feat: make piscina entrypoint easier for typescript

[danielpza]

This is technically a breaking change in the typescript api.

Changed the generics from

const pool = new Piscina<Parameters<typeof import("./worker.js").default>[0], ReturnType<typeof import("./worker.js").default>>({ filename: "./worker.js" })

// or specified manually
const pool = new Piscina<{a: number, b: number}, number>({ filename: "./worker.js" })

to

const pool = new Piscina<typeof import("./worker.js")>({ filename: "./worker.js" })

// or specified manually
const pool = new Piscina<{default: (payload: { a: number, b: number }) => number}>({ filename: "./worker.js" })

Also works with multiple exports:

// worker.ts
export function greet({ name }: { name: string }): string {
  return `Hello ${name}!`;
}

export default function add({ a, b }: { a: number; b: number }): number {
  return a + b;
}

// main.ts
const pool = new Piscina<typeof import("./worker.js")>({ filename: "./worker.js" })

pool.run({
  // types are checked for default export
}) // returns Promise<number>

pool.run({
  // types are checked for greet export
}, { name: 'greet' }) // returns Promise<string>

This (imo) improves the typescript usage by reducing the amount of code needed to properly type the worker function.

[metcoder95]

I really like this, but sadly it imposes a breaking change in the current version; can we aim to add a fallback to the current behavior if the generic passed is not an object?

[danielpza]

I really like this, but sadly it imposes a breaking change in the current version; can we aim to add a fallback to the current behavior if the generic passed is not an object?

I played around with the code a bit to see if it was possible, unfortunately I think it's not possible (or I don't know enough TS) to make it backwards compatible.

[metcoder95]

I'm thinking on function overloads, but not sure if they can be applied to classes 🤔

Otherwise, we can point this feature directly to v6, but will take some time to land as it will be aimed to next year.

[danielpza]

I'm thinking on function overloads, but not sure if they can be applied to classes 🤔

I thought the same but couldn't get it to work, seems it's not possible.

Otherwise, we can point this feature directly to v6, but will take some time to land as it will be aimed to next year.

I'm fine with this going to v6.

If someone later finds a way to backport it without breaking compatibility that would be great.

[metcoder95]

Perfect, then let's do this. I'll sync next branch to contain latest changes and we point this PR to that branch instead.

If willing, we can give it a try to backport it aftewards, otherwise we can open an Issue for someone that might be interested in contribute