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