Re: [RFC] [DISCUSSION] Immutable/final/readonly properties

> On Feb 23, 2020, at 12:06 PM, Larry Garfield <larry@garfieldtech.com> wrote: > > On Sun, Feb 23, 2020, at 2:39 AM, Mike Schinkel wrote: >>> On Feb 21, 2020, at 6:17 PM, Larry Garfield <larry@garfieldtech.com> wrote: >>> I'm totally on board for better value object support, so that's a good motive >>> for me. The question I have is whether this is really a good stepping stone in that direction or if >>> it would lead down a wrong path and lock us into too much TIMTOWTDI (for the Perl fans in the room). >>> So let's think that through down that path. How would write-once properties lead into >>> properly immutable value objects? Or do they give us that themselves? >>> >>> The biggest challenge for immutable objects, IMO, is evolving them. Eg, >>> $result->withContentType(...) to use the PSR-7 example. Would we expect people to do it with a >>> method like that, or would there be some other mechanism? If the properties are public, would we >>> offer a more syntactic way to modify them directly? >>> >>> The with*() method style requires cloning the object. What happens to the locked >>> status of a set property if the object is cloned? Are they then settable again, or do they come >>> pre-locked? >>> >>> Neither of those seem good, now that I think about it. If they come pre-locked, then >>> you really can't clone, change one property, and return the new one (as is the standard >>> practice now in that case). If they don't come pre-locked, then the newly created object can >>> have everything on it changed, once, which creates a loophole. I'm not sure what the right >>> answer is here. >>> >>> My other concern is a public property (the most likely use case) would have to be set >>> in the constructor. If it's not, then callers cannot rely on it having been set yet if >>> it's set lazily. And if code inside the class tries to set it lazily, it may already have been >>> set by some external code (rightly or wrongly) and cause a failure. >> >>> >>> How do we address that? There's absolutely use cases where setting everything in >>> the constructor ahead of time is what you'd do anyway, but there are plenty where you >>> wouldn't want to, either, which creates a race condition for who sets it first, or tries to >>> access it before it gets set, etc. (This is where my repeated questions about lazy initialization >>> come from.) >> >> >> I have struggled to follow this RFC thread fully, so if I am getting >> something out of context, please note that and I apologize in advance. >> >> However, it would see that rules for write once properties >> to support >> lazy loading would be rather simple: >> >> 1. Write-once properties can only be updated once. >> 2. Write-once properties can only be updated within the class where >> they are declared. > > This is the common use case I think many envision, but nothing in the proposal requires that. > A public write-once property (as currently written) would be world-readable, and world-writeable, > once. > Separate visibility for internal and external access is a separate matter. (Also potentially > useful, but not part of the write-once proposal at the moment.) This just hit me, so I think I will mention it. The culture on the list seems not to be one of collaboration on RFC to find solutions to improve PHP but instead waiting for someone to stick their neck out with an RFC and then see who can shoot the most holes through it. I did not actually expect that. I would have hoped for a collaborate culture of the nature I am used to in local startup-oriented meetups where most everyone is trying to help each other rather than just pointing out that the work someone else has done is deficient and not worthy of moving forward. If that is the desired culture that most want to have on this list, then I will accept it even though I will lament how much better it could be otherwise. If I misunderstand, please help me understand how wording like "but nothing in the proposal requires that" and "not part of the write-once proposal at the moment" rather than "the proposal does not require that but lets's explore how adding that requirement can make the proposal better" should not leave the impression I just mentioned above? Or maybe the problem is the mailing list is just not a mechanism for collaborative work and we need a new mechanism. Like Github, comments, and pull requests? Moving on... >> 3. If you want to update a property from outside the class, create a >> set_<property>() method to allow it to happen. >> 4. If you do not want it to be set externally, do not implement a >> set_<property>() method. >> 5. If you want it to be implemented externally sometimes but not >> others, implement guard classes inside the set_<property>() method. >> >> I think that addresses all scenarios, no? >> >> -Mike > > It does not. > > 1) Race condition if I assume that a public write-once property is a materialized value, but > access it before it gets materialized. Can you please explain what "materialized value" means in this context? Are you speaking in Scala again? If you mean that a write-once property just has not yet been assigned, I am not sure how that differs from just accessing a property that has not yet been assigned today? > 2) Race condition if internal non-constructor code wants to set the value, but some external > routine has set it first. As I said, don't let the external routine set it directly. Which you did note. (I am interested in finding solutions in the same solution space as selected proposals and not just limiting myself to the exact specifics of the proposal because IMO limiting ourselves in that way has all the bad aspects of bureaucracy and none of the good aspects.) > 3) Cloning creates an interesting and complicated case of both of the above. Does a cloned > object start with its write-once bits reset or no? There's problems both ways. If there are problems, let us find solutions. 1. Only allow internal methods to change write-once properties. 2. Options: 2a. Extend clone statement to have clone rewrite $foo that would allow rewriting the write-once properties. 2b. Allow rewriting once of write-once object properties until some operation is performed to seal or lock the object (operation being a method call, a function call with the object as param, or a statement with the object as argument.) 2c. Don't allow rewriting of cloned objects. See if it is really a problem. Fix it in a future RFC if so. 2d. Something else I am someone we can think of. -Mike