Traversal

A Traversal<S, A> focuses zero or more values A inside a source S.

type Traversal<S, A> = {
  readonly _tag: 'traversal'
  readonly getAll: (s: S) => ReadonlyArray<A>
  readonly modify: (f: (a: A) => A) => <T extends S>(s: T) => T
}

Traverse An Array

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

type Team = { members: string[] }

const members = Lens<Team>().prop('members')
const allMembers = compose(members, each<string>())

allMembers.getAll({ members: ['Ada', 'Grace'] })
// ['Ada', 'Grace']

allMembers.modify((name) => name.toUpperCase())({
  members: ['Ada', 'Grace'],
})
// { members: ['ADA', 'GRACE'] }

Traversals are for uniform updates across every focus. If you want exactly one array slot, use index.

Notes

• getAll returns every focus.
• modify updates every focus.
• Traversals through missing prism branches are no-ops.
• each is the built-in array traversal.