3 ms·
Cut means "if you matched this far, don't consider other options even if the current option fails to parse". With this grammar: expr: expr '+' term | term
by drothlis 7y ago
Cut means "if you matched this far, don't consider other options even if the current option fails to parse".
With this grammar:
expr: expr '+' term | term
And this input:
5 +
It will first consider the first alternative (expr '+' term) but will fail to match that branch because it's missing the "term" after the "+". That's fine because there's another alternative to try (term). That alternative doesn't match either, and you'll get the error message from the last alternative that it tried (probably something like "unexpected +").
But with this grammar:
expr: expr '+' <cut> term | term
Then with the same input ("5 +") it will consider the first alternative, fail to match, and then fail immediately with the error message from that first alternative (something like "missing term after +").
- mamcx 7y agoyeah, I get the purposed point of a "cut" but how I put one? Exist a basic sample? (ie: Using a peg parser how define what is a "cut"???) expr: expr '+' API_call_here? term | term
- pacaro 7y agoHmm, if you're using a peg parser that doesn't directly support this you can emulate it by choosing between the rhs of the cut and a production that will always match so expr : atom (op <cut> expr)* Would be replaced with expr : atom (op (expr / cut))* cut : "XXX"? or perhaps simpler expr : atom (op expr?)* Either way when you process the expr rule there will be an empty match where the rhs should be, and an appropriate error can be generated