Fold

A Fold<S, A> extracts zero or more values from S.

type Fold<S, A> = {
  readonly _tag: 'fold'
  readonly getAll: (s: S) => ReadonlyArray<A>
}

Create A Fold

import { Fold } from '@fuiste/optics'

const words = Fold<string, string>((text) => text.split(' '))

words.getAll('hello world')
// ['hello', 'world']

Folds From Composition

Read-only and partial composition often produces a Fold.

import { Getter, Lens, Prism, compose } from '@fuiste/optics'

const address = Prism<Person>().of({
  get: (person) => person.address,
  set: (address) => (person) => ({ ...person, address }),
})
const city = Getter<Address, string>((value) => value.city)

const maybeCity = compose(address, city)
// Fold<Person, string>

Notes

• Folds do not have set or modify.
• Anything composed with a Fold becomes a Fold.
• Use a fold when extraction is the point and mutation would be fiction.