PHP: rfc:dbc2

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====== PHP RFC: Native DbC support as definition ======
  * Version: 0.1
  * Date: 2015-02-10
  * Author: Yasuo Ohgaki <yohgaki@ohgaki.net>, Dmitry Stogov <dmitry@zend.com>, Joe Watkins <pthreads@pthreads.org>
  * Status: Under Discussion
  * First Published at: http://wiki.php.net/rfc/dbc2

===== Important Note =====

This RFC is part of "Design by Contract Introduction" RFC
  * https://wiki.php.net/rfc/introduce_design_by_contract

This RFC is an alternative approach to "Native DbC support" RFC.
  * http://wiki.php.net/rfc/dbc

Contracts can be defined without modification to the language, however, the D documentation calls the resulting implementation "clumsy and inconsistent".

Other advantages of a native implementation, also taken from the D manual:

  * a consistent look and feel for the contracts
  * tool support
  * compiler optimizations
  * easier management and enforcement of contracts
  * handling of contract inheritance

All of the above applies to PHP.

===== Introduction =====

The D manual contains the following succinct definition of contracts:

> The idea of a contract is simple - it's just an expression that must evaluate to true. If it does not, the contract is broken, and by definition, the program has a bug in it. Contracts form part of the specification for a program, moving it from the documentation to the code itself. And as every programmer knows, documentation tends to be incomplete, out of date, wrong, or non-existent. Moving the contracts into the code makes them verifiable against the program.

This should be easily comprehensible, it further validates the argument that the best implementation is a native one, so that the programmer really can "move the specification from the documentation to the code".

===== Proposal =====

Support for the following contracts will be introduced:

  * precondition       "require"( **precondition-expression** [,'Error msg'])
  * postcondition      "return"( **return_value**, **postcondition-expression** [,'Error msg'])
  * invariant          "require"( **invariant-expression** [,'Error msg'])

===== Pre and Post Condition =====

These contracts should be defined after the function declaration, but before the function body.

Multiple precondition and postcondition contracts may be used. The expressions are just a regular PHP expressions. They are evaluated in the function scope, and can access arguments and other scope variables, and return value (via a reserved name). Pre and post-conditions can't be defined for abstract methods and methods defined in interfaces.

<code>
function add(int $a, int $b) : int
	require($a > 0)
	require($b > 0)
	return($ret, $ret > 0, "something wrong")
{
	return $a + $b;
}
</code>

this code is going to be evaluated as

<code>
function add(int $a, int $b) : int
{
	assert($a > 0);
	assert($b > 0);
	$ret = $a + $b;
	assert($ret > 0, "something wrong");
	return $ret;
}
</code>

===== Invariant =====

Invariant contracts are declared using **require** inside class body. Multiple invariant contracts may be used. They may access object or static properties through **$this** and **self**. Invariant contracts may be used for classes, abstract classes and traits, but not for interfaces.

<code>
class Accumulator {
	private $sum = 0;
	
	function add(int $a)
		require($a > 0)
	{
		$this->sum += $a;
	}

	require($this->sum > 0, "overflow detected");
}
</code>

this code is going to be evaluated as

<code>
class Accumulator {
	private $sum = 0;
	
	function add(int $a)
	{
		assert($a > 0);
		assert($this->sum > 0, "overflow detected");
		$this->sum += $a;
		assert($this->sum > 0, "overflow detected");
	}
}
</code>

**Invariant contracts are not evaluated when object properties are changed from outside the class scope.**

====Contracts Inheritance Rules====

Contracts are constant, this has the following implications:

  * a derived class must satisfy invariant contracts of it's parent
  * a derived class overriding a method must satisfy the pre and post condition contracts of it's prototype.

Thus, given the following code:

<code>
class Child {
    require ($this->age < 18);
    
    public function someMethod($input) 
        require(somethingAbout($input)) {
        /* ... */
    }

    /* ... */
}

class Monster extends Child {
    require ($this->odour == OBNOXIOUS);

    public function someMethod($input) 
        require(somethingElseAbout($input)) {
        /* ... */
    }

    /* ... */
}
</code>

//Monster// must not break **any** contract in //Child//.

====Execution Control====

A new "dbc" INI switch is going to be introduced. It may get the following values:

  * dbc=on        - generate code for contracts and check them at run-time. Program, at any time, may change this settion to dbc=off through ini_set().
  * dbc=off       - generate code for contracts but don't check them at run-time. Program, at any time, may change this settion to dbc=on through ini_set().
  * dbc=zero_cost - don't generate code for contracts. This may be set only in php.ini and can't be changed through ini_set().

  The default value is "off".

===Contracts Execution Order===

If "dbc" is set to "on", the order of contracts validation is the following:

  - all require() contracts (precondition) defined for this function (and prototype where applicable)
  - all require() contracts (invariant) for this class and parents
  - method body
  - all require() contracts (invariant) for this class and parents
  - all return() contracts (postcondition) defined for this function (and prototype where applicable)

**Invariant and Special Methods**

  * <nowiki>__constructs()/__wakeup()/__set_state()</nowiki> will NOT execute invariant before method body.
  * <nowiki>__destruct()/__sleep()</nowiki> will NOT execute invariant after method body.

**Static Call**

  * Only pre and post conditions are executed.

===== Backward Incompatible Changes =====

None

  * No additional keyword

===== Proposed PHP Version(s) =====

  * PHP7

===== RFC Impact =====
==== To SAPIs ====
None

==== To Existing Extensions ====
None

==== To Opcache ====
Opcache will have to be extended to support contracts and store them in shared memory.

==== New Constants ====
None

==== php.ini Defaults ====
dbc=Off for all (INI_ALL)

  * hardcoded default values
  * php.ini-development values
  * php.ini-production values

===== Open Issues =====
  * Contracts inheritance rules
  * Consider introduction of **static require()** as class invariant for static methods
  * Need to discuss syntax
  * How to manage votes for 2 RFCs

===== Unaffected PHP Functionality =====

This RFC does not affect any existing features

===== Future Scope =====

Documentation systems may adopt native DbC syntax for documentation purpose.

===== Vote =====
This project requires a 2/3 majority

  * Option would be YES/NO only


===== Patches and Tests =====
Links to any external patches and tests go here.

If there is no patch, make it clear who will create a patch, or whether a volunteer to help with implementation is needed.

Make it clear if the patch is intended to be the final patch, or is just a prototype.

===== Implementation =====
After the project is implemented, this section should contain 
  - the version(s) it was merged to
  - a link to the git commit(s)
  - a link to the PHP manual entry for the feature

===== References =====

  * http://wiki.php.net/rfc/dbc
  * http://wiki.php.net/rfc/expectations

===== Rejected Features =====
Keep this updated with features that were discussed on the mail lists.
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