TypeScript Lens implementation with property proxy

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TypeScript Lens implementation with property proxy


Lens is composable abstraction of getter and setter. For more detail of Lens, I recommend reading the following documents.


Via npm:
npm i lens.ts


// import a factory function for lens
import { lens } from 'lens.ts';

type Person = {
  name: string;
  age: number;
  accounts: Array<Account>;

type Account = {
  type: string;
  handle: string;

const azusa: Person = {
  name: 'Nakano Azusa',
  age: 15,
  accounts: [
      type: 'twitter',
      handle: '@azusa'
      type: 'facebook',
      handle: 'nakano.azusa'

// create an identity lens for Person
const personL = lens<Person>();

// key lens with k()
personL.k('name') // :: Lens<Person, string>
personL.k('accounts') // :: Lens<Person, Array<Account>>
personL.k('hoge') // type error, 'hoge' is not a key of Person
personL.k('accounts').k(1) // :: Lens<Person, Account>
personL.k(1) // type error, 'i' cannot be used for non-array type

// You can use property proxy to narrow lenses
personL.name // :: Lens<Person, string>
personL.accounts // :: Lens<Person, Array<Account>>
personL.accounts[1] // :: Lens<Person, Account>
personL.hoge // type error

// get and set with Lens
personL.accounts[0].handle.get()(azusa) // -> '@azusa'
personL.accounts[0].handle.set('@nakano')(azusa) // -> { ... { handle: '@nakano' } ... }
personL.age.set(x => x + 1)(azusa) // -> { age: 16, ... }

// Lens composition
const fstAccountL = lens<Person>().accounts[0] // :: Lens<Person, Account>
const handleL = lens<Account>().handle // :: Lens<Account, string>
fstAccountL.compose(handleL) // :: Lens<Person, string>

// Getter/Setter composition
fstAccountL.get(handleL.get())(azusa) // -> '@azusa'
fstAccountL.set(handleL.set('@nakano'))(azusa) // -> { ... { handle: '@nakano' } ... }

You can find similar example code in /test.


lens.ts exports the followings:
import {
} from 'lens.ts';


A function lens is a factory function for an identity lens for a type. It returns a Lens instance.
lens<Person>() // :: Lens<Person, Person>

Getter, Setter

They are just a type alias for the following function types.
export type Getter<T, V> = (target: T) => V;
export type Setter<T>    = (target: T) => T;

Basically, Getter is a function to retrieve a value from a target. Setter is a function to set or update a value in a provided target and return a new object with a same type as the target.
Any Setter returned from Lens has immutable API, which means it doesn't modify the target object.

Lens<T, U>

An instance of Lens can be constructed with a getter and setter for a source type T and a result type U.
Lens is not just a class, but it's internally a product type of LensImpl and Proxy types, so you cannot just create one with new Lens(). A recommended way to create an instance is lens<X>(), but you can also manually provide a getter and a setter with createLens().
function createLens<T, U>(
  _get: Getter<T, U>,
  _set: (value: U) => Setter<T>
): Lens<T, U> {
  return proxify(new LensImpl(_get, _set));

Lens provides the following methods.

.k<K extends keyof U>(key: K)

Narrow the lens for a property of U.
// we will use these types for the following examples
type Person = {
  name: string;
  age: number;
  accounts: Account[];

lens<Person>.k('name') // :: Lens<Person, string>
lens<Person>.k('accounts') // :: Lens<Person, Account[]>
lens<Person>.k('accounts').k(0) // :: Lens<Person, Account>


It is polymorphic.
  • .get(): Getter<T, U>
  • .get<V>(f: Getter<U, V>): Getter<T, V>

.get() returns a getter, which can be applied to an actual target object to retrive an actual value. You can optionally provide another getter (or mapping function) to retrieve a mapped value.
const target = { age: 15, ... };

const ageL = lens<Person>().k('age');

ageL.get()(target) // -> 15
ageL.get(age => age + 10)(target) // -> 25


It is polymorphic.
  • .set(val: U): Setter<T>
  • .set(f: Setter<U>): Setter<T>

.set() returns a setter, which can set or update an internal value and returns an updated (and new) object. Setters here should be all immutable. You can provide a value to set, or optionally a setter for value.
const target = { age: 15, ... };

const ageL = lens<Person>().k('age');

ageL.set(20)(target) // -> { age: 20, ... }
ageL.set(age => age + 1)(target) // -> { age: 16, ... }

.compose(another: Lens<U, V>): Lens<U, V>

Compose 2 lenses into one.
let lens1: Lens<T, U>;
let lens2: Lens<U, V>;

let accountsL = lens<Person>().k('accounts');
let firstL = <T>() => lens<T[]>().k(0);

let firstAccountL =
  accountsL.compose(firstL()); // :: Lens<Person, Account>

FYI: The reason firstL becomes a function with <T> is to make it polymorphic.

Proxied properties

Lens<T, U> also provides proxied properties for the type U.
objL.name // same as objL.k('name')
arrL[0] // same as arrL.k(0)


Property proxy couldn't have been implemented without @ktsn's help.
  • https://github.com/ktsn/lens-proxy