Confusing error message for failed positive lookahead expression

[vsemozhetbyt]

Issue type

• Bug Report: yes
• Feature Request: maybe
• Question: no
• Not an issue: no

Prerequisites

• Can you reproduce the issue?: yes
• Did you search the repository issues?: yes
• Did you check the forums?: no (cannot find the URL)
• Did you perform a web search (google, yahoo, etc)?: no (seems unneeded)

Description

Error message for failed positive lookahead expression does not mention this expression

Steps to Reproduce

Consider a rule where double hyphens are not allowed:

const pegResult = require('pegjs/')
  .generate('root = ([^-] / "-" !"-")+')
  .parse('a-b--c');

console.log(pegResult);

Expected behavior:

Something like: peg$SyntaxError: Expected "-" !"-", [^\-], or end of input but "--" found.

Actual behavior:

peg$SyntaxError: Expected "-", [^\-], or end of input but "-" found.

Software

• PEG.js: 0.11.0-master.b7b87ea
• Node.js: 14.0.0-v8-canary202002123546737a75
• NPM or Yarn: npm 6.13.7
• Browser: -
• OS: Windows 7 x64
• Editor: -

[StoneCypher]

Hi, I'm not very good at peg yet. I want to make sure I'm understanding this correctly.

It seems like what you're saying is "I have a rule where abcd is forbidden because d is disallowed as a lookahead in a rule about cd. When it complains, I'd prefer to hear about cd but I only hear about d."

Is that roughly correct?

[vsemozhetbyt]

I would rather say: "I have a rule where abcc is forbidden because c is disallowed as a lookahead for a c in a rule about abc. When it complains, I'd prefer to hear about cc but I only hear about c." Otherwise, the error message is not useful or clear: Expected "c"... but "c" found.

[StoneCypher]

Oh, the error message is confusing because the forbiddance is a repetition?

[vsemozhetbyt]

I would say the message is confusing because

1. Expected and Found is the same in this case and
2. the real cause of failure is not clear.

Maybe it would be useful to mention lookahead expressions if they are the cause of failure. Forbidden repetitions are just a case, compare this without a forbidden repetition.

const pegResult = require('pegjs/')
  .generate('root = ("a" / "b" !"a")+')
  .parse('aba');

console.log(pegResult);

peg$SyntaxError: Expected "a", "b", or end of input but "b" found.

[vsemozhetbyt]

Sorry, I am a bit clumsy, as I've just started to learn the PEG.

BTW, I've tried about dozen other parser generators (I am a newbie in this area) and this seems to be my favorite now)

[StoneCypher]

Yeah, peg's almost the only parser generator for javascript that I consider acceptable

Don't worry about being clumsy. We all are

[Mingun]

@vsemozhetbyt, actually, you grammar should issue a info/warning at build stage, because it accept only inputs, which are accepted by grammar

root
  = "a"+ "b"*
  / "b"+

but compiler not (yet?) smart enough to infer that. I even don't know, may be it is NP-complete problem...

[StoneCypher]

the thing about np complete problems is that you can often still do a pretty decent job heuristically

if it's often right and never wrong, then it's a defacto improvement