The type is right-biased for {@link #map(Function)}: mapping transforms only + * the {@link Right} branch.
+ * + * @param{@code Option} is a sealed algebraic data type with exactly two variants: + * {@link Some}, containing a value, and {@link None}, representing absence.
+ * + * @param contained value type + */ +public sealed interface Option permits Some, None { + + /** + * Creates an option containing a non-null value. + * + * @param value value to wrap + * @param value type + * @return a {@link Some} + * @throws NullPointerException when {@code value} is {@code null} + */ + static Option some(final A value) { + return new Some<>(Objects.requireNonNull(value, "value")); + } + + /** + * Creates an empty option. + * + * @param value type + * @return an empty {@link None} + */ + static Option none() { + return new None<>(); + } + + /** + * Creates an option from a nullable reference. + * + * @param value nullable value + * @param value type + * @return {@link None} for {@code null}; otherwise {@link Some} + */ + static Option fromNullable(final A value) { + return value == null ? none() : some(value); + } + + /** + * Transforms a present value while preserving absence. + * + * @param mapper transformation applied to a present value + * @param resulting value type + * @return transformed option + */ + Option map(Function mapper); + + /** + * Eliminates this ADT by supplying one function for each variant. + * + * @param onNone computation used for an empty option + * @param onSome computation used for a present value + * @param resulting type + * @return result produced by the matching branch + */ + B fold(Supplier onNone, Function onSome); + + /** + * Returns whether this option contains a value. + * + * @return {@code true} for {@link Some}, otherwise {@code false} + */ + default boolean isDefined() { + return fold(() -> false, ignored -> true); + } + + /** + * Returns whether this option is empty. + * + * @return {@code true} for {@link None}, otherwise {@code false} + */ + default boolean isEmpty() { + return !isDefined(); + } + + /** + * Returns the contained value when present or computes a fallback. + * + * @param fallback fallback computation + * @return contained value or fallback value + */ + default A getOrElse(final Supplier fallback) { + Objects.requireNonNull(fallback, "fallback"); + return fold(fallback, Function.identity()); + } +} diff --git a/src/main/java/io/codeswarm/typeclasses/data/Right.java b/src/main/java/io/codeswarm/typeclasses/data/Right.java new file mode 100644 index 0000000..46b6299 --- /dev/null +++ b/src/main/java/io/codeswarm/typeclasses/data/Right.java @@ -0,0 +1,43 @@ +package io.codeswarm.typeclasses.data; + +import java.util.Objects; +import java.util.function.Function; + +/** + * Right branch of {@link Either}. + * + * @param value right value + * @paramThis milestone intentionally models validation without introducing an + * Applicative typeclass. Error accumulation across independent computations is + * deferred until the project has a generic Applicative abstraction.
+ * + * @param