Mastering JavaScript Functional Programming - Second Edition

Implementing lenses with objects

The simplest way to implement a lens is by representing it with an object with just two properties: a getter and a setter. In this case, we'd have something like this:

const lens = (getter, setter) => ({getter, setter});

This is easy to understand: given a getter and a setter, lens() just creates an object with those two attributes. With this definition, lensProp() would be as follows:

const lensProp = attr => lens(getField(attr), setField(attr));

The first function, lensProp(), creates a getter/setter pair by using getField() and setField(); very straightforward. Now that we have our lens, how do we implement the three basic functions that we saw in the previous section? Viewing an attribute just requires ...