A lens is a pair of functions (getter and setter) that are under the data structure. It's really that simple. There is currently a library for them,
type ('s,'a) t = { get : -> 'a; set : 'a -> -> 's; }
Example (using the ocaml library above) for a tailor,
type measurements = { inseam : float; } type person = { name : string; measurements : measurements; } let lens_person_measurements = { get = (fun x -> x.measurements); set = (fun ax -> {x with measurements = a}); } let lens_measurements_inseam = { get = (fun x -> x.inseam); set = (fun ax -> {x with inseam = a}); } let lens_person_inseam = compose lens_measurements_inseam lens_person_measurements
When creating lenses together, you can see this as a way to avoid having to write with constantly when working with records. You can also see that ppx for creating these lenses will be very useful. Yaron, recently posted on a caml list , they are working on what would look like a lens.
An important understanding in the van Larhoven Lens definition (PDF) shows how one of the functions ( fmap ) of a particular Functor can do these operations (install and receive and very useful update function).