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What are the use cases for integrated object to object (ie, class to class) conversion in PHP?
I'm not entirely clear on what the use case is in PHP. When would that be superior to "just write an asFoo() method and move on with life"?

Reading your question triggered the following thoughts. They are not use-case's per-se because I can't remember them, but I know I have felt the need for these each at least once, and especially the last one.

Note that you cannot currently do any of these with an asFoo() method, at least not directly.

1. It would be nice to be able to create a new instance of a parent, grandparent class, etc. class given the properties of a child class instance. Maybe:

class Progenitor {
public $surname;
function __construct($surname) {
$this-> surname = $surname;
}
}
class Offspring extends Progenitor {}
$offspring = new Offspring('Schinkel');

$progenitor = clone $offspring asinstanceof Progenitor
echo get_class($progenitor); // prints: Progenitor
echo $progenitor->surname; // prints: Schinkel

2. Similarly it would be nice to be able to treat an object as an instance of its parent/grandparent/etc. where (parentof $instance) === $instance

$progenitor = $offspring asinstanceof Progenitor
echo get_class($progenitor); // prints: Progenitor
echo $progenitor === $offspring ? 'frue' : 'false; // prints: true

Note I have no idea if having two instances where references are equal but they are different classes would have unintended consequences, so this might not be a good idea, not sure. But it would be nice to be able to treat an object as its parent from time to time, if it is possible.

3. It would also be nice to create a child class starting with an instance of a parent as its base.

See example for #5. It should work the same.

4. Similarly it might be nice to be able to (somehow) assign a child class' identity and properties to an instance of its parent/grandparent/etc. where (childof $instance) === $instance although I have absolutely no idea how it this would work syntactically.

???

5. And probably the functionality I've wanted most in this area — which is admittedly only tangentially-related — is to be able to assign to $this so we could do something like clone an object to get an equivalent object. Or if case #1 or #2 above was possible, by cloning an instance of its parent/grandparent/etc. would initialize for that object. For example:

class Foo {
public $value;
function __construct(int $value) {
$this->value = $value;
}
function fromFoo(Foo $foo) {
$this = clone $foo; // <-- assigning $this replaces all state to be equal to the cloned instance
}
}
$f1 = new Foo(100);
$f2 = new Foo(0);
$f2->fromFoo($f1);
echo $f2->value; // prints 100

Scalar conversion I can see, but every time someone suggests adding siblings to __toString(), there's major pushback.

Look ma, no magic methods! :-)

-Mike

I like Rust's From and TryFrom traits, which allow types to define
conversions into or from other types. The From trait always succeeds,
and TryFrom can fail. However, it's not a "cast" -- the user calls an
into/try_into or from/try_from methods.

I would be supportive of defining official ways to convert types from
one to another, not necessarily casts, though. I think these qualities
are important:

1. Would have failable and non-failable variants. It's nice to know
   that a particular conversion cannot or can fail.
2. It should be amenable to static analysis. Tools should be able to
   warn about missing paths for handling failable conversions, or that
   conversion from A to B is possible but A to C is not.

I'm not sure how to technically achieve this for PHP. I don't think
any casting proposal I've seen meets these characteristics, so I would
personally vote no on them.

What are the use cases for integrated object to object (ie, class to class) conversion in PHP? Rust's type system is a very different beast, so its casting logic makes more sense there. I'm not entirely clear on what the use case is in PHP. When would that be superior to "just write an asFoo() method and move on with life"?

Scalar conversion I can see, but every time someone suggests adding siblings to __toString(), there's major pushback.

--Larry Garfield

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The features I was referring to are not casting; casting does exist in
Rust but this is not it. Rather, it's a generic way to convert
objects. It's nice to have a standard way to do it, instead of every
project doing something bespoke. It also can enable some use cases
that aren't relevant in PHP (yet) such as taking any generic type
which can be converted into a T, because you only want to do that
transformation conditionally on something else.

I was planning to propose something closer to

On Mon, Jul 12, 2021 at 2:19 AM Larry Garfield larry@garfieldtech.com
wrote:

What are the use cases for integrated object to object (ie, class to
class) conversion in PHP? Rust's type system is a very different beast, so
its casting logic makes more sense there. I'm not entirely clear on what
the use case is in PHP. When would that be superior to "just write an
asFoo() method and move on with life"?

Scalar conversion I can see, but every time someone suggests adding
siblings to __toString(), there's major pushback.

I was thinking of something akin to many compiled languages' approach of
"consider this expression is now of that type, and throw an exception if
it's not". An example of this approach from Benjamin's proposal of old^

$service = (EmailService) $diContainer->get('email.service');

Instead of

/** @var EmailService $service */
$service = $diContainer->get('email.service');
if (!$service instanceof EmailService) {
throw new TypeError('Expected instance of EmailService, ...');
}

--
Best regards,
Max Semenik