[css-syntax-3] The `+` in `<an+b>` confuses parsing tools

[tidoust]

The <an+b> type is defined with a production rule that starts with <an+b> = .... This type is now referenced from the definition of :nth() in the CSS Generated Content for Paged Media Module.

Problem is that the + confuses grammar parsers, including CSSTree and the highlighting mechanism in Bikeshed (which creates a link to the definition of +), as they parse + as a component value multiplier.

The Value Syntax Definition is not entirely clear about what characters can be used to define basic data types between < and > but it says:

Other non-keyword literal characters that may appear in a component value, such as “+”, must be written enclosed in single quotes.

I think that's the only occurrence in CSS specs of a construct that parsers find problematic. Tools could be fixed of course, I'm wondering whether it would be ok to defined the type as <an-plus-b> instead.

(If so, I'm happy to prepare a PR to update the specs that define or reference the type)

[cdoublev]

I think those tools should be able to parse any <ident> between < and >, like in the syntax descriptor.

https://drafts.css-houdini.org/css-properties-values-api-1/#consume-a-data-type-name

[tabatkins]

I think it's fine to just rename it to <an-b>, honestly. The plus isn't necessary.

[fantasai]

Question about bikeshedding: as we're getting further away from the actual form of the notation "2n+1", it's getting more confusing to refer to the notation. An+B was somewhat understandable, because it pattern-matches against the notation. But <an-b> just seems like weird 1970s computer garbage naming.

Some possible replacements:

• <An-B> at least helps it parse as three components (a, n, b), rather than two (an, b).
• <nth> or <ith> or <index> or something like that (it is called :nth-child())
• other ideas?

[cdoublev]

other ideas?

<position-pattern>.

I assume escaping + in an+b, which would allow to be a valid <ident> parsed by CSSTree, is excluded.

[tabatkins]

Yeah, we need to name that's useful to read and write; there's no reason to contort ourselves to preserve a specific name. ^_^

[Psychpsyo]

I like <a-n-plus-b> and <nth> the most. Maybe <nth-notation> so that it at least feels like a noun.
Anything that involves fully lowercase an reads like the word "an" which feels weird and a bit ugly to read to me.

<nth> or <nth-notation> would also acknowledge the fact that these can be of the form An-B as well. (or any other form that isn't strictly An+B)