View source for 2021/11/25/PHP documentation gripe

{{nav/codeblog}}
==Executive Summary==
{{box/hilite|
I got confused on a couple of points:
:'''1.''' PHP7.4 doesn't recognize "mixed" as a type; it assumes it is a class name.
:'''2.''' I find it confusing that ''return'' types are allowed to get ''more'' specific, but ''parameter'' types are only allowed to become ''less'' specific.
}}
Item 2 especially just doesn't make sense to me; it seems to me that you would only ever want to get ''more'' specific in descendant classes.

I'd say that this ended up being primarily a PHP Gripe rather than a documentation gripe, but the documentation did use unnecessarily complex examples to illustrate the point... and they really should have had a table or something to clarify what is meant by "specific", especially with regard to non-objects.
==The Investigation==
The [https://www.php.net/manual/en/language.oop5.variance.php official PHP documentation] says:
<blockquote>Covariance '''allows a child's method to return a more specific type than the return type of its parent's method'''. Whereas, contravariance allows a parameter type to be less specific in a child method, than that of its parent.</blockquote>
...and then goes on to note that both are supported in {{hashtag|PHP7.4}} (which is what I am using).

However:
<syntaxhighlight lang=php line start=2 highlight=5>
abstract class cClass1 {
    abstract function GetIt() : mixed;
}
abstract class cClass2 extends cClass1 {
    abstract function GetIt() : object;
}
</syntaxhighlight>
<blockquote>PHP Fatal error:  Declaration of cClass2::GetIt(): object must be compatible with cClass1::GetIt(): mixed in /home/htnet/site/git/ferreteria/base/tests/php2.php on line 6</blockquote>
...and also...
<syntaxhighlight lang=php line start=2 highlight=5>
abstract class cClass1 {
    abstract function SetIt($v);
}
abstract class cClass2 extends cClass1 {
    abstract function SetIt(object $v);
}
</syntaxhighlight>
<blockquote>PHP Fatal error:  Declaration of cClass2::SetIt(object $v) must be compatible with cClass1::SetIt($v) in /home/htnet/site/git/ferreteria/base/tests/php2.php on line 6</blockquote>
So, kinda no.

The examples given in the documentation seem to be only about specificity of ''object'' types -- so I should be able to do this:
<syntaxhighlight lang=php line start=2 highlight=7>
class cClassA {}
class cClassB extends cClassA {}
abstract class cClass1 {
    abstract function SetIt(cClassA $v);
}
abstract class cClass2 extends cClass1 {
    abstract function SetIt(cClassB $v);
}
</syntaxhighlight>
...but, umm:
<blockquote>PHP Fatal error:  Declaration of cClass2::SetIt(cClassB $v) must be compatible with cClass1::SetIt(cClassA $v) in /home/htnet/site/git/ferreteria/base/tests/php2.php on line 8</blockquote>

Looking at the actual example in the docs (condensed down a bit for readability):
<syntaxhighlight lang=php>
abstract class Animal {
    protected string $name;
    public function __construct(string $name) { $this->name = $name; }
    abstract public function speak();
}
class Dog extends Animal {
    public function speak() { echo $this->name . " barks"; }
}
class Cat extends Animal {
    public function speak() { echo $this->name . " meows"; }
}
//---
interface AnimalShelter { public function adopt(string $name): Animal; }
class CatShelter implements AnimalShelter {
    public function adopt(string $name): Cat { return new Cat($name); } // returns Cat instead of Animal
}
class DogShelter implements AnimalShelter {
    public function adopt(string $name): Dog { return new Dog($name); } // returns Dog instead of Animal
}

$kitty = (new CatShelter)->adopt("Ricky");
$kitty->speak();
echo "\n";

$doggy = (new DogShelter)->adopt("Mavrick");
$doggy->speak();
echo "\n";
</syntaxhighlight>
This works -- so what's the difference? Tentatively, it's the fact that the parent function is defined in an '''interface''', not a class or trait -- but a test shows that this isn't the problem. Modifying AnimalShelter to be an abstract class instead of an interface gives us this:
<syntaxhighlight lang=php>
abstract class AnimalShelter { abstract public function adopt(string $name): Animal; }
class CatShelter extends AnimalShelter {
    public function adopt(string $name): Cat { return new Cat($name); } // returns Cat instead of Animal
}

class DogShelter extends AnimalShelter {
    public function adopt(string $name): Dog { return new Dog($name); } // returns Dog instead of Animal
}
</syntaxhighlight>
...which also works just fine.

Let's rewrite the doc example into the form in my examples at the top:
<syntaxhighlight lang=php>
class ClassA {}
class ClassB extends ClassA {}

abstract class Class1 { abstract public function GetIt(): ClassA; }
abstract class Class2 extends Class1 { abstract public function GetIt(): ClassB; }
</syntaxhighlight>
This works... and apparently the difference is that I'm dealing with return types rather than argument types. Changing the code to work with argument types produces the expected error:
<syntaxhighlight lang=php>
class ClassA {}
class ClassB extends ClassA {}

abstract class Class1 { abstract public function SetIt(ClassA $o); }
abstract class Class2 extends Class1 { abstract public function SetIt(ClassB $o); }
</syntaxhighlight>
<blockquote>PHP Fatal error:  Declaration of Class2::SetIt(ClassB $o) must be compatible with Class1::SetIt(ClassA $o) in /home/htnet/site/git/ferreteria/base/tests/php2.php on line 7</blockquote>
Reversing the parameters does produce results that agree with the documentation, i.e. this works:
<syntaxhighlight lang=php>
class ClassA {}
class ClassB extends ClassA {}

abstract class Class1 { abstract public function SetIt(ClassB $o); }
abstract class Class2 extends Class1 { abstract public function SetIt(ClassA $o); }
</syntaxhighlight>
Also, at some point I was clued in to the fact that PHP was parsing "mixed" as a specific type (class?), not a pseudoclass. Removing "mixed" from the return-type works:
<syntaxhighlight lang=php>
abstract class cClass1 {
    abstract function GetIt();
}
abstract class cClass2 extends cClass1 {
    abstract function GetIt() : object;
}
</syntaxhighlight>
The same is true for function parameters:
<syntaxhighlight lang=php>
abstract class cClass1 {
    abstract function SetIt(object $o);
}
abstract class cClass2 extends cClass1 {
    abstract function GetIt($o);
}
</syntaxhighlight>