Re: RFC: Comparable interface

On Thu, 30 Sep 2010 16:25:28 +0100, Adam Harvey <aharvey@php.net> wrote:

The initial version of the patch actually did just that: it simply modified zend_std_compare_objects. That's pretty much the cleanest way you could do it. :) (...) Additionally, there are already semantics built into compare_function to handle object/non-object comparisons, although they're of very limited use in practice. The specific use case I had in mind was allowing comparisons of objects representing monetary amounts against numeric values (since that was a particular itch I had a couple of years ago), but I'm sure there would be others.

It's true that the engine already allows comparison of objects with scalars, but that can only happen after a cast. At least you now have the guarantee that if you compare a scalar x of type u with an object y and they're equal, then x === (u) y is also true*. With your patch, you could make $obj == "jjj" be true and (string) $obj yield a completely different thing. Obviously you wouldn't do this on purpose, my point is that this could lead to a hard to debug problem. This is what makes me very reluctant to support this more general approach of allowing scalar-object comparisons. * (maybe there's some edge case where this wouldn't happen because the engine is inconsistent with its preferences towards get, cast_object and __toString, but my point stands)

In terms of the specific question as to whether equality is symmetric: certainly it should be, but we would have no way of enforcing it, obviously. Java doesn't either, though: there's simply a note in the documentation saying that implementations should be symmetric.

Obviously it must be the programmer's responsibility to write a compare() function that is symmetric (and in general, establish a total order). My point about about symmetry was another: you know have potentially different code running in these two cases: $a == $b $b == $a The current implementation is "symmetric" in the sense the compare_objects handler is called only if both objects have the same handler (only the arguments are exchanged). -- Gustavo Lopes