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Message-ID: <D4639B26-25E5-4404-855A-6FB80AD4E727@gmail.com>
Cc: PHP internals <internals@lists.php.net>
Date: Thu, 28 Jun 2012 23:22:31 +1000
To: Sebastian Krebs <krebs.seb@gmail.com>
Subject: Re: [PHP-DEV] List comprehensions and generator expressions for PHP
From: davidkmuir@gmail.com (David Muir)

I'd assume that array_map() only works with arrays, while list comprehension=
 should work with anything traversable.

David


On 28/06/2012, at 10:43 PM, Sebastian Krebs <krebs.seb@gmail.com> wrote:

> Hi,
>=20
> Whats the difference to the (already existing) function array_map() (excep=
t
> the syntax and one more new keyword)?
>=20
>> $firstNames =3D array_map(function($user){return $user->firstname;},
> $users);
>=20
> Don't want to rewrite every example you gave, but you probably see my poin=
t.
>=20
> Regards,
> Sebastian
>=20
> 2012/6/28 Nikita Popov <nikita.ppv@gmail.com>
>=20
>> Hi internals!
>>=20
>> Python and several other languages include support for list
>> comprehensions and generator expressions and I'd like to see something
>> similar in PHP too.
>>=20
>> 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.
>>=20
>> Currently I'm using the following syntax:
>>=20
>>   $firstNames =3D [foreach ($users as $user) yield $user->firstName];
>>=20
>> This code is roughly equivalent to writing:
>>=20
>>   $firstNames =3D [];
>>   foreach ($users as $user) {
>>       $firstNames[] =3D $user->firstName;
>>   }
>>=20
>> 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:
>>=20
>>   $firstNames =3D [foreach ($users as $user) yield $user->firstName];
>>=20
>>   $firstNames =3D [foreach ($users as $user) yield $user->getFirstName()]=
;
>>=20
>>   $firstNames =3D [foreach ($users as $user) yield $user['firstName']];
>>=20
>> It's also possible to explicitly specify a key:
>>=20
>>   $firstNames =3D [foreach ($users as $user) yield $user->id =3D>
>> $user->firstName];
>>=20
>> It is also possible to filter elements using list comprehensions:
>>=20
>>   $underageUsers =3D [foreach ($users as $user) if ($user->age < 18)
>> yield $user];
>>   // or just the names
>>   $underageUserNames =3D [foreach ($users as $user) if ($user->age <
>> 18) yield $user->firstName];
>>=20
>> It is also possible to nest multiple foreach loops:
>>=20
>>   $aList =3D ['A', 'B'];
>>   $bList =3D [1, 2];
>>   $combinations =3D [foreach ($aList as $a) foreach ($bList as $b)
>> yield [$a, $b]];
>>   // gives: [ ['A', 1], ['A', 2], ['B', 1], ['B', 2] ]
>>=20
>> All the above are list comprehensions (or in PHP rather array
>> comprehensions), i.e. they create an array as the result.
>>=20
>> If this is not needed it is also possible to compute the values lazily
>> using generator expressions, which use () instead of [].
>>=20
>>   $firstNames =3D (foreach ($users as $user) yield $user->firstName);
>>=20
>> In this case $firstNames will no longer be an array of first names,
>> but instead will be a generator producing first names.
>>=20
>> This is handy if you only need to iterate the resulting "list" only
>> once as it saves you holding the whole list in memory.
>>=20
>> Also it allows you to work with infinite lists easily:
>>=20
>>   function *naturalNumbers() {
>>       for ($i =3D 0; ; ++$i) {
>>           yield $i;
>>       }
>>   }
>>=20
>>   // all natural numbers
>>   $numbers =3D naturalNumbers();
>>   // only the odd ones
>>   $oddNumbers =3D (foreach ($numbers as $n) if ($n % 2) yield $n);
>>   // ...
>>=20
>> (At this point I wonder whether one should include support for
>> for-loops in list comprehensions. So the naturalNumbers() function
>> could be replaced with (for ($i =3D 0;; ++$i) yield $i), etc)
>>=20
>> So, what do you think? Do we want something like this in PHP?
>>=20
>> Nikita
>>=20
>> --
>> PHP Internals - PHP Runtime Development Mailing List
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>>=20
>>=20