Support for logical not ! operator in binding to boolean values

[thisispete]

Describe the problem

I'm trying to bind to a boolean value from a store, but I actually want the inverse boolean value to be bound
ie:

<Toggle bind:checked={!$unChecked}>Label</Toggle>

I get the error Can only bind to an identifier (e.g. 'foo') ...

Describe the proposed solution

would be cool to support the ! syntax

<Toggle bind:checked={$isChecked}>Label</Toggle>
<Toggle bind:checked={!$unChecked}>Label</Toggle>

Alternatives considered

current workaround:

$: isChecked = !$unChecked
$: unChecked.set(!isChecked)

<Toggle bind:checked={isChecked}>Label</Toggle>

Importance

nice to have

[Prinzhorn]

This is a very specific special case of #3937 and #7265 so you could do sth. like bind:checked|negated

[bluwy]

I'd second the issues linked by @Prinzhorn too. Svelte only supports binding to variables, not expressions.

[samuelstroschein]

I suggest not using the ! operator anyways. First, using an equality check is more readable, and second, the bang operator (if I recall correctly) can have unexpected outcomes.

Hence your example with an equality check becomes:

<Toggle bind:checked={$isChecked}>Label</Toggle>
<Toggle bind:checked={$isChecked === false}>Label</Toggle>

[tanhauhau]

Since this issue is about supporting binding with !variable, eg:

<Toggle bind:checked={!$unChecked}>Label</Toggle>

and this is not going to happen. So I am going to close this issue.

What is more likely to happen would be

<Toggle bind:checked|negated={$unChecked}>Label</Toggle>

as @Prinzhorn mentioned. So if you are interested on that, do check out the issue #3937 and #7265.

[francoisschwarzentruber]

I agree with @Prinzhorn that it could be done with an input modifier |negated. However, using ! is super natural and would fit the spirit of the elegance of Svelte. Contrary to other modifiers like trim, note that the negation operator ! is bijective. If from !b we can get the value of b, while from s.trim() you can not get the value of s.

On the logical point of view, after bind:checked you could in principle have any literal see https://en.wikipedia.org/wiki/Literal_(mathematical_logic). The current version of Svelte supports positive literal, i.e. a boolean variable. But I do not see a reason why negative literals are not supported.