RFC: Records

Hi Rob and Ilia

Thank you both for your detailed proposals and the efforts you put into
shaping this important feature for PHP It is clear a lot of thought has
gone into both approaches and I appreciate the opportunity to share my
thoughts.

niedz., 17 lis 2024 o 01:17 Ilija Tovilo tovilo.ilija@gmail.com
napisał(a):

Hi Rob

Hello internals,

I'm ready as I'm going to be to introduce to you: "Records"
https://wiki.php.net/rfc/records!

Thanks for your proposal. I very much agree that value semantics are a
highly desirable feature. I sent out my concept proposal on how this
may be achieved through structs some months ago. [1] There are some
remaining technical challenges, but the PoC looks promising. [2]

I personally do not think immutable data structures are a good
solution to the problem, and I don't feel like we need another,
slightly shorter way to do the same thing. In my proposal, I mentioned
that there are two primary issues with immutable data structures:

1. They are expensive. Any modification of the class requires a clone
   of the object by design, even when the cloned object is immediately
   discarded. This is most noticeable for lists and other growable data
   types, because they are large and change frequently. An O(n) insertion
   becomes infeasible very fast.
2. The clone is explicit, which gets old pretty quickly. Many of PHPs
   quality-of-life features like op-assign (e.g. +=) become unusable.

Structs would instead be fully-fledged objects that adopt CoW
(copy-on-write) semantics, which is how PHP already implements arrays.
Essentially, objects are automatically cloned when they are changed
and when the object is referenced from multiple places. If the
object is not referenced anywhere else, it may be safely mutated
without a clone, as there is nobody to observe this side-effect. When
a shared object is mutated multiple times (e.g. an append to a list in
a loop), only the first mutation requires a clone. It's also something
you don't need to think about, it just happens.

As mentioned, the main motivation of my proposal is to implement new
dedicated data structures like Vectors, Maps, Sets, etc. in PHP
(userland and extensions) without sacrificing ergonomics or
performance. However, they would also solve the same challenges for
simple data objects. IMO, immutability was never a great solution to
the problem of "spooky action at a distance". Mutability itself is not
a problem, just shared mutability.

Note that this concept is heavily inspired by Swift, Rust and likely
other languages I don't know. Chris Lattner (the original author of
LLVM and Swift) has shared very similar thoughts in an interview
recently. [3]

If the primary motivation of your RFC is to reduce boilerplate, then
it's true that structs would not help much. I do wonder whether 1. we
need even shorter code and 2. if we do, whether it's something that
could be added to classes alike, rather than tying it to records when
it could be more generic instead.

A small note: The $test = &Test(); syntax is ambiguous, as it's
already legal. https://3v4l.org/CE5rt

Ilija

[1] https://externals.io/message/122845
[2] https://github.com/php/php-src/pull/13800
[3] https://youtu.be/JRcXUuQYR90?si=p51_x3wkfeeeGfq-&t=3319

The goal of Records as I see it is to provide a concise syntax for defining
immutable value objects optimized for simplicity and common use cases
and I strongly align with this goal. The concise syntax in your proposal is
a significant advantage for small immutable types

record User(string $name, int $age)

This simplicity eliminates boilerplate making it an excellent fit for
common scenarios like DTOs or configuration objects

Structs are designed to provide immutability with Copy-on-Write semantics
ensuring efficiency when dealing with large or nested structures

While I prefer Rob’s syntax Ilia’s focus on performance through
Copy-on-Write is compelling. For example CoW optimization could make
scenarios like this more efficient

struct Line {
public Point $start
public Point $end
}

$line1 = new Line(start: new Point(0, 0), end: new Point(1, 1))
$line2 = $line1->with(end: new Point(2, 2))

Could CoW be integrated into Records for performance-critical cases without
sacrificing simplicity?

On Rob’s Records proposal the Syntax Simplicity and Automatic Equality make
equality straightforward and intuitive

$user1 = new User("Alice", 30)
$user2 = new User("Alice", 30)
var_dump($user1 === $user2) // true

On Ilia’s Structs proposal Copy-on-Write Semantics offers performance
advantages for large or nested structures making it a great fit for
optimizing immutability but the Explicit Equality - while useful for
complex cases explicit equality checks introduce additional boilerplate for
simple scenarios.

Rob’s Records proposal is exceptionally well-suited for lightweight
immutable types and aligns with PHP’s evolution toward simplicity and
developer experience. That said, Ilia's CoW optimizations are valuable for
performance and could complement Records in specific cases.

