fp-magic-lens

Usually when defining our functionally pure lenses for our data object, we define a getter and a setter methods explicitly.

If I show you that my getter for lens is user => user.name and ask you to tell me about it's setter,

You would define it like this (name, user) => { ...user, name }

What if I tell you that fp-magic-lens can save you this effort and generate the setter() for you based on the getter provided?

npm install fp-magic-lens
or
yarn add fp-magic-lens

import setter from 'fp-magic-lens'

const getNameAge = user => [user.name, user.age]

const setNameAge = setter(getNameAge)

// equivalent to
// const setNameAge = ([name, age], user) => ({ ...user, name, age })

setNameAge(['Bishwajit', 21], {})
// { name: "Bishwajit", age: 21 }

Mathematical Definition -

 data Lens a b = Lens { getter :: a -> b, setter :: b -> a -> a }

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.

Getter is a simple function that receives an object (whole) and returns a piece of it (part).

With the setter, we are looking to generate a new whole with the new part that we have passed to it.

Lenses are basically the get/set functions we’re used to but functionally pure!