Lenses have been called "jQuery for data types". They allow to access

and modify certain parts of data types. Lenses are compositional, that

is one may build up complicated data accessor from a few simpler ones.

An idea behind lenses gives rise to a number of useful compositional

constructions (folds, traversals, isomorphisms, first-class patterns).

We discuss semantics and pragmatics of these notions in Haskell.