effect-optics

Define an optic once with Optic.id<S>().key('a').key('b') and call .replace(value, state) or .modify(fn) to produce an updated copy. This replaces repetitive nested spread syntax with a reusable, type-safe path.

A Lens focuses an always-present field, a Prism focuses a union variant that may not match, an Optional handles cases where both reading and writing can fail, and a Traversal operates over zero or more collection elements. Composing two optics yields the weaker kind.

Use .tag('Variant') to narrow the union before chaining .key() into the variant's fields. Replace and modify become no-ops on non-matching variants, returning the original value unchanged.

Optics work with plain JavaScript objects, structs, records, tuples, and arrays. Class instances cause runtime errors on replace or modify unless the optic is generated via Schema.toIso on a class schema.

The replace method returns the original S unchanged when the optic cannot focus, such as a missing key or non-matching tag. Use replaceResult or getResult to receive an explicit Result with failure details instead.

Use .forEach() combined with .check() or .notUndefined() to filter focused elements, then apply .modifyAll(fn) to transform only matching items. Non-matching elements pass through unchanged.