Immutability

All writable optics return updated values. They do not mutate their inputs.

const next = name.set('Grace')(person)

next !== person
next.name === 'Grace'
person.name !== 'Grace'

Functional Updates

Lens#set and Prism#set both accept either a concrete value or an updater function.

name.set((value) => value.toUpperCase())(person)

For prisms, updater functions are no-ops when the target is missing.

optionalCity.set((city) => city.toUpperCase())({ name: 'Ada' })
// { name: 'Ada' }

Reference Identity

When an update does not change the focused value, the library preserves the existing reference.

const same = name.set(person.name)(person)

Object.is(same, person)
// true

That matters for React, memoization, caches, and the general avoidance of ambient sadness.