Lens

A Lens<S, A> focuses exactly one required value A inside a source S.

type Lens<S, A> = {
  readonly _tag: 'lens'
  readonly get: (s: S) => A
  readonly set: (a: A | ((a: A) => A)) => <T extends S>(s: T) => T
}

Create A Property Lens

import { Lens } from '@fuiste/optics'

type Person = {
  name: string
  age: number
}

const name = Lens<Person>().prop('name')

Read And Write

name.get({ name: 'Ada', age: 36 })
// 'Ada'

name.set('Grace')({ name: 'Ada', age: 36 })
// { name: 'Grace', age: 36 }

name.set((value) => value.toUpperCase())({ name: 'Ada', age: 36 })
// { name: 'ADA', age: 36 }

Notes

• get always succeeds.
• set never mutates the input.
• Unchanged values preserve reference identity.
• Array numeric property lenses are supported, but index and each are usually clearer.