True suppression of errors or "Am I holding try_ wrong?"

[jevinskie]

I added the else_ to make a branch-rule out of a rule. Is that at all sensible?

In this example I'd like to match either hello world or hello :: world and not produce an error since it still makes a successful production and I'd like is_error to represent that. Am I on the right path or do I need a new approach to branches, conditions, and errors?

https://lexy.foonathan.net/playground/?id=KsjjTseno&mode=tree

[foonathan]

So I think there is a bit of a misunderstanding with dsl::try_: it is only used for error recovery. If you have a situation where a token is required by the grammar, but parsing can continue if it's omitted, you can use dsl::try_ for that. The error will still be reported, but instead of canceling the production, parsing can continue to check the rest of the input.

Also, dsl::opt(dsl::else_ >> x) is never correct, as that is equivalent to x: dsl::else_ is a branch rule that is unconditionally taken and is meant in combination with |.

I didn't quite understand what you want, so here are possible interpretations:

1. You want to parse hello world or hello::world, where either is correct. Use dsl::opt() (or dsl::if_()) with LEXY_LIT("::") (dsl::colon + dsl::colon isn't a branch rule): https://lexy.foonathan.net/playground/?id=1dz7vEofc&mode=tree
2. You want to parse hello::world but recover when the user omitted the ::. This is where dsl::try_() comes into a play, as a missing :: is still an error: https://lexy.foonathan.net/playground/?id=GEKh3zMex&mode=tree Note that a single : confuses it, since recovery just skips parsing the :: and continues with parsing world, which fails as there is trailing whitespace on the input: https://lexy.foonathan.net/playground/?id=43G5df3KG&mode=trace
3. You want to parse hello::world but recover when the user omitted the :: or used a single : instead. You can either use dsl::try_(dsl::colon) + dsl::try_(dsl::colon), or use a special recovery rule to skip trailing whitespace: https://lexy.foonathan.net/playground/?id=Gvxz8ax4j&mode=trace

(I've just now noticed that there is dsl::double_colon as well, which is a shorthand for LEXY_LIT("::")).

I should probably make the behavior in 3 the default, i.e. skip whitespace after error recovery; I haven't considered that situation before.

[foonathan]

I should probably make the behavior in 3 the default, i.e. skip whitespace after error recovery; I haven't considered that situation before.

This has been implemented in 30eb800.