For a long time I've tried to get my head wrapped around the visitor pattern, and somehow this thing keeps being rather fuzzy to me.
I'm currently under the impression it is only useful to apply operations on objects that implement the Composite pattern. At least as far as PHP is concerned. Is that an accurate observation, or am I missing something?
I'm in the process of reimplementing some functionality, and this includes computing a diff between two Entities. These Entities contain various value objects. Different derivatives in the type hierarchy of these Entities have different value objects. Originally the diff code was contained in the entities themselves, though this caused quite some clutter, so I'd rather move it out into dedicated service objects. The Visitor pattern sprung to mind, though I do not see how I can actually sanely apply it. Would all the value objects need to implement some EntityElement interface? That seems bad. Is this a case where the Visitor pattern indeed does not apply, or am I simply failing to see how it would be applied nicely here?