This API is part of the Hack Standard Library, not HHVM itself.
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This function is available as Dict\gen_from_keys() in Facebook's www repository.

Returns a new dict where each value is the result of calling the given async function on the corresponding key


namespace HH\Lib\Dict;

function from_keys_async<Tk as arraykey, Tv>(
  Traversable<Tk> $keys,
  (function(Tk):Awaitable<Tv>) $async_func,
): Awaitable<dict<Tk, Tv>>;

For non-async functions, see Dict\from_keys().