Optic Kinds

An optic describes a relationship between a source type S and a focused target type A.

The kind tells you what operations are lawful.

Kind	Cardinality	Writable	Use it for
`Lens<S, A>`	exactly one	yes	required fields
`Prism<S, A>`	zero or one	yes	optional fields, union branches
`Iso<S, A>`	exactly one	yes	invertible representations
`Traversal<S, A>`	zero or more	yes	arrays and repeated focuses
`Getter<S, A>`	exactly one	no	derived values
`Fold<S, A>`	zero or more	no	read-only extraction

Total Vs Partial

A Lens is total. If you have a Lens<Person, string>, you can always get a string from a Person.

A Prism is partial. If you have a Prism<Person, Address>, get may return undefined.

const result = addressPrism.get(person)
// Address | undefined

Getter and Fold deliberately have no write operations. Once a read-only optic enters composition, the result is read-only too.

This is not a limitation. It is the type system preventing you from setting a computed value and calling it a plan.