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To: "'Nikita Popov'" <nikita.ppv@gmail.com>,
	"'PHP internals'" <internals@lists.php.net>
Date: Thu, 28 Jun 2012 14:24:35 +0100
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Subject: RE: [PHP-DEV] List comprehensions and generator expressions for PHP
From: jared.williams1@ntlworld.com ("Jared Williams")
References: <CAF+90c8M9nxhGTXXh5dkKYaM75iYaPuKiSxfr7pC3KDkLyEwrA@mail.gmail.com>

 

> -----Original Message-----
> From: Nikita Popov [mailto:nikita.ppv@gmail.com] 
> Sent: 28 June 2012 11:49
> To: PHP internals
> Subject: [PHP-DEV] List comprehensions and generator 
> expressions for PHP
> 
> 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?

No mention of yielding keys in the comprehensions. Presume that would
work?

I would definitely like to see generators in, the amount of
boilerplate 
code required for the current Iterator interface is not nice.

Also have been playing with your addGeneratorsSupport branch, trying
to see
If can get any benefits from using asynchronous operations.

The simplest example I could think of using Memcached
(assuming libmemcached fetch() does return as soon as it recieves an
item)

Eg.

function *getMulti(Memcached $memcached, array $keys)
{
    if ($memcached->getDelayed($keys)) {
        while ($item = $memcached->fetch())
          yield $item['key'] => $item['value'];
    }
}

foreach(getMulti($memcached, ['foo', 'bar', ... ]) as $key => $value)
{
	doSomeWork();
}

So doSomeWork would be called as soon as a single value is available,
rather
than having to wait til all values have returned.

Should in theory work right?

Jared