Stream

Stream is lazy sequence of elements which may be infinitely long.

Stream is composed of a head and a tail. Head contains first value, tail is lazy evaluated - subsequent values are calculated as needed. The stream has two implementations available:

• EmptyStream which stand for empty stream (Empty is reserved keyword in PHP)
• Cons which stand for a stream with one or more elements (consisting of a head and tail)

Construction

Stream::empty();

Stream::of(1, 2, 3);

Stream::ofAll(['a', 'b', 'c']);

Stream::range(1, 10); // stream with 10 subsequent numbers from 1

Because a stream can be infinite there are several other interesting ways to create it:

Stream::from(1); // infinite stream which emits subsequent numbers from 1

Stream::continually(function () {return mt_rand(1, 10); }); // creates stream which emits random numbers from 1 to 10

Stream::iterate(1, function (int $i) {return $i * 2; }); // creates stream, use 1 as start value

// and call given supplier with prev value each time to calculate next value

Collectors

Common use case for stream is to iterate over some kind of source (potentially infinite) filter it and collect values to list:

Stream::continually($dataProvider)

->filter(function($value) {return $value%2===0;})

->take(5)

->collect(Collectors::toList());

Examples

Find the sum of the first ten squares of even numbers (from php 7.4):

Stream::from(1)

->filter(fn($n) => $n%2===0)

->map(fn($n) => $n**2)

->take(10)

->sum();