How can I return from parsing ahead of time?

[shuizaiku7]

Hi, I'm parsing big files, but somethings I don't need to parse them all.
Maybe I just need to collect a certain number of items from the file.

Is there any mechanism to return from parsing ahead of time?

[ColinH]

Sure, your grammar doesn't have to include the eof rule in a way that a successful parse finishes with it. Consider this edge case, if your grammar matches a single a via tao::pegtl::one< 'a' > and does nothing else your parsing run will finish after reading only the first character of the file and, depending on whether the first character exists and is an 'a', will either succeed or fail.

[ColinH]

That sounds like you need a custom rule, some dynamic_rep that is similar to tao::pegtl::rep but takes the number of repetitions from the state instead of a template parameter.

[ColinH]

PS: You could throw an exception when you are done, but I wouldn't go down that path since it's neither elegant nor efficient to end a successful parsing run like that. The PEGTL is designed around an exception during parsing being an unrecoverable "global" failure.

[ColinH]

Alternatively you could use the tao::pegtl::until rule, increment a counter in the action for each item, and check whether the counters match in a separate action attached to the tao::pegtl::success rule that you use as termination condition for the until. This works if you use a slightly less often used feature, namely that actions can return bool instead of void; if an action returns false then the rule that it is attached to will be considered as not matched, "as if" the rule had returned false itself (though in that case the action wouldn't have been called).

[shuizaiku7]

Yeah, i agree that using exception to implement a regular logic is not elegant.
I've considered returning bool from action's apply function, but at first i thought it was used for rewinding sth instead of stop match according to my understanding from our documentation.

Thank you, i will try the seond way.

[ColinH]

Whether a rule (or action) returning false stops the parsing run or lets it continue somewhere else (in the grammar) depends fully on the position of this rule in, and the structure of, the overall grammar.

Anyhow, glad to have been of help, let me know if you get stuck.

[ColinH]

Alternatively you could use the tao::pegtl::until rule, increment a counter in the action for each item, and check whether the counters match in a separate action attached to the tao::pegtl::success rule that you use as termination condition for the until. This works if you use a slightly less often used feature, namely that actions can return bool instead of void; if an action returns false then the rule that it is attached to will be considered as not matched, "as if" the rule had returned false itself (though in that case the action wouldn't have been called).