The Optic Types

"The best way to predict the future is to invent it... The second best way is to fund it. The third best way is to map it."

– Neal Stephenson, Cryptonomicon

That ends the everyday pages. This group and the ones after it are On demand: read a page when a task needs it, and skip it until then.

Every Focus path is built from optics, and this group gives each single-value optic a page of its own. Read one when a path does something you want to understand, or when you need an optic the Focus DSL does not generate for you. Here is the destination, before any theory: a reusable path from a User down to the name of the street they live on, composed by hand from generated lenses, and two one-line updates through it. Every line compiles against the real library on every build:

var streetName = UserLenses.address()
    .andThen(AddressLenses.street())
    .andThen(StreetLenses.name());

User moved = streetName.set("Baker Street", user);
User shouted = streetName.modify(String::toUpperCase, user);
// moved.address().street().name()   -> "Baker Street"
// shouted.address().street().name() -> "FLEET STREET"
// the whole graph is rebuilt for you; user itself is untouched

A Lens focuses a field that is always there. A Prism focuses one variant of a sealed type, which might not match. An Affine focuses a value that may be absent, such as an Optional field. An Iso converts between two shapes of the same information. Decision Trees picks one for your data, and Composition Rules says what type two of them compose to.

Why this matters

Three things separate these optics from a bag of getter helpers. They are generated: annotate a record and the boilerplate is the processor's problem, forever in sync with the fields. They are effect-ready: the same path that does a pure set today runs a validating, accumulating, or asynchronous update tomorrow through modifyF, because every settable optic is generic over an Applicative. And they are lawful: the round-trip laws each optic must satisfy are published in hkj-test and checked, not assumed.

Pages in this group

  1. Lenses: A field that is always there
  2. Prisms: One variant of a sealed type
  3. Affines: A value that may be absent
  4. Isomorphisms: Two shapes of the same information

Hands-On Learning

Practise this group in the Lens & Prism Journey (30 exercises).

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