How to use dsl::partial_combination and specifiying that some branch are mandatory

[sandwoodK]

Hello,
First of all, congratulation for the amazing job done. I am evaluating if I could change a big giant parser from boost spirit to lexy. My first finding are promising ( particularly build time, and much easier error compilation messages + very nice trace feature which really help to understand what's happen behind the hood).

Now; i have some stuff where I have not found solution/answer so far; here is a first one

From the documentation , i am trying to solve the case where for example 'b' is mandatory; 'a' and 'c' optional.
struct production { static constexpr auto rule = dsl::partial_combination(dsl::lit_c<'a'>, dsl::lit_c<'b'>, dsl::lit_c<'c'>); };
is is possible ? post checking after parsing can be undesirable , for multiple reason :

• efficiency ( let's stop to parse as soon as we know that overall parsing is anyway false)
• value produced by 'b' branch is of some B type where distinction between missing value and default value of B is not possible, and B type can not be changed for external constraints.

[foonathan]

This isn't possible using the existing combinators and I don't want to complicate them as much, as I think it is a somewhat niche feature, and if I start adding that, you might also want to parse something between n-m times but only if X is not parsed, etc.

If you don't want to do validation afterwards, which I would really recommend - it is a semantic error, not a parse error, and if the parser accepts as much as possible, you can do better error messages during semantic validation, you can use a lexy::scan_production: https://lexy.foonathan.net/playground/?id=sq9TY9P17&mode=tree You can then implement whatever logic you want, albeit more verbosely.