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Subject: [PATCH] [RFC] Closures and lambda functions in PHP
From: chris_se@gmx.net (Christian Seiler)

Hi,

As a followup to the discussion in January, I'd like post a revised patch to
this list that implements closures and anonymous functions in PHP.

INTRODUCTION
------------

Closures and lambda functions can make programming much easier in 
several ways:

  1. Lambda functions allow the quick definition of throw-away functions
     that are not used elsewhere. Imaging for example a piece of code that
     needs to call preg_replace_callback(). Currently, there are three
     possibilities to acchieve this:

      a. Define the callback function elsewhere. This distributes code that
         belongs together throughout the file and decreases readability.

      b. Define the callback function in-place (but with a name). In 
that case
         one has to use function_exists() to make sure the function is only
         defined once. Example code:

          <?php
             function replace_spaces ($text) {
               if (!function_exists ('replace_spaces_helper')) {
                 function replace_spaces_helper ($matches) {
                   return str_replace ($matches[1], ' ', '&nbsp;').' ';
                 }
               }
               return preg_replace_callback ('/( +) /', 
'replace_spaces_helper',
                                             $text);
             }
          ?>

         Here, the additional if() around the function definition makes the
         source code difficult to read.

      c. Use the present create_function() in order to create a function at
         runtime. This approach has several disadvantages: First of all, 
syntax
         highlighting does not work because a string is passed to the 
function.
         It also compiles the function at run time and not at compile 
time so
         opcode caches can't cache the function.

  2. Closures provide a very useful tool in order to make lambda 
functions even
     more useful. Just imagine you want to replace 'hello' through 
'goodbye' in
     all elements of an array. PHP provides the array_map() function which
     accepts a callback. If you don't wan't to hard-code 'hello' and 
'goodbye'
     into your sourcecode, you have only four choices:

      a. Use create_function(). But then you may only pass literal values
         (strings, integers, floats) into the function, objects at best as
         clones (if var_export() allows for it) and resources not at 
all. And
         you have to worry about escaping everything correctly. 
Especially when
         handling user input this can lead to all sorts of security issues.

      b. Write a function that uses global variables. This is ugly,
         non-reentrant and bad style.

      c. Create an entire class, instantiate it and pass the member function
         as a callback. This is perhaps the cleanest solution for this 
problem
         with current PHP but just think about it: Creating an entire 
class for
         this extremely simple purpose and nothing else seems overkill.

      d. Don't use array_map() but simply do it manually (foreach). In this
         simple case it may not be that much of an issue (because one simply
         wants to iterate over an array) but there are cases where doing
         something manually that a function with a callback as parameter 
does
         for you is quite tedious.

     [Yes, I know that str_replace also accepts arrays as a third 
parameter so
     this example may be a bit useless. But imagine you want to do a more
     complex operation than simple search and replace.]

PROPOSED PATCH
--------------

I now propose a patch that implements compile-time lambda functions and
closures for PHP while keeping the patch as simple as possible. The patch is
based on a previous patch on mine which was based on ideas discussed here
end of December / start of January.

Userland perspective
--------------------

1. The patch adds the following syntax as a valid expression:

function & (parameters) { body }

(The & is optional and indicates - just as with normal functions - that the
anonymous function returns a reference instead of a value)

Example usage:

$lambda = function () { echo "Hello World!\n"; };

The variable $lambda then contains a callable resource that may be called
through different means:

$lambda ();
call_user_func ($lambda);
call_user_func_array ($lambda, array ());

This allows for simple lambda functions, for example:

function replace_spaces ($text) {
   $replacement = function ($matches) {
     return str_replace ($matches[1], ' ', '&nbsp;').' ';
   };
   return preg_replace_callback ('/( +) /', $replacement, $text);
}

2. The patch implements closures by defining an additional keyword 'lexical'
that allows an lambda function (and *only* an lambda function) to import
a variable from the "parent scope" to the lambda function scope. Example:

function replace_in_array ($search, $replacement, $array) {
   $map = function ($text) {
     lexical $search, $replacement;
     if (strpos ($text, $search) > 50) {
       return str_replace ($search, $replacement, $text);
     } else {
       return $text;
     }
   };
   return array_map ($map, array);
}

The variables $search and $replacement are variables in the scope of the
function replace_in_array() and the lexical keyword imports these variables
into the scope of the closure. The variables are imported as a reference,
so any change in the closure will result in a change in the variable of the
function itself.

3. If a closure is defined inside an object, the closure has full access
to the current object through $this (without the need to use 'lexical' to
import it seperately) and all private and protected methods of that class.
This also applies to nested closures. Essentially, closures inside 
methods are
added as public methods to the class that contains the original method.