Thank you both again for your hard work and thoughtful proposals. To move
this discussion forward perhaps we could organize a community vote to gauge
which capabilities are preferred overall. This could help the team better
understand the direction the community supports and explore possible hybrid
solutions if needed.

On a related note I was recently asked about my old draft RFC proposal for
Structs[1] and whether it was abandoned or forgotten. While it didn’t
address many of the ideas you have both raised, it was built with
composition in mind from the start. I hope that such capabilities could
also be explored in future extensions as they would bring additional
flexibility to these immutable types.

[1] https://wiki.php.net/rfc/structs#fields_composition

Best regards

Michał Marcin Brzuchalski

Hello Ilija and Larry,

You both touch upon some interesting and similar thoughts, and it may be worth sharing how I arrived here; at least so we have some shared context:

Personally, I feel that classes are quite bloated with features and what feature works with what is quite confusing for new developers; I would rather see features removed than added to them at this point. This isn't why I chose the "record" keyword or a new syntax, to be clear. However, it was one of many strong reasons as to why I felt it would be an ok deviation from "tacking on more features to classes."

One of the main reasons for the alternative creation syntax is because I felt that "new" was misleading at best, and just plain wrong at worst. It is also why I chose "&", to make it clear you are not getting "a new one" but one that "just happens to exist with the values you asked for." I'd be open to a different keyword or something else entirely. It's just that "new" is the wrong one for records.

I did explore "data classes" to a degree, and I can see why Ilija created the new "mutating" syntax for their RFC. It gets really weird, really fast. Records are the other side of structs, though. They are (nearly literally) arrays with a class entry on them, and thus, essentially, typed structured arrays with behavior. In fact, I was originally going to pitch it as such, but decided it would either stand on its own or not; but it is what they were designed to be from the beginning; hence the short declaration syntax.

In other words, I see this being used anywhere you'd normally use a structured array but want some type safety, without all the boilerplate of classes or dealing with equality. For example, DTO's, configuration, etc. They solve a different problem than Ilija's structs, which makes more sense for collections, but I see records as being good candidates for values in those collections.

As I shopped this RFC around to coworkers, old coworkers, other maintainers on the projects I work on, and (nearly) random strangers on the internet, it became clear that people liked it and understood them, but wanted more power. Things like better custom (de)serialization than what we have with readonly classes, custom initializers for computed properties, etc.

Furthermore, as I thought about these new features for records, I also thought about how people would use them. They would most likely start with a simple record (or not), but when changing them, I also thought about the diffs they would create in code reviews. In fact, this is a large reason behind the rules for traditional constructors. Adding a traditional constructor to an already existing record should look exactly like that in the diff, without shifting around a bunch of properties.

Eventually, we ended up with the RFC you are reading today.

(To be 100% transparent, some people also thought it was pointless and wondered what was wrong with readonly classes, but I'll come back to that).

Now, with that context in mind...

Plus, having another fixed type creates questions any time a new feature is added.

I think this would happen even if I scoped out some of the RFCs you hinted at. I don't think there is any way around this. Adding new features complicates future features, and it doesn't matter where you put them. Or rather, it matters, but not as much as you'd think.

For example, my nameof RFC is basically on hiatus until we have pattern matching. We simply lack the grammar necessary to do it correctly, and it doesn't seem like the place to define that grammar because then it may pigeonhole pattern matching. I think that is fine, but these are normal things to go through when working on an RFC, in my limited experience.

My point is, we constantly have to make this decision process, and we just have to 'figure it out' as we go.

I personally do not think immutable data structures are a good
solution to the problem, and I don't feel like we need another,
slightly shorter way to do the same thing.

As Ilija notes, immutable objects are not always the answer. I like them, and use them frequently, but they're not always appropriate. And we already have them with either readonly classes or now private(set) (which is close enough to immutable 99.4% of the time)

To be clear, this is another tool in the developer's toolbox and not meant to replace classes (even readonly classes). They are strictly immutable value objects, which makes sense for numbers, time, custom values (see: the UserId example in the RFC), etc. These aren't as good for generic collections like maps, sets, vectors, etc. or even things like services or controllers. For these types of things, classes (or structs) make more sense.

Like Ilija mentioned in their email, there are significant performance optimizations to be had here that are simply not possible using regular (readonly) classes. I didn't go into detail as to how it works because it feels like an implementation detail, but I will spend some time distilling this and its consequences, into the RFC, over the coming days. As a simple illustration, there can be significant memory usage improvements:

100,000 arrays: https://3v4l.org/Z4CcV
100,000 readonly classes: https://3v4l.org/1vhNp

and what we would expect from records: https://3v4l.org/4nYXG which is on par with the array example.

