote:

> 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.

I can write
  mapMaybe f . mapMaybe g
 but not
  mapEither f . mapEither g
 that's why I think, this kind of mapEither is not as much 'map' as
'mapMaybe' is. :-)