Prism Toolkit

Convenience methods and common patterns with the Prisms utility

This page is the lookup catalogue for the prism API. The conceptual introduction, mental models, and worked examples live in Prisms; use this page when you already know what a prism is and need to find the right method.

Prism Convenience Methods

Streamlined Operations for Common Patterns

Whilst getOptional() and build() are the core operations, the Prism interface provides several convenience methods that make everyday tasks more ergonomic and expressive.

Quick Reference:

Type Checking with matches()

The matches() method provides a clean alternative to getOptional(source).isPresent():

Prism<JsonValue, JsonString> stringPrism = JsonValuePrisms.jsonString();

// Clear, declarative type checking
if (stringPrism.matches(value)) {
    // Process as string
}

// Useful in streams and filters
List<JsonValue> onlyStrings = values.stream()
    .filter(stringPrism::matches)
    .collect(Collectors.toList());

Real-World Example: Filtering polymorphic domain events:

@GeneratePrisms
sealed interface DomainEvent permits UserEvent, OrderEvent, PaymentEvent {}

// Business logic: process only payment events
public void processPayments(List<DomainEvent> events) {
    Prism<DomainEvent, PaymentEvent> paymentPrism =
        DomainEventPrisms.paymentEvent();

    long paymentCount = events.stream()
        .filter(paymentPrism::matches)
        .count();

    logger.info("Processing {} payment events", paymentCount);

    events.stream()
        .filter(paymentPrism::matches)
        .map(paymentPrism::getOptional)
        .flatMap(Optional::stream)
        .forEach(this::processPayment);
}

Default Values with getOrElse()

When you need fallback values, getOrElse() is more concise than getOptional().orElse():

Prism<ApiResponse, SuccessResponse> successPrism =
    ApiResponsePrisms.successResponse();

// Extract success data or use default
String data = successPrism.getOrElse(
    new SuccessResponse("fallback", 200),
    response
).data();

// Particularly useful for configuration
Config config = Prisms.some()
    .getOrElse(Config.DEFAULT, optionalConfig);

Real-World Example: Parsing user input with graceful degradation:

@GeneratePrisms
sealed interface ParsedValue permits IntValue, StringValue, InvalidValue {}

public int parseUserQuantity(String input, int defaultQty) {
    ParsedValue parsed = parseInput(input);

    Prism<ParsedValue, IntValue> intPrism = ParsedValuePrisms.intValue();

    // Extract integer or use sensible default
    return intPrism.getOrElse(
        new IntValue(defaultQty),
        parsed
    ).value();
}

// Application settings with fallback
public DatabaseConfig getDatabaseConfig(ApplicationConfig config) {
    Prism<ConfigSource, DatabaseConfig> dbConfigPrism =
        ConfigSourcePrisms.databaseConfig();

    return dbConfigPrism.getOrElse(
        DatabaseConfig.DEFAULT_POSTGRES,
        config.source()
    );
}

Transforming Matches with mapOptional()

The mapOptional() method transforms matched values without building them back into the source type:

Prism<JsonValue, JsonNumber> numberPrism = JsonValuePrisms.jsonNumber();

// Extract and transform in one operation
Optional<String> formatted = numberPrism.mapOptional(
    num -> String.format("%.2f", num.value()),
    jsonValue
);

// Compose transformations
Optional<Boolean> isLarge = numberPrism.mapOptional(
    num -> num.value() > 1000,
    jsonValue
);

Real-World Example: ETL data transformation pipeline:

@GeneratePrisms
sealed interface SourceData permits CsvRow, JsonObject, XmlNode {}

public List<CustomerRecord> extractCustomers(List<SourceData> sources) {
    Prism<SourceData, CsvRow> csvPrism = SourceDataPrisms.csvRow();

    return sources.stream()
        .map(source -> csvPrism.mapOptional(
            csv -> new CustomerRecord(
                csv.column("customer_id"),
                csv.column("name"),
                csv.column("email")
            ),
            source
        ))
        .flatMap(Optional::stream)
        .collect(Collectors.toList());
}

// Extract business metrics from polymorphic events
public Optional<BigDecimal> extractRevenue(DomainEvent event) {
    Prism<DomainEvent, OrderCompleted> orderPrism =
        DomainEventPrisms.orderCompleted();

    return orderPrism.mapOptional(
        order -> order.lineItems().stream()
            .map(LineItem::totalPrice)
            .reduce(BigDecimal.ZERO, BigDecimal::add),
        event
    );
}

Simple Modifications with modify()

Instead of manually calling getOptional().map(f).map(build), use modify():

Prism<JsonValue, JsonString> stringPrism = JsonValuePrisms.jsonString();

// Clean modification
JsonValue uppercased = stringPrism.modify(
    str -> new JsonString(str.value().toUpperCase()),
    jsonValue
);

// Verbose alternative
JsonValue verboseResult = stringPrism.getOptional(jsonValue)
    .map(str -> new JsonString(str.value().toUpperCase()))
    .map(stringPrism::build)
    .orElse(jsonValue);

If the prism doesn't match, modify() safely returns the original structure unchanged.

