This library is an extract from <http://hackage.haskell.org/package/lens lens>
(with no dependencies). It's not a toy lenses library, unsuitable for “real
world”, but merely a small one. It is compatible with lens, and should have
same performance. It also has better documentation.
There's a longer readme <https://github.com/monadfix/microlens#readme on
Github>. It has a migration guide for lens users, a description of other
packages in the family, a discussion of other lens libraries you could use
instead, and so on.