4. Closures may live longer as the methods that declared them. It is 
perfectly
possible to have something like this:

function getAdder($x) {
   return function ($y) {
     lexical $x;
     return $x + $y;
   };
}

Zend internal perspective
-------------------------

The patch basically changes the following in the Zend engine:

When the compiler reaches a lambda function, it creates a unique name 
for that
function ("\0__compiled_lambda_FILENAME_N" where FILENAME is the name of the
file currently processed and N is a per-file counter). The use of the 
filename
in the function name ensures compability with opcode caches. The lambda
function is then immediately added to the function table (either the global
function table or that of the current class if declared inside a class 
method).
Instead of a normal ZEND_DECLARE_FUNCTION opcode the new
ZEND_DECLARE_LAMBDA_FUNC is used as an opcode at this point. The op_array
of the new function is initialized with is_lambda = 1 and is_closure = 0.

When parsing a 'lexical' declaration inside an anonymous function the parser
saves the name of the variable that is to be imported in an array stored
as a member of the op_array structure (lexical_names).

The opcode handler for ZEND_DECLARE_LAMBDA_FUNC does the following: First of
all it creates a new op_array and copies the entire memory structure of the
lambda function into it (the opcodes themselves are not copied since they
are only referenced in the op_array structure). Then it sets is_closure = 1
on the new op_array, and for each lexical variable name that the compiler
added to the original op_array it creates a reference to that variable from
the current scope into a HashTable member in the new op_array. It also saves
the current object pointer ($this) as a member of the op_array in order to
allow for the closure to access $this. Finally it registers the new op_array
as a resource and returns that resource.

The opcode handler of the 'lexical' construct simply fetches the variable
from that HashTable and imports it into local scope of the inner function
(just like with 'global' only with a different hash table).

Some hooks were added that allow the 'lambda function' resource to be 
called.
Also, there are several checks in place that make sure the lambda function
is not called directly, i.e. if someone explicitely tries to use the 
internal
function name instead of using the resource return value of the declaration.

The patch
---------

The patch is available here:
<http://www.christian-seiler.de/temp/closures-php-5.3-2008-06-16-1.diff>

Please note that I did NOT include the contents of zend_language_scanner.c
in the patch since that can easily be regenerated and just takes up enormous
amounts of space.

The patch itself applies against the 5.3 branch of PHP.

If I understand the discussion regarding PHP6 on this list correctly, some
people are currently undergoing the task of removing the unicode_semantics
switch and if (UG(unicode)). As soon as this task is finished I will also
provide a patch for CVS HEAD (it doesn't make much sense adopting the patch
now and then having to change it again completely afterwards).

BC BREAKS
---------

  * Introduction of a new keyword 'lexical'. Since it is very improbable 
that
    someone should use it as a function, method, class or property name, I
    think this is an acceptable break.

Other that that, I can find no BC breaks of my patch.

CAVEATS / POSSIBLE WTFS
-----------------------

  * On writing $func = function () { }; there is a semicolon necessary. 
If left
    out it will produce a compile error. Since any attempt to remove that
    necessity would unecessarily bloat the grammar, I suggest we simply keep
    it the way it is. Also, Lukas Kahwe Smith pointed out that a single
    trailing semicolon after a closing brace already exists: do { } 
while ();

  * The fact that 'lexical' creates references may cause certain WTFs:

      for ($i = 0; $i < 10; $i++) {
        $arr[$i] = function () { lexical $i; return $i; };
      }

    This will not work as expected since $i is a reference and thus all
    created closures would reference the same variable. In order to get this
    right one has to do:

      for ($i = 0; $i < 10; $i++) {
        $loopIndex = $i;
        $arr[$i] = function () { lexical $loopIndex; return $loopIndex; };
        unset ($loopIndex);
      }

    This can be a WTF for people that don't expect lexical to create an
    actual reference, especially since other languages such as JavaScript
    don't do it. On the other hand, global and static both DO create
    references so that behaviour is consistent with current PHP.

    But complex constructions such as this will probably not be used by
    beginners so maintaining a good documentation should solve this.

  * The fact that 'lexical' is needed at all may cause WTFs. Other languages
    such as JavaScript implicitely have the entire scope visible to child
    functions. But since PHP does the same thing with global variables, I
    find a keyword like 'lexical' much more consistent than importing the
    entire scope (and always importing the entire scope costs unnecessary
    performance).

FINAL THOUGHTS
--------------

My now proposed patch addresses the two main problems of my previous patch:
Support for closures in objects (with access to $this) and opcode caches. My
patch applies against PHP_5_3 and does not break any tests. It adds a 
valuable
new language feature which I'd like to see in PHP.

Regards,
Christian