Hello, So following Carter Schonwald suggestion of creating a new thread, but not literally, since the amount of combinations would be huge, here is my own humble attempt to gather all hypothesis discussed into separate proposals, for your voting pleasure. (Some are mutually exclusive, some are not) Proposal 0: ---------------- Do nothing Proposal 1: ---------------- Add no new code, but document Data.Either to mention how to use `left` and (+++) from Control.Arrow Proposal 2: ---------------- Document Control.Arrow to exemplify the (->) instance usage. Proposal 3: ---------------- Implement mapLeft in Data.Either 3a) ... as (a -> b) -> Either a c -> Either b c (so that is easy to follow) 3b) ... as Control.Arrow.left 3c) ... and also redefine the -> instance of Control.Arrow.left as mapLeft Proposal 4: ---------------- Implement mapBoth in Data.Either 4a) ... as (a -> b) -> (c -> d) -> Either a c -> Either b d (so that is easy to follow) 4b) ... as Control.Arrow.(+++) 4c) ... and also redefine the -> instance of Control.Arrow.(+++) as mapBoth Proposal 5: ---------------- == Proposal 4, using the mapEither name instead. Proposal 6: ---------------- Implement mapRight in Data.Either 6a) ... as (b -> c) -> Either a b -> Either a c 6b) ... as fmap Proposal 7: ---------------- Don't implement mapRight, but include documentation in Data.Either stating that it is just fmap. Proposal 8: ---------------- Bring bifunctors to base.