Optics: Lens & Prism Journey

Duration: ~40 minutes | Tutorials: 4 | Exercises: 30

Journey Overview

This journey teaches the fundamental optics: Lens, Prism, and Affine. By the end, you'll never write verbose immutable update code again.

Lens (product types) → Lens Composition → Prism (sum types) → Affine (optional)

The Optics Hierarchy (Preview)

         Lens (single required field)
              ↓
         Prism (one variant of sum type)
              ↓
         Affine (zero or one element)
              ↓
         Traversal (zero or more elements)

When you compose a Lens with a Prism, you get an Affine. This journey builds that intuition.

Tutorial 01: Lens Basics (~10 minutes)

File: Tutorial01_LensBasics.java | Exercises: 7

Learn immutable field access and modification with Lenses, the foundation of the optics library.

What you'll learn:

• The three core operations: get, set, modify
• Using @GenerateLenses to auto-generate lenses for records
• Manual lens creation with Lens.of()
• Lens composition with andThen

Key insight: A Lens is a first-class getter/setter. You can pass it around, compose it, and reuse it across your codebase.

Before and After:

// Without lenses (verbose, error-prone)
var updated = new User(user.name(), newEmail, user.address());

// With lenses (clear, composable)
var updated = UserLenses.email().set(newEmail, user);

Real-world application: User profile updates, configuration management, any nested record manipulation.

Links to documentation: Lenses Guide

Hands On Practice

Tutorial 02: Lens Composition (~10 minutes)

File: Tutorial02_LensComposition.java | Exercises: 7

Learn to access deeply nested structures by composing simple lenses into powerful paths.

What you'll learn:

• Composing lenses with andThen to create deep paths
• Updating nested fields in a single expression
• Creating reusable composed lenses
• The associative property: (a.andThen(b)).andThen(c) == a.andThen(b.andThen(c))

Key insight: Composition is the superpower of optics. Combine small, reusable pieces into complex transformations.

Before and After:

// Without lenses (nightmare)
var newUser = new User(
    user.name(),
    user.email(),
    new Address(
        new Street("New St", user.address().street().number()),
        user.address().city()
    )
);

// With lenses (one line)
var newUser = userToStreetName.set("New St", user);

Real-world application: Updating deeply nested JSON, modifying complex domain models, configuration tree manipulation.

Links to documentation: Composing Optics

Hands On Practice

Tutorial 03: Prism Basics (~10 minutes)

File: Tutorial03_PrismBasics.java | Exercises: 9

Learn to work with sum types (sealed interfaces) safely using Prisms.

What you'll learn:

• The three core operations: getOptional, build, modify
• Pattern matching on sealed interfaces
• Using @GeneratePrisms for automatic generation
• Using matches() for type checking and doesNotMatch() for exclusion filtering
• The nearly prism for predicate-based matching
• Prism composition

Key insight: Prisms are like type-safe instanceof checks with built-in modification capability.

Example scenario: An OrderStatus can be Pending, Processing, or Shipped. A Prism lets you safely operate on just the Shipped variant.

// Safely extract tracking number only if Shipped
Optional<String> tracking = shippedPrism
    .andThen(trackingLens)
    .getOptional(orderStatus);

Real-world application: State machine handling, discriminated unions, API response variants, event processing.

Links to documentation: Prisms Guide | Advanced Prism Patterns

Hands On Practice

Tutorial 04: Affine Basics (~10 minutes)

File: Tutorial04_AffineBasics.java | Exercises: 7

Learn to work with optional fields and nullable properties using Affines.

What you'll learn:

• The core operations: getOptional, set, modify
• Using Affines.some() for Optional<T> fields
• Why Lens.andThen(Prism) produces an Affine, not a Traversal
• Using matches() and getOrElse() convenience methods
• Composing Affines for deep optional access
• When to use Affine vs Lens vs Prism vs Traversal

Key insight: An Affine is more precise than a Traversal when you know there's at most one element. It's what you get when you compose a guaranteed path (Lens) with an uncertain one (Prism).

Decision guide:

Real-world application: User profiles with optional contact info, configuration with optional sections, nullable legacy fields.

Links to documentation: Affines Guide

Hands On Practice

Running the Tutorials

./gradlew :hkj-examples:test --tests "*Tutorial01_LensBasics*"
./gradlew :hkj-examples:test --tests "*Tutorial02_LensComposition*"
./gradlew :hkj-examples:test --tests "*Tutorial03_PrismBasics*"
./gradlew :hkj-examples:test --tests "*Tutorial04_AffineBasics*"

Common Pitfalls

1. Forgetting andThen for Composition

Problem: Trying to access nested fields without composing lenses.

Solution: Chain lenses with andThen:

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

2. Using Prism.get Instead of getOptional

Problem: Expecting get() on a Prism when the variant doesn't match.

Solution: Prisms return Optional. Always use getOptional():

Optional<Shipped> shipped = shippedPrism.getOptional(orderStatus);

3. Expecting Traversal When You Get Affine

Problem: Thinking Lens + Prism = Traversal.

Solution: Lens + Prism = Affine (zero-or-one, not zero-or-many). Use asTraversal() if needed.

What's Next?

After completing this journey:

1. Continue to Traversals & Practice: Learn bulk operations on collections
2. Jump to Focus DSL: Use the ergonomic path-based API
3. Explore Real Examples: See Auditing Complex Data

Previous: Scope & Resource Next: Optics: Traversals & Practice