[Merged by Bors] - Add condition negation

[Shatur]

Objective

Closes #7202

Solution

Introduce a not helper to pipe conditions. Opened mostly for discussion. Maybe create an extension trait with not method? Please, advice.
Introduce not(condition) condition that inverses the result of the passed.

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Changelog

Added

• not condition.

[afonsolage]

I just find strange how it feels using this adapter:

my_system.run_if(always_false.pipe(system_adapter::not))

A user would expect something like:

my_system.run_if(system_adapter::not.pipe(always_false))

Or even:

my_system.run_if(not(always_false)) // More ergonomic but more hacky

But the pipe outputs to the system given as arguments, so it's like working with Unix piping:

my_system.run_if(always_false || system_adapter::not) // It's a bad syntax, just showing tho.

This may be out of scope of this PR, but I feel like the piping logic is a bit different from what users would expect.

So maybe changing from not to negate would be a better idea? For me, whenever I see not I think in terms of condition, not in terms of transformation/mutating the value, which is what the adapter is doing.

my_system.run_if(always_false.pipe(system_adapter::negate))

[Shatur]

@afonsolage agree with you. I also still not sure if its better to create an extension trait instead to allow the following:

my_system.run_if(always_false.negate())

[JoJoJet]

But the pipe outputs to the system given as arguments, so it's like working with Unix piping:

I don't quite understand this objection. System piping is supposed to work like Unix piping, no?

[alice-i-cecile]

> my_system.run_if(not(always_false)) // More ergonomic but more hacky

This is my preference. I think it's by far the most readable, and extends much better to OR and AND.

[Shatur]

Agree, looks best. Any idea how it could be achieved?

[alice-i-cecile]

fn not<C: Condition<Params>>(condition: C) -> impl Condition {
   || { !condition }
}

Maybe something like this?

[Shatur]

I thought about something like this, but Condition doesn't implement Not, that's the issue :(

[Shatur]

I think it could be done via piping, but I can't figure out how to write the return type:

pub fn not<Params, C: Condition<Params>>(condition: C) -> impl Condition {
    condition.pipe(|In(val): In<bool>| !val)
}

The returned condition requires parameter, but I don't know how to define it.

[JoJoJet]

The correct Params type to use for PipeSystem is AlreadyWasSystem, so that function would look like

pub fn not<Params, C: Condition<Params>>(condition: C) -> impl Condition<AlreadyWasSystem>
where
    C::System: ReadOnlySystem,
{
    condition.pipe(|In(val): In<bool>| !val)
}

I'm not entirely sure why the extra where clause is required, it might be due to a limitation of the current compiler.

[JoJoJet]

Alternatively you could return impl ReadOnlySystem<In = (), Out = bool>, though you'd still need the C::System: ReadOnlySystem where clause.

[Shatur]

Thank you a lot, did as you suggested.

[alice-i-cecile]

Removing the controversial tag: I think we've converged on the bikeshedded syntax nicely.

[inodentry]

Good. :)

[alice-i-cecile]

bors r+

[bors]

Pull request successfully merged into main.

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