Proposal: Add isLeft/isRight to Data.Either

[Edward Z. Yang]

+1

Excerpts from Simon Hengel's message of Fri Nov 30 12:38:59 -0800 2012:
> Hi,
> I propose to add isLeft/isRight to Data.Either, with the obvious
> definitions:
> 
>     isLeft :: Either a b -> Bool
>     isLeft (Left  _) = True
>     isLeft (Right _) = False
> 
>     isRight :: Either a b -> Bool
>     isRight (Left  _) = False
>     isRight (Right _) = True
> 
> There has been a discussion on that before [1].  While I agree that
> fromLeft/fromRight are moot, I don't see issues with isLeft/isRight.
> 
> Personally I care mostly about isLeft, but for orthogonality I propose
> to add both isLeft and isRight.
> 
> Here is a (possibly incomplete) list of packages that come with their
> own definition of isLeft:
> 
>     snap-core, multifocal, PriorityChansConverger, tamarin-prover-utils,
>     Agda, PCLT, cmdtheline, scyther-proof, xmlhtml, hspec-expectations,
>     Glob, language-glsl, Craft3e, hledger-lib, narc, nemesis,
>     type-settheory, PCLT-DB, RJson, bio, errors, rss2irc, heist
> 
> Discussion period: 2 Weeks
> 
> Cheers,
> Simon
> 
> [1] http://www.haskell.org/pipermail/libraries/2006-October/006098.html
>