async function as type in hacklang

Question

I am trying to build a shape where key is string and value is function. This works fine for regular functions but gives error for async functions. Following is what I am trying to do

  const type TAbc = shape(
    'works' => (function (): string),
    'doesnt_work' => (async function(): Awaitable<string>)
  );

This results in error A type specifier is expected here.Hack(1002)

Encountered unexpected text async, was expecting a type hint.Hack(1002)

Is this illegal in Hacklang? If yes then would like to know why?

Answer 1

async function (): T is illegal in Hacklang. The suggested way to do this is by defining it in the return type, Awaitable<T>.

So to get this working we can do this

const type TAbc = shape(
  'sync' => (function (): string),
  'async' => (function(): Awaitable<string>)
);

And while initialising an instance of this type we can internally call an async function. For eg:

$abc = shape(
'async' => function(): Awaitable<string> {
  return someAsyncFunction();
});