Change comparator

In the last section we saw how the dispatch is made by default for a multimethod of dimension two. The default comparator is the asymmetric using the lexicographic order to select the best match. In this section we show that the default method comparator can be changed. The library provides another method comparator which is the symmetric comparator. Let’s see how it behave on a simple example.

We define a multimethod with the symmetric comparator:

Method2<Float> foo2 = new Method2<Float>()
		.symmetricComparator()
		.add(new Cases() {
	
	float match(A x, A y, String s) {
		return x.id;
	}
	
	float match(A x, B y, String s) {
		if (s.equals("multiply")) {
			return x.id * y.fd;
		}
		return 0;
	}
	
	float match(A x, C y, String s) {
		if (s.equals("add")) {
			return x.id + y.fd;
		}
		return 0;
	}
	
	float match(D x, B y, String s) {
		if (s.equals("add")) {
			return 2 * x.id + 3 * y.fd;
		}
		return 0;
	}
});

We configured the multimethod by simply calling the symmetricComparator() method. Let’s try it:

try {
	print(foo2, a, c); // 12.3
	print(foo2, d, b); // 31.9
	print(foo2, d, c); // error
}
catch (Throwable e) {
	e.printStackTrace();
}

There is still a perfect match for the (A, C) and (D, B) however for the couple (D, C), the two compatible methods match(A, C, String) and match(D, B, String) lead to an exception in the invoke() call.
Indeed to select a couple of types, the symmetric comparator is using a product order and the couple must be “better” or “equals” for each dimension. For the couple of objects (D, C), match(D, B, String) is better than match(A, C, String) for the first dimension but match(A, C, String) is better than match(D, B, String) for the second dimension.
Then the comparator considers match(D, B, String) and match(A, C, String) equals for (D, C) so that the multimethod throws an exception: the call is ambiguous.

In the next section, we see how to resolve ambiguous calls.