Composing Software

Lenses

A lens is a composable pair of pure getter and setter functions which focus on a particular field inside an object, and obey a set of axioms known as the lens laws. Think of the object as the whole and the field as the part. The getter takes a whole and returns the part of the object that the lens is focused on.

1 // view = whole => part

The setter takes a whole, and a value to set the part to, and returns a new whole with the part updated. Unlike a function which simply sets a value into an object’s member field, Lens setters are pure functions:

1 // set = whole => part => whole

Note: In this text, we’re going to use some naive lenses in the code examples just to give you a beneath-the-hood peek at the general concept. For production ...