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Add Has query parameter #3088

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alice-i-cecile opened this issue Nov 9, 2021 · 4 comments
Closed

Add Has query parameter #3088

alice-i-cecile opened this issue Nov 9, 2021 · 4 comments
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A-ECS Entities, components, systems, and events C-Feature A new feature, making something new possible

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@alice-i-cecile
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alice-i-cecile commented Nov 9, 2021

What problem does this solve or what need does it fill?

A common pattern is to use Option<&C> query parameters, then check if it's .is_some.

Unfortunately, this is slightly more verbose than needed (and the intent is less clear). More critically, it pointlessly locks the resource to a higher permissions level than needed.

What solution would you like?

Add a Has<C: Component> query parameter, which returns true if the entity has the particular component. This does not require any read permissions, as components cannot be changed during the execution of a parallel stage.

What alternative(s) have you considered?

Use the existing strategy.

Additional context

Idea blatantly borrowed from @Ralith's HECS, where it is called Satisfies, and can also be used to check whether an entity matches a given query. We don't need that particular bit of functionality: if anything, a simple Query::contains(entity) method would work well (but should be a seperate PR). See #3089 for that issue.

A Not query parameter, used for this and query filters, might be helpful for clarity but is not essential.

@alice-i-cecile alice-i-cecile added C-Feature A new feature, making something new possible A-ECS Entities, components, systems, and events labels Nov 9, 2021
@TheRawMeatball TheRawMeatball self-assigned this Nov 9, 2021
@BoxyUwU
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BoxyUwU commented Nov 9, 2021

Option<With<T>> and then calling .is_some() is the same behaviour as Has<T> isnt it?

@alice-i-cecile
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alice-i-cecile commented Nov 9, 2021

Option<With> and then calling .is_some() is the same behaviour as Has isnt it?

Yes, but the intent to the reader is dramatically less clear. I don't think I've ever seen With<T> being used in a non-filtering position (and had to verify that this in fact works).

@MinerSebas
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Another "problem" with Option<With<T>> is, that the Some(false) will never occur, because either the Entity has it (Some(true)) or it doesn't (None). #2665 (comment)

@alice-i-cecile
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Closing this out; I don't think it's useful enough to be worth expanding the API.

If we can only have one, I think it needs to be With because of the symmetry with Without. Has and DoesNotHave is a no-go.

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