Argument of type '[string]' is not assignable to parameter of type 'U'

Question

I have these below types declaried,

import { ReadonlyDeep } from 'type-fest';
import { AsyncFunction } from 'type-fest/source/async-return-type';

// We don't allow null. undefined is required instead.
export type SimpleArgs = (string | number | boolean | undefined)[];

export type BasicAsyncFunc<U extends SimpleArgs, R> = (...args: U) => Promise<ReadonlyDeep<R>>;

interface Memoized<U extends SimpleArgs, R> extends BasicAsyncFunc<U, R> {
  cache_size: () => number;
  clear_cache: () => void;
}


const memoize_async = <R, U extends SimpleArgs>(
  options: { ttl: number; size: number },
  f: BasicAsyncFunc<U, R>,
): Memoized<U, R> => {
  return () => {
     // some more logic
     return f(arg) // error is here
  }
}

Argument of type '[string]' is not assignable to parameter of type 'U'. '[string]' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint '(string | number | boolean | undefined)[]'.

Also, please can anyone explain the type declared above?

What is the signature of f from the above type?

Also, the signature of memoize_async ?

I have a basic understanding of generics. I am confused here.

I tried as bellow,

1. export type SimpleArgs = (string | number | boolean | undefined)[];

• type SimpleArgs should be an array of string or number or boolean and undefined is also allowed.

2. export type BasicAsyncFunc<U extends SimpleArgs, R> = (...args: U) => Promise<ReadonlyDeep<R>>;

   BasicAsyncFunc type accepts two generics:

   • U, extends SimpleArgs What U can be here?
   • What is R here?

3. interface Memoized<U extends SimpleArgs, R> extends BasicAsyncFunc<U, R> { cache_size: () => number; clear_cache: () => void; }

   No idea at all here:(

Answer 1

My best answer is that U in memoized_async is not actually (string | number | boolean | undefined)[] .

If I did this:

const foo = memoize_async<void,number[]>({ttl:0,size:0},async ()=>{
  
})

arg , which is apparently a string, would be invalid because the function expects a number. I'm not exactly sure what you would need to do as a quick fix but if I were you I would just slap a bunch of @ts-ignore's or type assertions to fix the job.