Compile-time data assembly and mapping with Higher-Kinded-J. Use whenever the task involves: mapping a domain record to/from a wire DTO, request, response, entity or payload (@GenerateMapping, MappingSpec, @MapField); mapping a sealed domain hierarchy to a sealed DTO hierarchy; hand-written toDto()/fromDto()/toDomain()/toEntity() mapper or converter classes; replacing MapStruct/ModelMapper/Dozer or a reflective bean mapper; building one target record by merging several source records (@GenerateMerge); constructing a record from independently-validated parts, collecting every error (@GenerateAssembly, fields(), accumulate()); or giving a sealed error hierarchy a structured envelope with a typed context record (@GenerateErrorEnvelope, ErrorEnvelope, editContext). Also covers the arity rules (the hand-written fields()/accumulate() ladder stops at 16; @GenerateAssembly has no ceiling), parse/build law-checking with MappingLaws, and -parameters.
Higher-Kinded-J Spring Boot integration (hkj-spring-boot-starter). Use PROACTIVELY whenever the working file or task involves: (1) a Spring controller method (@RestController / @GetMapping / @PostMapping / @PutMapping / @DeleteMapping) that returns Either, Validated, EitherOrBoth, EitherOrBothPath, EitherPath, MaybePath, ValidationPath, TryPath, IOPath, CompletableFuturePath, VTaskPath, VStreamPath, or FreePath; (2) a sealed interface or sealed hierarchy named DomainError / *Error / *Failure used as the Left of an Either; (3) Jackson 3.x / tools.jackson serialization of HKJ types, including EitherOrBoth and NonEmptyList; (4) any application.yml / application.properties key under hkj.web.* (including hkj.web.either-or-both-path-enabled), hkj.json.*, hkj.validation.*, hkj.async.*, hkj.virtual-threads.*, hkj.actuator.*, hkj.security.*, or hkj.effect-boundary.*; (5) mapping a domain error to an HTTP status code, including 4xx/5xx codes outside the heuristic table (409 Conflict, 422 Unprocessable Entity, 429 Too M
Functional core imperative shell architecture with Higher-Kinded-J and Java 25: separate pure business logic from side effects, boundary design, records and sealed interfaces for domain models, where to call unsafeRun, testing without mocks, push effects to edges, TimeSource, injecting the clock, deterministic time, testing time without sleeping, SteppableClock
Integrate HKJ effects with optics: .focus() on EitherPath/MaybePath/TryPath/IOPath/ValidationPath, FocusPath.toEitherPath(), AffinePath.toMaybePath(), TraversalPath.toListPath(), ValidatedPrism parse/parsePath smart constructors (parse-don't-validate), Edits.accumulate().applyPath() and Edits.combine() for sparse multi-edits and REST PATCH, validation pipelines combining Focus DSL with Effect Path error handling
Build HKJ effect handlers: @EffectAlgebra sealed interfaces, @ComposeEffects composition, Free monad programs, interpreters, natural transformations, foldMap, program analysis, multiple interpretation modes (production, test, audit, dry-run), mock-free testing
Help with Higher-Kinded-J (HKJ) Java library: choose Path types (MaybePath, EitherPath, TryPath, ValidationPath, IOPath, VTaskPath, VResultPath, EitherOrBothPath), set up Gradle/Maven, migrate imperative Java code to Effect Paths, fix compiler errors, understand the railway model. Also: NonEmptyList, EitherOrBoth (inclusive-or, success with warnings), open-arity accumulating assembly (fields()/accumulate()), path-native resilience (withRetry/withTimeout/withCircuitBreaker/withBulkhead), TimeSource
Generate and compose HKJ optics: @GenerateLenses, @GenerateFocus, @GenerateTraversals, @GeneratePrisms, @ImportOptics, @OpticsSpec. Focus DSL navigation, FocusPath, AffinePath, TraversalPath, Lens, Prism, Iso, deep immutable record updates, collection navigation. Also: ValidatedPrism (parse-don't-validate smart constructors), Edits multi-edit builder and REST PATCH, Update<S> and Monoids.update(), optic-path labelling (segments()/pathString())
Write tests against Higher-Kinded-J types using hkj-test: assertThatEither / assertThatMaybe / assertThatTry / assertThatIO / assertThatVTask / assertThatVStream / assertThatVTaskPath / assertThatVStreamPath / assertThatVTaskContext / assertThatVResultPath / assertThatLazy / assertThatValidated / assertThatNonEmptyList / assertThatEitherOrBoth / assertThatFieldError / assertThatErrorEnvelope / assertThatReader / assertThatWriter / assertThatStateTuple / assertThatList / assertThatOptionalKind / assertThatStream / assertThatId / assertThatEitherT / assertThatMaybeT / assertThatOptionalT / assertThatReaderT / assertThatStateT / assertThatWriterT / assertThatFree / assertThatEitherF, AssertJ fluent assertions for HKJ types, transformer assertion unwrappers, Kind-narrowing wrappers (List, OptionalKind, Stream, Id), Path/context assertions (VTaskPath, VStreamPath, VTaskContext, VResultPath), validation-accumulation and FieldError path assertions, optic laws / law harness (IsoLaws, LensLaws, PrismLaws, AffineLaws,