Change rune parameter types from void functions to unknown functions

[WillsterJohnson]

Describe the problem

An abstract example;

Sometimes an effect is a single expression

$effect(() => {
  goDoStuffWith(myState);
})

Me, and probably a good few others, like to shorten single-statement callbacks into one line

() => doDoStuffWith(myState)

However, this callback cant always be passed to $effect because of an incompatible return type

declare function $effect(fn: () => void | (() => void)): void
//                                 ^ if the single expression resolves to a value,
//                                   it violates the `void` type

---

A more tangible example;

I have three boolean states, call them A, X, and Y.

When X changes, set A = X, when Y changes, set A = Y.

I can't do this with a derived, as I can't know which of X or Y changed

let A = $derived.by(() => {
  return X
  return Y // you can't even express the above idea
})

I can do it with effects though;

$effect(() => {
  A = X
})
$effect(() => {
  A = Y
})

I would prefer to write it as follows to save a bit on vertical space, as I don't find the readability to be impaired by collapsing the function body (tbh, I find the following more readable);

$effect(() => A = X)
$effect(() => A = Y)

But assignment expressions resolve to the assigned value, and in this case thats always a boolean, not assignable to void.

Describe the proposed solution

Use unknown instead;

declare function $effect(fn: () => unknown | (() => unknown)): void

The same for other effects; $effect.root, $effect.pre

---

Workarounds;

just use javascript void. I find this reduces readability, as it adds more boilerplate and forces brackets to stack up at the end.

$effect(() => void (A = X))
$effect(() => void (A = Y))

or...

Adding this to app.d.ts (or other global declaration) applies the proposed change as a patch

declare global {
  function $effect(fn: () => unknown | (() => unknown)): void
}

Importance

nice to have

[paoloricciuti]

The return value of an effect can be void but if you return something it needs to be the cleanup function. I think this change will be confusing because then nothing stops you from doing

$effect(()=>{
    return 3;
});