ignore output of part of a rule

[larsimmisch]

First of all: this is great. I haven't had that much fun writing a grammar for ages.

I want to use PEGTL for something relatively straightforward, parsing Lua tables.

I've got something working based upon your Lua example and I'm happy with the progress I've made in just a few hours.

But as I am getting into attaching actions to the rules (to construct values from the parser run), I see that I will hit a snag with the unary plus/minus operator that I need for numerals. Unfortunately, - --[[ this is silly --]] 2 is valid Lua and -2.

In my rule:

	struct decimal_signed : p::seq< p::one< '+', '-' >, seps, decimal > {};

I'd like to ignore the seps, i.e. I want to match it, but not have it produce any output. What is the best way to achieve this?

[ColinH]

First of all: Thanks for the kind words, we are glad that you enjoy using the PEGTL :-)

Regarding your question, there is (currently) no way to obtain only part of the matched input, short of attaching multiple actions to the sub-rules, which should be the easiest way to achieve what you need:

Let the action attached to thep::one< '+', '-' >-rule remember the sign in a state, then do everything else in the action attached to decimal.

(Presumably you need the sign and the digits together in order to to call a standard string-to-integer function; in that case, since the data might not be contiguous in the input, it will have to be somehow "copied together". Or you can roll your own string-to-integer function spread over the two actions...)

[d-frey]

Adding another idea:

struct decimal_plus : decimal {};
struct decimal_minus : decimal {};
struct decimal_plus_full : p::seq< p::one< '+' >, seps, decimal_plus > {};
struct decimal_minus_full : p::seq< p::one< '-' >, seps, decimal_minus > {};
struct decimal_signed : p::sor< decimal_plus_full, decimal_minus_full > {};

Then attach your actions to decimal_plus and decimal_minus. In the latter case, just convert the input to a number and negate that (positive) number after a proper range check.

[larsimmisch]

Nice! Thank you!

I think I'll use a variant from @d-frey with actions attached to decimal_minus and decimal_nosign:

struct decimal_nosign : numeral_one< p::digit, p::one< 'e', 'E' > > {};
struct decimal_minus : decimal_nosign {};
struct decimal_plus_full : p::seq< p::one< '+' >, seps, decimal_nosign > {};
struct decimal_minus_full : p::seq< p::one< '-' >, seps, decimal_minus > {};
struct decimal : p::sor< decimal_plus_full, decimal_minus_full, decimal_nosign > {};

[d-frey]

You're welcome. If similar problems come up more often in the future and the work-arounds get out of hand, feel free to re-open this issue.