#👉What is Lens in Functional Programming? A lens is a composable pair of pure getter and setter functions which focus on a particular field

[js-functional-programming]

👀👓🕶 👉What is Lens in Functional Programming? 👀👓🕶

👉What is Lens in Functional Programming? 

 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. 

👉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: 

 🤚🤚🤚Why Lens? 

 👉Lenses allow you to abstract state shape behind getters and setters. Instead of littering your codebase with code that dives deep into the shape of a particular object, import a lens. 

 👉If you later need to change the state shape, you can do so in the lens, and none of the code that depends on the lens will need to change. 

 🤚🤚🤚Lens Laws 

 👉view(lens, set(lens, a, store)) ≡ a — If you set a value into the store, and immediately view the value through the lens, you get the value that was set. 

👉set(lens, b, set(lens, a, store)) ≡ set(lens, b, store) — If you set a lens value to a and then immediately set the lens value to b, it's the same as if you'd just set the value to b.

 👉set(lens, view(lens, store), store) ≡ store — If you get the lens value from the store, and then immediately set that value back into the store, the value is unchanged.