Checking for the left-recursion

[evilguest]

The left recursion handling in Ohm is marvellous. It helps handling PEG-problematic tasks like RPN or left-associativity with ease.
However, most of the other PEG libraries are unable to handle the left recursion.
What would be the best way to detect the presence of the left recursion in the Ohm grammar without re-implementing half the Ohm?
I've tried to perform the graph search on the Grammar object, but quickly found that I don't understand the object topology. Despite the class types declared, most of the PExpr instances seem to be just PExprs, with no hints on their actual types.

[pdubroy]

Hi @evilguest —

I'm not sure what you mean by "most of the PExpr instances seem to be just PExprs, with no hints on their actual types". If you are using a recent version of Ohm, the pexprs will be instances of the appropriate subclass of PExpr. Take this example:

import * as ohm from "ohm-js";

const arithmetic = ohm.grammar(`
  Arithmetic {
    Exp = Exp "+" Term -- plus
        | Term
    Term = Term "*" number -- times
         | number
    number = digit+
  }`);

for (const [ruleName, ruleInfo] of Object.entries(arithmetic.rules)) {
  const { body } = ruleInfo;
  console.log(body.constructor.name);
}

This produces

Seq
Alt
Seq
Alt
Plus

Unfortunately I don't think we have a generic way to recursively walk PExprs, but's it's not too bad to implement yourself.

Does that help?

[evilguest]

Thanks, this helps a lot. I've misinterpreted the debugger output. The root cause is Jest screwing the imports, so any attempt to perform (body instanceof Apply) fails.