Add optional callback argument to set() method for the repeating complex actions with state

[PaulMaly]

@Rich-Harris
Right now, if we need to make some multiple manipulations with the state we need doing something like:

const { foo, bar } = this.get();
++foo;
bar = 'baz';
this.set({ foo, bar });

I think it'll be nice if we'll able to create some function which will receive the current state as an argument and return a new state, and use this function as an argument in methods set():

export function changeFooBar({ foo, bar }) => {
   ++foo;
   bar = 'baz';
   return { foo, bar };
}
...
import { changeFooBar } from '...';
...
this.set(changeFooBar);

I think I can provide a PR for that change. Seems it's just a few lines of code.

[mrkishi]

Even though it'd be a small change, wouldn't it still complicate set unnecessarily? I'm generally against polymorphic functions whenever possible.

Other options include:

// explicitly passing state
this.set(changeFooBar(this.get()))

// abandoning functional helper
export function mutateFooBar(target) {
    let { foo, bar } = target.get()
    ++foo
    bar = 'baz'
    target.set({ foo, bar })
}

[Conduitry]

Yeah I'm definitely leaning against including this in core of Svelte. It does sound like a reasonable enough contribution for svelte-extras though.

[PaulMaly]

@Conduitry Yes and right now I use it like an external method which I need to include in every component in my app (!!!) because of 90% of my state changes are too complex to just using built-in set(). So, I suppose one line of code (!!!), without breaking changes, sound like a reasonable price to simplify my work. I'm sure it'll be useful for the others too.

[mrkishi]

I don't understand. Even with the change, you'd still be including the external utilities?

[PaulMaly]

@mrkishi It’s not mandatory with this change.

But we’ll be able to import complex state manipulations just like we do it with events, actions and transitions.

[PaulMaly]

Not relevant to v3.