How to define mutually exclusive states?

[ceving]

I would like to define three states such that when one is set to true the other two should be set to false. I need to prevent an endless loop. The thee states should not react unconditionally to any state change of the others. Instead only a change to true should cause a reaction in the other states. I Is this possible with VanJS?

[Tao-VanJS]

Try this https://jsfiddle.net/1cjuLt8e/1/?

[ceving]

I think this could be easier, if a derived state would be writable. I think fan.derive should accept another argument for an optional setter.

A derived state is similar to a view in a database. A view does not have its own storage, but calculates its contents with a function querying some real tables. In the same way a derived state calculates its value with a function based on states containing real values.

But sometimes it is useful to write a view in a database. Because a view has no storage, this is not possible. To solve this it is possible to define an "instead-of" trigger, which does what needs to be done instead of writing to the non existing storage of the view. In VanJS this would be a setter for a derived state.

Example with setters for derived states:

let real_state = van.state(0);

let fst_mex_state = van.derive(() => { return real_state.val == 0 },
                               (v) => { if (v) real_state.val = 0 });
let snd_mex_state = van.derive(() => { return real_state.val == 1 },
                               (v) => { if (v) real_state.val = 1 });
let trd_mex_state = van.derive(() => { return real_state.val == 2 },
                               (v) => { if (v) real_state.val = 2 });

By setting the 3rd state to true the first two will automatically become false:

trd_mex_state.val = true;

[Tao-VanJS]

You can set the val property of derived states, in the same way as you would do with regular states.

[ceving]

What does it mean to modify the value of a derived state?

I think a derived state can not have its own value. A derived state can just have a memoization of a calculation, it is just a cache. And therefor I think it does not make any sense to set the value of a derived state. In order to modify a derived state, it is necessary to modify the values, the derived state depends on, such that the calculation results into a different memoization.

[Tao-VanJS]

See the example here: #217 (comment)