darcs patch: Adding isLeft, isRight, fromLeft, fromRight, splitEithers

[Henning Thielemann]

On Tue, 31 Oct 2006, Ross Paterson wrote:

> On Tue, Oct 31, 2006 at 02:20:04PM +0300, Bulat Ziganshin wrote:
> > nevertheless, we want to use them all :)   "don't teach me a life,
> > just give some money :)"
> 
> Sure.  Meanwhile over at http://hackage.haskell.org/trac/ghc/ticket/960
> we have yet another discussion of how to track down the runtime errors
> that result from this style.

Today I'm also struggling with early uses of fromJust.
   fromMaybe (error "function X was not called corectly")
  is much better. I vote for not adding isLeft and fromLeft, 'either' is a
good and safe thing.