Re: List comprehensions and generator expressions for PHP

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, > > 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 >> >> -- >> PHP Internals - PHP Runtime Development Mailing List >> To unsubscribe, visit: http://www.php.net/unsub.php >> >>