====== Request for Comments: Prototype checks ======
  * Version: 1.0
  * Date: 2011-09-19
  * Author: Etienne Kneuss <colder@php.net>
  * Status: Inactive
  * First Published at: http://wiki.php.net/rfc/prototype_checks

===== Introduction =====

Past discussions on the mailing lists have shed some light on the various ways we handle prototype checks and what may be done to improve PHP in that area.
This RFC summarizes the current state (5.3/5.4) of prototype checks and possible improvements to it

===== Prototype checks =====

Prototype checks occur in different contexts:

==== Implementing abstract method ====
=== Normal methods ===
The prototype is checked with current normal rules (see Current rules). Any mismatch with current rules generates a FATAL error.
=== Constructors ===
  * In 5.3: No checks are performed
  * In 5.4: The prototype is checked with current normal rules (see Current rules). Any mismatch with current rules generates a FATAL error.

==== Implementing interface method ====
=== All Methods ===
The prototype is checked with current normal rules (see Current rules). Any mismatch with current rules generates a FATAL error.


==== Overriding concrete method ====
=== Normal methods ===
The prototype is checked with current normal rules (see Current rules). Any mismatch with current rules generates a STRICT error.
=== Constructors ===
No checks are performed.

==== Overriding abstract method ====
Scenario:

  abstract class ParentAbs {
      abstract public function foo($a);
  }
  abstract class SubAbs extends ParentAbs {
      abstract public function foo($a);
  }
  
NOT allowed, always result in a fatal error even if the prototypes are compatible.

==== Overriding interface method ====
Scenario:

  interface ParentIface {
      function foo($a);
  }
  interface SubIface extends ParentIface {
      function foo($a);
  }
  
NOT allowed, always result in a fatal error even if the prototypes are compatible.

  
===== Current rules =====

==== Allowed ====
This part specifies what is currently considered as valid signature modifications:

=== Adding new optional arguments ===

  function foo($a)
is compatible with, in a sub class:
  function foo($a, $b = 42)

=== Adding a return-by-ref ===

  function foo($a)
is compatible with, in a sub class:
  function &foo($a)

==== Mismatch ====
This part specifies what is currently considered as invalid signature modifications:

=== Strenghtening the type hint ===
Given:
  class A {}
  class B extends A{}

  function foo(A $a)
is imcompatible with, in a sub class:
  function foo(B $a)

=== Removing a return by ref ===

  function &foo()
is imcompatible with, in a sub class:
  function foo()


=== Adding a mandatory argument ===

  function foo()
is imcompatible with, in a sub class:
  function foo($a)

==== Mismatch but theoretically compatible ====

This part specifies what is currently considered as invalid modifications, despite being theoretically sound:

=== Removing the type hint ===

  function foo(Array $a)
is currently imcompatible with, in a sub class:
  function foo($a)

=== Weakening the type hint ===
Given:
  class A {}
  class B extends A{}

  function foo(B $a)
is currently imcompatible with, in a sub class:
  function foo(A $a)

=== Arg no longer by ref ===

  function foo(&$a)
is currently imcompatible with, in a sub class:
  function foo($a)


=== Requiring less arguments ===

  function foo($a)
is currently imcompatible with, in a sub class:
  function foo()


===== Topics worth discussing =====

==== Allow more theoretically valid modifications ====

We have three wrong errors for modifications that should be accepted. Some of those might require more sophisticated checks than others, so not all might be worth including.

==== Clarify the constructor problem ====
Constructors can be seen as pseudo-static methods, for this reason, the same checks for normal methods do not always apply for constructors.

It is however unclear whether we really want, for constructors, to be more strict (and how strict) if they are defined via an abstract class. So:

  - Do we really want that check to be performed on 5.4 for constructors coming from abstract methods
  - Do we really want the mismatch to result in a FATAL error (potential BC break coming from 5.3 where no checks were done)


==== Similar prototypes from different interfaces ====
It would be better to allow multiple interfaces to define the same intersection of prototype. It is currently not allowed in any case.

For example:

  interface A {
    function apply($a, $b);
    // ...
  }
  
  interface B {
     function apply($a, $b);
     // ...
  }
  
  class C implements A, B { .. }

This is currently not allowed, but there is no reason why it shouldn't be.