Although I provide "isLeft" and "isRight" in my "errors" package, count me against adding them to Data.Either. I only include them in "errors" because it is supposed to be a "everything and the kitchen sink" utility library. The API of standard libraries should provide guidance to new users for how to program idiomatically in Haskell and including those two functions will very likely lead new users astray from pattern matching, giving them a poor initial impression of Haskell.