Mf

:::info[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 an Awaitable of Map after a filtering operation has been applied to each value in the provided KeyedTraversable

namespace HH\Asio;

function mf<Tk as arraykey, Tv>(
  KeyedTraversable<Tk, Tv> $inputs,
  (function(Tv): Awaitable<bool>) $callable,
): Awaitable<Map<Tk, Tv>>;

This function is similar to Map::filter(), but the filtering of the values is done using Awaitables.

This function is called mf because we are returning a map, and we are doing a filtering operation.

$callable must return an Awaitable of bool.

The keys and values in the Map of the returned Awaitable are not available until you await or join the returned Awaitable.

Parameters

• KeyedTraversable<Tk,Tv> $inputs - The KeyedTraversable of values to map.
• (function(Tv): Awaitable<bool>) $callable - The callable containing the Awaitable operation to apply to $inputs.

Returns

• Awaitable<Map<Tk,Tv>> - An Awaitable of Map after the filtering operation has been applied to the values in $inputs.

Examples

enum COLOR: int {
  RED = 1;
  ORANGE = 2;
  YELLOW = 3;
  GREEN = 4;
  BLUE = 5;
  INDIGO = 6;
  VIOLET = 7;
}

<<__EntryPoint>>
async function basic_usage_main(): Awaitable<void> {
  $fruits = ImmMap {
    'Apple' => COLOR::RED,
    'Banana' => COLOR::YELLOW,
    'Grape' => COLOR::GREEN,
    'Orange' => COLOR::ORANGE,
    'Pineapple' => COLOR::YELLOW,
    'Tangerine' => COLOR::ORANGE,
  };

  // Similar to $times->filter(...)
  // But awaits the awaitable result of the callback
  // rather than using it directly
  $orange_fruits =
    await \HH\Asio\mf($fruits, async ($color) ==> ($color == COLOR::ORANGE));

  foreach ($orange_fruits as $fruit => $color) {
    echo $fruit, 's are ', COLOR::getNames()[$color], "\n";
  }
}