Mfk

:::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 key and value in the provided KeyedTraversable

namespace HH\Asio;

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

This function is similar to mf(), but passes element keys to the callable as well.

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

This function is called mfk because we are returning a map, doing a a filtering operation that includes keys.

$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 keys and values to filter.
• (function(Tk, 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 both the keys and 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
  $not_self_named = await \HH\Asio\mfk(
    $fruits,

    // Exclude fruits who's name is the same as their color
    async ($name, $color) ==> \strcasecmp($name, COLOR::getNames()[$color]),
  );

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