Conditional Operations with modifyWhen() and setWhen()

These methods combine matching with predicate-based filtering:

Prism<ConfigValue, StringConfig> stringConfig =
    ConfigValuePrisms.stringConfig();

// Only modify non-empty strings
ConfigValue sanitised = stringConfig.modifyWhen(
    str -> !str.value().isEmpty(),
    str -> new StringConfig(str.value().trim()),
    configValue
);

// Only update if validation passes
ConfigValue validated = stringConfig.setWhen(
    str -> str.value().length() <= 255,
    new StringConfig("validated"),
    configValue
);

Real-World Example: Business rule enforcement in order processing:

@GeneratePrisms
sealed interface OrderStatus permits Draft, Submitted, Approved, Rejected {}

public class OrderProcessor {
    private static final Prism<OrderStatus, Submitted> SUBMITTED =
        OrderStatusPrisms.submitted();

    // Only approve orders above minimum value
    public OrderStatus approveIfEligible(
        OrderStatus status,
        BigDecimal orderValue,
        BigDecimal minValue
    ) {
        return SUBMITTED.setWhen(
            submitted -> orderValue.compareTo(minValue) >= 0,
            new Approved(Instant.now(), "AUTO_APPROVED"),
            status
        );
    }

    // Apply discount only to high-value draft orders
    public OrderStatus applyVipDiscount(OrderStatus status, Order order) {
        Prism<OrderStatus, Draft> draftPrism = OrderStatusPrisms.draft();

        return draftPrism.modifyWhen(
            draft -> order.totalValue().compareTo(VIP_THRESHOLD) > 0,
            draft -> draft.withDiscount(VIP_DISCOUNT_RATE),
            status
        );
    }
}

Use Cases:

• Conditional validation: Update only if current value meets criteria
• Guarded transformations: Apply changes only to valid states
• Business rules: Enforce constraints during updates
• Workflow automation: Apply state transitions based on business logic

Fallback Matching with orElse()

The orElse() method chains prisms to try multiple matches:

Prism<JsonValue, JsonNumber> intPrism = JsonValuePrisms.jsonInt();
Prism<JsonValue, JsonNumber> doublePrism = JsonValuePrisms.jsonDouble();

// Try int first, fall back to double
Prism<JsonValue, JsonNumber> anyNumber = intPrism.orElse(doublePrism);

Optional<JsonNumber> result = anyNumber.getOptional(jsonValue);
// Matches either integer or double JSON values

// Building always uses the first prism's constructor
JsonValue built = anyNumber.build(new JsonNumber(42)); // Uses intPrism.build

Real-World Example: Handling multiple error types in API responses:

Prism<ApiResponse, String> errorMessage =
    ApiResponsePrisms.validationError()
        .andThen(ValidationErrorLenses.message())
        .orElse(
            ApiResponsePrisms.serverError()
                .andThen(ServerErrorLenses.message())
        );

// Extracts error message from either error type
Optional<String> message = errorMessage.getOptional(response);

Common Prism Patterns with the Prisms Utility

Ready-Made Prisms for Standard Types

The Prisms utility class (in org.higherkindedj.optics.util) provides factory methods for common prism patterns, saving you from writing boilerplate for standard Java types.

Quick Reference:

Working with Optional: Prisms.some()

import org.higherkindedj.optics.util.Prisms;

Prism<Optional<String>, String> somePrism = Prisms.some();

Optional<String> present = Optional.of("hello");
Optional<String> value = somePrism.getOptional(present); // Optional.of("hello")

Optional<String> empty = Optional.empty();
Optional<String> noMatch = somePrism.getOptional(empty); // Optional.empty()

// Useful for nested Optionals
Optional<Optional<Config>> nestedConfig = loadConfig();
Optional<Config> flattened = somePrism.getOptional(nestedConfig)
    .flatMap(Function.identity());

Either Case Handling: Prisms.left() and Prisms.right()

Prism<Either<String, Integer>, String> leftPrism = Prisms.left();
Prism<Either<String, Integer>, Integer> rightPrism = Prisms.right();

Either<String, Integer> error = Either.left("Failed");
Optional<String> errorMsg = leftPrism.getOptional(error); // Optional.of("Failed")
Optional<Integer> noValue = rightPrism.getOptional(error); // Optional.empty()

// Compose with lenses for deep access
record ValidationError(String code, String message) {}
Lens<ValidationError, String> messageLens = ValidationErrorLenses.message();

Prism<Either<ValidationError, Data>, String> errorMessage =
    Prisms.<ValidationError, Data>left()
        .andThen(messageLens);

Either<ValidationError, Data> result = validate(data);
Optional<String> msg = errorMessage.getOptional(result);

