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To: Josh Bruce <josh@joshbruce.dev>, Larry Garfield <larry@garfieldtech.com>
CC: php internals <internals@lists.php.net>
Thread-Topic: [PHP-DEV] [RFC][Discussion] Objects can be declared falsifiable
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Date: Sun, 9 Aug 2020 20:59:25 +0000
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Subject: Re: [PHP-DEV] [RFC][Discussion] Objects can be declared falsifiable
From: tysonandre775@hotmail.com (tyson andre)

Hi Josh,=0A=
=0A=
I'd recommend looking at https://github.com/php/php-src/pull/5156 (linked t=
o in your RFC's reference) when implementing this PR.=0A=
Additionally, I didn't see if this was brought up - PHP already has a class=
 which is falsifiable - SimpleXMLElement.=0A=
=0A=
You can see the source of ext/simplexml/simplexml.c for `_IS_BOOL`=0A=
=0A=
```=0A=
static int sxe_object_cast_ex(zend_object *readobj, zval *writeobj, int typ=
e)=0A=
{=0A=
	php_sxe_object *sxe;=0A=
	xmlChar        *contents =3D NULL;=0A=
	xmlNodePtr	    node;=0A=
	int rv;=0A=
=0A=
	sxe =3D php_sxe_fetch_object(readobj);=0A=
=0A=
	if (type =3D=3D _IS_BOOL) {=0A=
		node =3D php_sxe_get_first_node(sxe, NULL);=0A=
		if (node) {=0A=
			ZVAL_TRUE(writeobj);=0A=
		} else {=0A=
			ZVAL_BOOL(writeobj, !sxe_prop_is_empty(readobj));=0A=
		}=0A=
		return SUCCESS;=0A=
	}=0A=
```=0A=
=0A=
```=0A=
static php_sxe_object* php_sxe_object_new(zend_class_entry *ce, zend_functi=
on *fptr_count)=0A=
	intern->zo.handlers =3D &sxe_object_handlers;=0A=
// ... elsewhere in ext/simplexml/simplexml.c=0A=
	sxe_object_handlers.cast_object =3D sxe_object_cast;=0A=
```=0A=
=0A=
The default handlers would be overridden, so this would probably need to fa=
ll back to the original handlers for types other than bool.=0A=
Also, this might need to copy all of the handlers from the parent class ent=
ry's zend_object_handlers,=0A=
to meet the expected behavior of other cast types and other magic behaviors=
 from SimpleXMLElement and other classes continuing to work.=0A=
=0A=
From the perspective of static analysis, a few things to note:=0A=
- Static analyzers such as psalm/phan currently deliberately don't bother h=
andling the possibility that `object` can be false even after it was checke=
d for falsiness.=0A=
  There's hundreds of other things that could be implemented, and adding th=
e special casing and performance overhead checking for FFI objects and Simp=
leXMLElement, subclasses of those is currently low priority compared to tho=
se things.=0A=
=0A=
  Code using SimpleXMLElement/FFI is generally limited to a few files in pr=
actice.=0A=
=0A=
  Definitely possible for analyzers to support this for known base classes,=
 though, and the priority would increase if the RFC passed.=0A=
=0A=
  I am a maintainer of Phan.=0A=
- For BC reasons, internal data structures such as ArrayObject probably wou=
ldn't get changed in any php release=0A=
   (e.g. could break code using falsiness check instead of null check).=0A=
   So this might lead to inconsistencies with newer extensions if half of t=
he data structures treat emptiness as false and half don't.=0A=
- This feature may end up getting adopted in cases where it's convenient bu=
t turns out prone to accidental bugs and later is deprecated and removed.=
=0A=
  For example, https://stackoverflow.com/questions/25031236/if-elem-vs-elem=
-is-not-none was seen in python=0A=
=0A=
  (e.g. `if (!$this->resultSetObject) { $this->resultSetObject =3D slow_db_=
operation(); }`=0A=
  would not behave the way people would previously expect for most objects =
(executed repeatedly instead of once))=0A=
=0A=
```=0A=
<?php=0A=
function test(SimpleXMLElement $e) {=0A=
    // False positive RedundantCondition in psalm=0A=
    if ($e) {=0A=
    }=0A=
}=0A=
``` =0A=
=0A=
> And I don=92t know of a way to add an interface to new stdClass() - but t=
hought this might be a valid use case:=0A=
=0A=
`stdClass` isn't a final class. I assume they meant this=0A=
=0A=
```=0A=
class EmptyStdClass extends stdClass implements Falsifiable {=0A=
    public function __toBool() { return false; }=0A=
}=0A=
function example(stdClass $s) {=0A=
    if (!$s) { throw new Exception("Previously impossible"); }=0A=
}=0A=
```=0A=
=0A=
Thanks,=0A=
- Tyson=