ekmett/lens — pull request #361 by sonyandy (2013-10-29 23:06:33+00:00)
Title: Control.Lens.Union with Variant type classes

This pull request contains a few type classes that are for sum types what `Control.Lens.Tuple`'s `Field` type classes are to product types. Also included are `default` implementations requiring `Generic` instances. These implementations are used in the `VariantA` and `VariantB` instances for `Either`. If this looks useful, I would like to document the type classes and module prior to any merge.