Visitor.java

package fj.function;

import fj.Equal;
import fj.F;
import fj.F0;
import fj.Function;
import fj.Monoid;
import fj.P1;
import fj.P2;
import fj.data.List;
import fj.data.Option;
import static fj.Function.compose;
import static fj.Function.curry;
import static fj.data.List.lookup;

/**
 * The essence of the visitor design pattern expressed polymorphically.
 *
 * @version %build.number%
 */
public final class Visitor {
  private Visitor() {
    throw new UnsupportedOperationException();
  }

  /**
   * Returns the first value available in the given list of optional values. If none is found return the given default value.
   *
   * @param values The optional values to search.
   * @param def The default value if no value is found in the list.
   * @return The first value available in the given list of optional values. If none is found return the given default value.
   */
  public static <X> X findFirst(final List<Option<X>> values, final F0<X> def) {
    return Monoid.<X>firstOptionMonoid().sumLeft(values).orSome(def);
  }

  /**
   * Returns the first non-<code>null</code> value in the given list of optional values. If none is found return the given default value.
   *
   * @param values The potentially <code>null</code> values to search.
   * @param def The default value if no value is found in the list.
   * @return The first non-<code>null</code> value in the given list of optional values. If none is found return the given default value.
   */
  public static <X> X nullablefindFirst(final List<X> values, final F0<X> def) {
    return findFirst(values.map(Option.fromNull()), def);
  }

  /**
   * Returns the first value found in the list of visitors after application of the given value, otherwise returns the
   * given default.
   *
   * @param visitors The list of visitors to apply.
   * @param def The default if none of the visitors yield a value.
   * @param value The value to apply to the visitors.
   * @return The first value found in the list of visitors after application of the given value, otherwise returns the
   * given default.
   */
  public static <A, B> B visitor(final List<F<A, Option<B>>> visitors, final F0<B> def, final A value) {
    return findFirst(visitors.map(Function.apply(value)), def);
  }

  /**
   * Returns the first non-<code>null</code> value found in the list of visitors after application of the given value,
   * otherwise returns the given default.
   *
   * @param visitors The list of visitors to apply looking for a non-<code>null</code>.
   * @param def The default if none of the visitors yield a non-<code>null</code> value.
   * @param value The value to apply to the visitors.
   * @return The first value found in the list of visitors after application of the given value, otherwise returns the
   * given default.
   */
  public static <A, B> B nullableVisitor(final List<F<A, B>> visitors, final F0<B> def, final A value) {
    return visitor(visitors.map(k -> compose(Option.fromNull(), k)), def, value);
  }

  /**
   * Uses an association list to perform a lookup with equality and returns a function that can be applied to a default,
   * followed by the associated key to return a value.
   *
   * @param x The association list.
   * @param eq The equality for the association list keys.
   * @return A function that can be applied to a default value (there is no association) and an associated key.
   */
  public static <A, B> F<B, F<A, B>> association(final List<P2<A, B>> x, final Equal<A> eq) {
    return curry((def, a) -> lookup(eq, x, a).orSome(def));
  }

  /**
   * Uses an association list to perform a lookup with equality and returns a function that can be applied to a default,
   * followed by the associated key to return a value.
   *
   * @param x The association list.
   * @param eq The equality for the association list keys.
   * @return A function that can be applied to a default value (there is no association) and an associated key.
   */
  public static <A, B> F<P1<B>, F<A, B>> associationLazy(final List<P2<A, B>> x, final Equal<A> eq) {
    return curry((def, a) -> lookup(eq, x, a).orSome(def));
  }
}