Introspection for references

On Sun, Jul 8, 2018 at 10:42 AM, Nicolas Grekas < nicolas.grekas+php@gmail.com> wrote:

Thanks for the explanation. I think that the VarCloner use case needs two bits of functionality: 1. Detecting whether a variable is a reference, so you can handle this specially. 2. An efficient way of determining whether a variable is part of a reference that has already been seen (and which one). The second requirement is stronger than just the ability to detect whether two variables are part of the same reference. Given just a same_ref($v1, $v2) function, one would have to check against a list of all previously seen references one at a time, rather than only performing a hashtable lookup. Currently this functionality is implemented as: 1. Copying the array, assigning a cookie to the copy and seeing if the original array is modified. With an extra catch for TypeErrors, this is compatible with typed properties. 2. Replacing the reference with a Stub object, which can be looked up by object id. At the end the Stub objects are replaced with their values again. This is fundamentally incompatible with typed properties, as the type will likely not permit the Stub class. Here are my thoughts on possible APIs for this use case. Construction of reference-reflection objects ----- An issue already discussed in the other threads is that in PHP we need to specify whether a parameter is accepted by reference, by value or by preferred-reference. We don't have the possibility of accepting either a value or reference, whatever we get. This leaves us with a few options: 1. Introducing a VM-level primitive that is not subject to this limitation. The typed properties thread suggested a reflect_variable() language construct. I'm not too fond of this option because reference reflection seems like an awfully specific thing to introduce a new language construct for. 2. A ReflectionReference::fromVariable(&$var) constructor. Contrary to what was said in the other thread, this does not cause issues with the copy-on-write mechanism. Since PHP 7 references and non-references can share values (including immutablized values in SHM). However, this approach does have two issues: a) It is impossible to distinguish whether $var was a singleton reference or a value beforehand. Both will show up as rc=2 references inside ReflectionReference::fromVariable(). (This may also be an advantage, because from a language-design perspective, we treat singleton references as non-references.) b) In case the original $var was a variable, it will now be a reference, so this has a side-effect. 3. A ReflectionReference::fromArrayElem(array $array, string|int $key) constructor, as suggested by Nicolas. This avoids the reference/value problem and solves the specific VarCloner case efficiently and directly. On the other hand, introspection of references inside non-arrays requires some workarounds (e.g, casting objects to arrays). 4. A combination of these. For example we could have... ... ReflectionReference::fromArrayElem(array $array, string|int $key) for array items. ... ReflectionReference::fromObjectProp(object $object, string $key) for object properties. ... ReflectionReference::fromVariable(&$var) for any other special cases. This would allow to cover the common and interesting cases with specialized methods, and leave a less efficient fallback for the general case. This is probably the option I'd favor. Determining whether something is a reference ----- I think the best way to handle this (and the reason why I used named constructors above) is to return null if the value is not a reference. This should be the most common case and it would be best to avoid the overhead of constructing an unnecessary object in this case. One important question in this context would be whether we consider singleton references as references or not. If we do, then the ReflectionReference::fromVariable() constructor will always return a non-null value, as the variables will be turned into a singleton reference if it was a reference. If we consider them as references, we'll also want an API method to distinguish them. E.g. a specialized isSingleton() or more generally getNumUsers() == 1. The alternative would be to always construct a ReflectionReference object which may or may not be a reference and has an isReference() method. I don't see any advantages to that approach though. Reference equality ----- A couple of approaches: 1. Have an isEqual(ReflectionReference $other): bool method, which determined whether two references are the same. The disadvantage is that this only allows pair-wise comparisons, so it does not fully solve the VarCloner use-case. 2. Make ReflectionReference constructor uniquing. That is, if a ReflectionReference for a certain reference already exists, then the constructor will return the same object. This means that references can be compared by identity $ref1 === $ref2. It also means that they can be used in hashtables via spl_object_id(). (Caveat: It's important to keep the ReflectionReference object alive for the during in which spl_object_id() is used, as usual.) 3. Some variation on 2 via a separate API. That is don't unique ReflectionReferences themselves, but provide a separate getId() API. The returned ID would only be meaningful as long as at least one ReflectionReference object for the reference is live, otherwise it may be reused. Actual API ----- If we go with null return value on non-reference and uniquing, then most of the functionality is already provided by the constructor. The only useful API method I can think of is something like getNumUsers(). So, my overall suggestion would be the following API: class ReflectionReference { // Constructors return null if not a reference, object is uniqued static function fromArrayElem(array $array, string|int $key): ?ReflectionReference; static function fromObjectProp(object $object, string $key): ?ReflectionReference; static function fromVariable(&$var): ReflectionReference; // Basically the reference count. Would subtract 1 for the // fromVariable() constructor, to make the values consistent. function getNumUsers(): int; } Thoughts? Nikita