PHP RFC: Parameter No Type Variance

• Version: 0.2
• Date: 2017-01-01
• Author: Niklas Keller me@kelunik.com
• Status: Under Discussion
• First Published at: http://wiki.php.net/rfc/parameter-no-type-variance

PHP doesn't currently allow variance for parameters as checking these for compatibility isn't possible on compile time. This limitation is caused by autoloading and doesn't allow widening the accepted parameters.

This RFC proposes to allow omitting the type entirely in a subclass, as dropping all parameter constraints is always valid according to the LSP principle.

Another practical use-case is, instead of widening the accepted type, allowing parent classes to introduce type declarations and having the sub-classes still compatible, i.e. this allows libraries to be upgraded to use (scalar) type declarations instead of manual checks without requiring an update for all sub-classes.

<?php
 
class ArrayClass {
  public function foo(array $foo) { /* ... */ }
}
 
class IterableClass extends ArrayClass {
  // This implementation also accepts Traversable.
  // Variance is already possible in this special case using "iterable".
  public function foo(iterable $foo) { /* ... */ }
}
 
class EverythingClass extends ArrayClass {
  // This implementation accepts all values.
  // Restrictions may be done via user code in the method body.
  // Variance is currently not allowed, it throws a warning if parent or child miss a type.
  public function foo($foo) { /* ... */ }
}

Current Result

Warning: Declaration of EverythingClass::foo($foo) should be compatible with ArrayClass::foo(array $foo) in %s on line 18

Proposed Result

Compiles without a warning.

Derick added a DateTimeZone type declaration for the third arg of the DateTime::createFromFormat() method. The method is already documented to accept only this class in the manual, but the type declaration is not actually present in the implementation.

However, this change had to be reverted, because all classes extending DateTime currently don't have this type declaration (and adding it would be illegal under LSP), so they started throwing a method signature mismatch warning.

None.

Next version, currently 7.2.

In the future there might be full support for contra-variance / co-variance for parameters / return types.

Requires a 2/3 majority.

TBD.

TBD.

None.