On Fri, 30 Nov 2012, Simon Hengel wrote:
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
It would be a good opportunity to check how these packages use isLeft and isRight. E.g. if they use them in connection with fromLeft and fromRight then this would be an argument for me to exclude isLeft and isRight as well. cmdtheline uses these functions in test/Main.hs for checking whether command options could be parsed or not. Maybe the tests could be stricter if they do (Left expectedValue ==) or (Right expectedValue ==) instead. snap-core uses these functions once in test/suite/Snap/Core/Tests.hs for testing whether Left or Right is returned as expected. multifocal uses isLeft once in src/Language/XML/Xml2Type.hs: map (\(Left x) -> x) . filter isLeft Functions like maybeLeft :: Either a b -> Maybe a maybeRight :: Either a b -> Maybe b in connection with mapMaybe would be more helpful in this case, or just the existing 'Data.Either.lefts'. PriorityChansConverger defines isLeft, isRight, fromLeft, fromRight but does not use them anywhere. tamarin-prover-utils defines isLeft, isRight but does not use them anywhere. Agda uses isLeft and isRight in some QuickCheck properties. And then there is this application: do -- ps0 :: [NamedArg ParseLHS] ps0 <- mapM classPat ps let (ps1, rest) = span (isLeft . namedArg) ps0 (p2, ps3) <- uncons rest -- when (null rest): no field pattern or def pattern found guard $ all (isLeft . namedArg) ps3 let (f, lhs) = fromR p2 (ps', _:ps'') = splitAt (length ps1) ps return $ Right (f, LHSProj x ps' lhs ps'') Looks at least interesting ... :-) I get tired ... So far it seems that isLeft and isRight are frequently used in testing.