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lazy

Note

This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information.

Returns a lazy, access elements only when needed view of the current Map

public function lazy(): KeyedIterable<Tk, Tv>;

Normally, memory is allocated for all of the elements of the Map. With a lazy view, memory is allocated for an element only when needed or used in a calculation like in map() or filter().

Guide​

Returns​

Examples​

This example shows you how to use lazy() on a rather large Map and the time for both a strict and non-strict version. Since we only need 5 of the elements in the end, the lazy view actually allows us to stop after we meet our required 5 without having to actually filter and allocate all 1000000 elements up front.

$map = new Map(\range(0, 1000000));

$s = \microtime(true);
$non_lazy = $map->filter($x ==> $x % 2 === 0)->take(5);
$e = \microtime(true);

\var_dump($non_lazy);
echo "Time non-lazy: ".\strval($e - $s).\PHP_EOL;

// Using a lazy view of the Map can save us a bunch of time, possibly even
// cutting this call time by 90%.
$s = \microtime(true);
$lazy = $map->lazy()->filter($x ==> $x % 2 === 0)->take(5);
$e = \microtime(true);

\var_dump(new Map($lazy));
echo "Time lazy: ".\strval($e - $s).\PHP_EOL;
```.hhvm.expectf
object(HH\Map) (5) {
  [0]=>
  int(0)
  [2]=>
  int(2)
  [4]=>
  int(4)
  [6]=>
  int(6)
  [8]=>
  int(8)
}
Time non-lazy: %f
object(HH\Map) (5) {
  [0]=>
  int(0)
  [2]=>
  int(2)
  [4]=>
  int(4)
  [6]=>
  int(6)
  [8]=>
  int(8)
}
Time lazy: %f
```.example.hhvm.out
object(HH\Map) (5) {
  [0]=>
  int(0)
  [2]=>
  int(2)
  [4]=>
  int(4)
  [6]=>
  int(6)
  [8]=>
  int(8)
}
Time non-lazy: 0.11302947998047
object(HH\Map) (5) {
  [0]=>
  int(0)
  [2]=>
  int(2)
  [4]=>
  int(4)
  [6]=>
  int(6)
  [8]=>
  int(8)
}
Time lazy: 0.0068733692169189