Sentinel Values: Prisms.only()

Perfect for matching specific constant values:

Prism<String, Unit> httpOkPrism = Prisms.only("200 OK");

// Check for specific status
if (httpOkPrism.matches(statusCode)) {
    // Handle success case
}

// Filter for specific values
List<String> onlyErrors = statusCodes.stream()
    .filter(Prisms.only("500 ERROR")::matches)
    .collect(Collectors.toList());

// Null sentinel handling
Prism<String, Unit> nullPrism = Prisms.only(null);
boolean isNull = nullPrism.matches(value);

Null Safety: Prisms.notNull()

Prism<String, String> notNullPrism = Prisms.notNull();

// Safe extraction
@Nullable String nullable = getDatabaseValue();
Optional<String> safe = notNullPrism.getOptional(nullable);

// Compose to filter null values in pipelines
Traversal<List<String>, String> nonNullStrings =
    Traversals.<String>forList()
        .andThen(Prisms.<String>notNull().asTraversal());

List<@Nullable String> mixedList = List.of("hello", null, "world", null);
List<String> filtered = Traversals.getAll(nonNullStrings, mixedList);
// Result: ["hello", "world"]

Type-Safe Casting: Prisms.instanceOf()

Elegant alternative to instanceof checks in type hierarchies:

sealed interface Animal permits Dog, Cat, Bird {}
record Dog(String name, String breed) implements Animal {}
record Cat(String name, int lives) implements Animal {}
record Bird(String name, boolean canFly) implements Animal {}

Prism<Animal, Dog> dogPrism = Prisms.instanceOf(Dog.class);

Animal animal = new Dog("Buddy", "Labrador");
Optional<Dog> maybeDog = dogPrism.getOptional(animal); // Optional.of(Dog(...))

// Compose with lenses for deep access
Lens<Dog, String> breedLens = DogLenses.breed();
Traversal<Animal, String> dogBreed =
    dogPrism.asTraversal().andThen(breedLens.asTraversal());

List<Animal> animals = List.of(
    new Dog("Rex", "German Shepherd"),
    new Cat("Whiskers", 9),
    new Dog("Max", "Beagle")
);

List<String> breeds = Traversals.getAll(
    Traversals.<Animal>forList().andThen(dogBreed),
    animals
);
// Result: ["German Shepherd", "Beagle"]

Collection Element Access

// First element (if list is non-empty)
Prism<List<String>, String> headPrism = Prisms.listHead();
List<String> names = List.of("Alice", "Bob", "Charlie");
Optional<String> first = headPrism.getOptional(names); // Optional.of("Alice")

// Last element
Prism<List<String>, String> lastPrism = Prisms.listLast();
Optional<String> last = lastPrism.getOptional(names); // Optional.of("Charlie")

// Element at specific index (read-only for queries)
Prism<List<String>, String> secondPrism = Prisms.listAt(1);
Optional<String> second = secondPrism.getOptional(names); // Optional.of("Bob")

// Safe access patterns
String firstOrDefault = headPrism.getOrElse("Unknown", names);
boolean hasList = headPrism.matches(names);

// ✅ For modifications, use proper traversals
Lens<List<String>, String> firstLens = listFirstElementLens();
List<String> updated = firstLens.modify(String::toUpperCase, names);

import org.higherkindedj.optics.util.ListPrisms;

// Cons pattern: decompose as (head, tail)
Prism<List<String>, Pair<String, List<String>>> cons = ListPrisms.cons();
Optional<Pair<String, List<String>>> decomposed = cons.getOptional(names);
// decomposed = Optional.of(Pair.of("Alice", ["Bob", "Charlie"]))

// Snoc pattern: decompose as (init, last)
Prism<List<String>, Pair<List<String>, String>> snoc = ListPrisms.snoc();

Composing Utility Prisms

The real power emerges when composing these utility prisms with your domain optics:

record Config(Optional<Either<String, DatabaseSettings>> database) {}
record DatabaseSettings(String host, int port) {}

// Build a path through Optional -> Either -> Settings -> host
Prism<Config, String> databaseHost =
    ConfigLenses.database()                    // Lens<Config, Optional<Either<...>>>
        .asTraversal()
        .andThen(Prisms.some().asTraversal())  // -> Either<String, DatabaseSettings>
        .andThen(Prisms.right().asTraversal()) // -> DatabaseSettings
        .andThen(DatabaseSettingsLenses.host().asTraversal()); // -> String

Config config = loadConfig();
Optional<String> host = Traversals.getAll(databaseHost, config)
    .stream().findFirst();

public class AppPrisms {
    public static final Prism<Optional<User>, User> SOME_USER = Prisms.some();
    public static final Prism<Response, SuccessResponse> SUCCESS =
        Prisms.instanceOf(SuccessResponse.class);
}

Previous: Prisms Next: Affines: Working with Optional Fields