Am 09.08.2013 17:31, schrieb Dan Doel:
Here's another smart-ass remark, then: use lens and bifunctors. They have great abstractions for these.
mapEither and mapPair are just two specializations of bimap. It also has first and second which generalize mapFst, etc. And by the way, everyone here (I think) has missed that you can also traverse these things, but bifunctors also has that covered, with bitraverse.
The individual parts are applications of lenses, too. over _1 and over _2 are mapFst and mapSnd. _1 and _2 function as traverseFst and traverseSnd.
These generalizations seem to be more natural to me than Control.Arrow. (However, identifiers with leading underscore are a bad idea, since they suppress "unused identifier" warnings in GHC. But I think I complained about that before.)