Data.Either.joinEither

Motivation

Nesting Eithers can be useful. Unnesting them with a join is as well but it could be handy to account for the left parameter.

Proposal

I'd like to propose this function for Data.Either:

joinEither :: (b -> c) -> Either c (Either b a) -> Either c a
joinEither f = either Left (either (Left . f) Right)

joinEither id would be just plain join. This function doesn't do much but the rest of functions in Data.Either are of similar scope.

joinEither2 :: (b -> Either a c) -> Either c (Either b a) -> Either c a would be more generic but that feels intuitively wrong to me since Lefts aren't supposed to just turn into Rights.