Prism

A Prism<S, A> focuses zero or one value A inside a source S.

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

Create A Prism

import { Prism } from '@fuiste/optics'

type Person = {
  name: string
  address?: { city: string }
}

const address = Prism<Person>().of({
  get: (person) => person.address,
  set: (address) => (person) => ({ ...person, address }),
})

Missing Targets

address.get({ name: 'Ada' })
// undefined

address.set((value) => ({ ...value, city: 'London' }))({ name: 'Ada' })
// { name: 'Ada' }

Concrete set values can materialize optional data when the prism’s own setter does so.

address.set({ city: 'London' })({ name: 'Ada' })
// { name: 'Ada', address: { city: 'London' } }

Notes

• get returns undefined for absent targets.
• Functional updates are no-ops when the target is absent.
• guard, at, and index are prism combinators for common cases.