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Date: Thu, 28 Jun 2012 14:43:04 +0200
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To: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] List comprehensions and generator expressions for PHP
From: krebs.seb@gmail.com (Sebastian Krebs)

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Hi,

Whats the difference to the (already existing) function array_map() (except
the syntax and one more new keyword)?

> $firstNames = array_map(function($user){return $user->firstname;},
$users);

Don't want to rewrite every example you gave, but you probably see my point.

Regards,
Sebastian

2012/6/28 Nikita Popov <nikita.ppv@gmail.com>

> Hi internals!
>
> Python and several other languages include support for list
> comprehensions and generator expressions and I'd like to see something
> similar in PHP too.
>
> I created a hacky proof of concept implementation here:
> https://github.com/nikic/php-src/tree/addListComprehensions. It's
> really dirty, but it implements both features in about ~150 lines of
> code.
>
> Currently I'm using the following syntax:
>
>    $firstNames = [foreach ($users as $user) yield $user->firstName];
>
> This code is roughly equivalent to writing:
>
>    $firstNames = [];
>    foreach ($users as $user) {
>        $firstNames[] = $user->firstName;
>    }
>
> You may notice that this particular list comprehension provides the
> same functionality as array_column(), just in a little more
> generalized way. E.g. you could use all of the following without
> having special functions for them all:
>
>    $firstNames = [foreach ($users as $user) yield $user->firstName];
>
>    $firstNames = [foreach ($users as $user) yield $user->getFirstName()];
>
>    $firstNames = [foreach ($users as $user) yield $user['firstName']];
>
> It's also possible to explicitly specify a key:
>
>    $firstNames = [foreach ($users as $user) yield $user->id =>
> $user->firstName];
>
> It is also possible to filter elements using list comprehensions:
>
>    $underageUsers = [foreach ($users as $user) if ($user->age < 18)
> yield $user];
>    // or just the names
>    $underageUserNames = [foreach ($users as $user) if ($user->age <
> 18) yield $user->firstName];
>
> It is also possible to nest multiple foreach loops:
>
>    $aList = ['A', 'B'];
>    $bList = [1, 2];
>    $combinations = [foreach ($aList as $a) foreach ($bList as $b)
> yield [$a, $b]];
>    // gives: [ ['A', 1], ['A', 2], ['B', 1], ['B', 2] ]
>
> All the above are list comprehensions (or in PHP rather array
> comprehensions), i.e. they create an array as the result.
>
> If this is not needed it is also possible to compute the values lazily
> using generator expressions, which use () instead of [].
>
>    $firstNames = (foreach ($users as $user) yield $user->firstName);
>
> In this case $firstNames will no longer be an array of first names,
> but instead will be a generator producing first names.
>
> This is handy if you only need to iterate the resulting "list" only
> once as it saves you holding the whole list in memory.
>
> Also it allows you to work with infinite lists easily:
>
>    function *naturalNumbers() {
>        for ($i = 0; ; ++$i) {
>            yield $i;
>        }
>    }
>
>    // all natural numbers
>    $numbers = naturalNumbers();
>    // only the odd ones
>    $oddNumbers = (foreach ($numbers as $n) if ($n % 2) yield $n);
>    // ...
>
> (At this point I wonder whether one should include support for
> for-loops in list comprehensions. So the naturalNumbers() function
> could be replaced with (for ($i = 0;; ++$i) yield $i), etc)
>
> So, what do you think? Do we want something like this in PHP?
>
> Nikita
>
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>

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