Functional Lenses

This package provides some basic types and functions for using lenses in OCaml. Functional lenses are based on F# implementation in FSharpX. See src/FSharpx.Core/Lens.fs for the original implementation. Written by Alessandro Strada.

See also:

• http://bugsquash.blogspot.com/2011/11/lenses-in-f.html Lenses in F#
• http://stackoverflow.com/questions/8179485/updating-nested-immutable-data-structures Stackoverflow question about Updating nested immutable data structures
• http://stackoverflow.com/questions/5767129/lenses-fclabels-data-accessor-which-library-for-structure-access-and-mutatio Haskell libraries for structure access and mutation
• http://www.youtube.com/watch?v=efv0SQNde5Q Functional lenses for Scala by Edward Kmett on YouTube
• http://patternsinfp.wordpress.com/2011/01/31/lenses-are-the-coalgebras-for-the-costate-comonad/ Lenses are the coalgebras for the costate comonad by Jeremy Gibbons

Examples

First load Lens in utop.

utop # #use "lens.ml";;

Given a couple of records

type car = {
    make : string;
    model: string;
    mileage: int;
  };;

type editor = {
    name: string;
    salary: int;
    car: car;
};;

type book = {
    name: string;
    author: string;
    editor: editor;
};;

Create a new nested record

let scifi_novel = {
   name =  "Metro 2033";
   author = "Dmitry Glukhovsky";
   editor =  {
     name = "Vitali Gubarev";
     salary =  1300;
     car =  {
       make = "Lada";
       model = "VAZ-2103";
       mileage = 310000
    }
  }
};;

Now to construct a few lenses to access some things

let car_lens = {
    get = (fun x -> x.car);
    set = (fun v x -> { x with car = v })
  };;

let editor_lens = {
    get = (fun x -> x.editor);
    set = (fun v x -> { x with editor = v })
};;

let mileage_lens = {
    get = (fun x -> x.mileage);
    set = (fun v x -> { x with mileage = v })

};;

Using these lenses we can modify the mileage without having to unpack the record

let a = compose mileage_lens (compose car_lens editor_lens) in
_set 10 scifi_novel a;;

Or using the Infix module we can do the same thing, only shorter.

_set 10 scifi_novel (editor_lens |-- car_lens |-- mileage_lens);;

(* or *)

((editor_lens |-- car_lens |-- mileage_lens) ^= 10) @@ scifi_novel;;