Naturally, real life won't see that kind of reduction in memory consumption (nobody is creating an array of all the same items, most of the time), but hopefully it gives you an idea of what can be gained.

I can see the benefit of an inline constructor. Kotlin has something similar. But I can see the benefit of it for all classes, even service classes, not just records. (In Kotlin, it's used for service classes all the time.)

I think this would be a good future scope. If people like it, an RFC to extend it to regular classes makes sense.

There's already been an RFC for clone-with that works on any object; it just never made it to a vote. I could see an argument for an even more dedicated syntax (eg, eliminate "clone" and just do "$foo with (bar: 'baz')"), but again, useful on all objects, not just records.

I feel like this is similar to my nameof RFC and doomed from the start. Classes are just too featureful to pin down what a "with" actually means. The discussion went into depth on this, and nothing definitive came out of it (IMHO). In a sense, records get a "fresh start" and can define exactly what it means and how it can be used.

The alternate creation syntax... OK, this one I can't really see a benefit to, and Ilija already noted it may cause conflicts.

And from Ilija:

A small note: The $test = &Test(); syntax is ambiguous, as it's
already legal. https://3v4l.org/CE5rt

I originally had something about this in the RFC but removed it at the last minute. I'll add it back! For now, it will live in "open issues" in case someone has a better idea. As to the reason for it, I covered it briefly above, but as a reminder, it boils down to my reluctance to use "new" to get a record. If anyone has any better ideas, I'm open to it.

And again, from Larry:

Value-based strong equality, in cases where I want that, I also want to be able to control it. That goes back to Jordan's operator overload RFC, and specifying a custom == and <=>. I'd rather just have that.

I originally wanted to add operators. For (I hope) obvious reasons, I decided to just wait for them. I think something like records (with value semantics built-in) would be much more palatable for people worried about the "abuse" of operators. I'm just going to steer clear of that topic and leave it "undefined" for now--with the expectation that someone (Jordan?) may come along to define operations on objects.

Value-style passing is the really interesting one, but I want to be able to use it without being forced into all of the other features here.

I don't think there is much way around it. You either need a special syntax (structs) or immutability (records). For regular classes, there is always "==" if you have control over how they are compared. Sadly, I don't think this is possible for regular classes, but I could be wrong.

And... that's a book. I'm really sorry about the length of this email, but hopefully I've addressed both your questions and concerns as best I can.

Sincerely,

— Rob

Hi Larry,

niedz., 17 lis 2024 o 08:24 Larry Garfield larry@garfieldtech.com
napisał(a):

...
I can see the benefit of an inline constructor. Kotlin has something
similar. But I can see the benefit of it for all classes, even service
classes, not just records. (In Kotlin, it's used for service classes all
the time.)

What visibility would you expect for inline constructor properties in
service classes?
For records it is clearly a public fields declaration, while for service
classes I'd expect private|protected - adding visibility increases the
boilerplate back to normal classes.

The alternate creation syntax... OK, this one I can't really see a benefit
to, and Ilija already noted it may cause conflicts.

I agree, personally I'd see "new" keyword used like for normal classes
still feasible.

I would far prefer assembling record-ish behavior myself, using the
smaller parts above. Eg:

final readonly data class Point(int $x, int $y);

"final" prevents extension. "readonly" makes it immutable. "data" gives
it value-passing semantics. Any class can use an inline constructor.
"with" is designed to work automatically on all objects. Boom, I've just
assembled a Record out of its constituent parts, which also makes it easier
for others to learn what I'm doing, because the features opted-in to are
explicit, not implicit.

It opens a bunch of concerns, questions like why would you use "data"
keyword without having "final"?
Although I like the "data" keyword very much.

Kind regards,

Michał Marcin Brzuchalski

I would far prefer assembling record-ish behavior myself, using the smaller parts above. Eg:

final readonly data class Point(int $x, int $y);

"final" prevents extension. "readonly" makes it immutable. "data" gives it value-passing semantics. Any class can use an inline constructor. "with" is designed to work automatically on all objects. Boom, I've just assembled a Record out of its constituent parts, which also makes it easier for others to learn what I'm doing, because the features opted-in to are explicit, not implicit.

Sorry for the radio silence, but I found myself saying something was impossible. This is software, anything is possible...

Anyway, I think I figured out how to implement a "data" modifier, and it is quite excellent (for the exact reasons you give).

I'll break this up into multiple RFCs, pending some further tests to try and break my proof-of-concept:

• a "data" modifier on class for value semantics
• an "inline constructor"

As you mention, at that point, a "record" or "value" keyword can just be shorthand for "final readonly data class".

— Rob