示例#1
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 /**
  * Maps this elements to a {@code Monoid} and applies {@code foldLeft}, starting with {@code
  * monoid.zero()}:
  *
  * <pre><code>
  *  foldLeft(monoid.zero(), (ys, x) -&gt; monoid.combine(ys, mapper.apply(x)));
  * </code></pre>
  *
  * @param monoid A Monoid
  * @param foldable A foldable
  * @param mapper A mapper
  * @param <T> type of the foldable elements
  * @param <U> Component type of the given monoid.
  * @return the folded monoid value.
  * @throws NullPointerException if {@code monoid} or {@code mapper} is null
  */
 static <T, U> U foldMap(
     Monoid<U> monoid, Foldable<T> foldable, Function<? super T, ? extends U> mapper) {
   Objects.requireNonNull(monoid, "monoid is null");
   Objects.requireNonNull(foldable, "foldable is null");
   Objects.requireNonNull(mapper, "mapper is null");
   return foldable.foldLeft(monoid.zero(), (ys, x) -> monoid.combine(ys, mapper.apply(x)));
 }
示例#2
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 /**
  * Folds the elements of {@code Foldable} from the left, starting with {@code monoid.zero()} and
  * successively calling {@code monoid::combine}.
  *
  * @param monoid A monoid, providing a {@code zero} and a {@code combine} function.
  * @param foldable A foldable
  * @param <T> type of the foldable elements
  * @return a folded value
  * @throws NullPointerException if {@code monoid} or {@code foldable} is null
  */
 static <T> T fold(Monoid<T> monoid, Foldable<T> foldable) {
   Objects.requireNonNull(monoid, "monoid is null");
   Objects.requireNonNull(foldable, "foldable is null");
   return foldable.foldLeft(monoid.zero(), monoid::combine);
 }
示例#3
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 /**
  * The monoid of endomorphisms under composition.
  *
  * @param <A> Value type
  * @return The monoid of endomorphisms of type A.
  */
 static <A> Monoid<Function<A, A>> endoMonoid() {
   return Monoid.of(Function.identity(), Function::compose);
 }