@Theory
public void unitLaw(Integer x) {
Integer left = sumMonoid.multiply(0, x);
Integer right = sumMonoid.multiply(x, 0);
assertThat(left, is(x));
assertThat(right, is(x));
}
/**
* Folds this tree using the given monoid.
*
* @param f A transformation from this tree's elements, to the monoid.
* @param m The monoid to fold this tree with.
* @return The result of folding the tree with the given monoid.
*/
public <B> B foldMap(final F<A, B> f, final Monoid<B> m) {
return m.sum(f.f(root()), m.sumRight(subForest()._1().map(foldMap_(f, m)).toList()));
}
@Theory
public void sumBinOp(Integer x, Integer y) {
Integer actual = sumMonoid.multiply(x, y);
assertThat(actual, is(x + y));
}
@Test
public void sumUnitIsZero() {
assertThat(sumMonoid.unit(), is(0));
}