rfc:prototype_checks

Request for Comments: 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:

  1. Do we really want that check to be performed on 5.4 for constructors coming from abstract methods
  2. 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.

rfc/prototype_checks.txt · Last modified: 2014/04/09 00:52 by levim