Uncaught SyntaxError: Identifier '_alt' has already been declared

[yulingtianxia]

This issue happends on JS runtime target with antlr 4.9, for example:

antlr4 -Dlanguage=JavaScript Java9.g4

The generated file Java9Parser.js run with this error.

[ericvergnaud]

Hi,
does not happen with all grammars.
Can you produce the smallest grammar that triggers the issue?

[yulingtianxia]

Hi,
does not happen with all grammars.
Can you produce the smallest grammar that triggers the issue?

Hello, I can produce this issue by Java9.g4.

[ericvergnaud]

That's not a small grammar.

[XStepien]

Hello, i can reproduce this issue with a "medium sized" grammar when trying to update from Antlr 4.8 to 4.9 (Javascript Runtime).

Also, it would be great if the documentation could give a concrete example of migration from 4.8 to 4.9 Javascript parser (visitor/listener).

[ericvergnaud]

@XStepien re docs feel free to submit a PR
re the issue, I'm looking for a 10 rules grammar where the issue can be reproduced.

[ericvergnaud]

could relate to a * and + in the same rule

[ericvergnaud]

alternately please post here the generated code fragment where this happens and the corresponding grammar fragment

[XStepien]

I have this kind of rule:

expr : expr WS? Calc_ope WS? expr
    | Parenthesis WS? expr WS? ParenthesisEnd
    | operand
    ;

Where "Calc_ope" is :

Calc_ope: Mult | Div | Add | Sub;

Mult: '*';
Div: '/';
Add: '+';
Sub: '-';

Hope this help

[ericvergnaud]

and that rule generates invalid javascript?

[XStepien]

I don't know if it's this rule, but it's related to your comment => "could relate to a * and + in the same rule"

I have to do some tests before being able to say which rule generate this issue ( Identifier '_alt' has already been declared )

[ericvergnaud]

there is no * or + in your rule (I'm referring to ANTLR operators * and +, not characters '*' and '+')

[XStepien]

Ok, sorry for the misunderstanding,I will try to find more information soon

[dbemiller]

I'm also seeing this. I can't post the whole grammar for IP reasons, but I do have a few tricky rules with + and * in them:

nestedPredicate :
    comparison
  | L_PAREN nestedPredicate (SPACE (OR_UPPER | OR_LOWER) SPACE nestedPredicate)+ R_PAREN
  | L_PAREN nestedPredicate (SPACE (AND_UPPER | AND_LOWER) SPACE nestedPredicate)+ R_PAREN
  ;

nestedMathable :
    integer
  | decimal
  | column
  | L_PAREN nestedMathable SPACE DIVIDE SPACE nestedMathable R_PAREN
  | L_PAREN nestedMathable SPACE MINUS SPACE nestedMathable R_PAREN
  | L_PAREN nestedMathable SPACE PLUS SPACE nestedMathable (SPACE PLUS SPACE nestedMathable)* R_PAREN
  | L_PAREN nestedMathable SPACE TIMES SPACE nestedMathable (SPACE TIMES SPACE nestedMathable)* R_PAREN
  ;

Hope this helps. It seems plausible since the error is a re-declared variable, and these rules are also recursive.

If not I can start deleting/anonymizing names to try to get a minimum reproducible grammar that I can share without exposing proprietary stuff though.

[ericvergnaud]

can you paste the generated fragment causing the trouble?

[dbemiller]

ahh... yeah now that I'm looking at the generated code, it's much clearer. The nestedPredicate() and nestedMathable() functions do not have the error, so ignore my last comment.

My plain old mathable() rule does, though:

mathable() {
  ...
  try {
    ...
    case 6:
      ...
      let _alt = this._interp.adaptivePredict(this._input,19,this._ctx)
      ...
    case 7:
      ...
      let _alt = this._interp.adaptivePredict(this._input,20,this._ctx)
      ...
}

Issue being that let declares the same var twice within the same block scope. The rule in my g4 file looks like:

mathable :
    integer
  | decimal
  | column
  | nestedMathable SPACE DIVIDE SPACE nestedMathable
  | nestedMathable SPACE MINUS SPACE nestedMathable
  | nestedMathable SPACE PLUS SPACE nestedMathable (SPACE PLUS SPACE nestedMathable)*
  | nestedMathable SPACE TIMES SPACE nestedMathable (SPACE TIMES SPACE nestedMathable)*
  ;

So... it looks like the issue is with non-recursive * rules. Recursive ones are ok.

I was able to fix it by tweaking the generated code to wrap { } braces around the missing offending case blocks, like:

case 6: {
  ...
}
case 7: {
  ...
}

[ericvergnaud]

In c87e55b I've switched let _alt... to var ...alt
It would help if you could try it out and tell if that fixes the issue

[dbemiller]

That should also be fine. let is spec'd to throw a SyntaxError when you re-declare it (see the "Redeclarations" section).

var is not

Duplicate variable declarations using var will not trigger an error, even in strict mode, and the variable will not lose its value, unless another assignment is performed.

[dbemiller]

followup: just tested by following the instructions here on your commit. It builds & runs fine again for me. Thanks for the quick response & fix!

Not sure what antlr's release schedule is... do you have any estimate on how long it'll be until a patch version gets released with the bug fix?

[XStepien]

Hello,

I just upgraded to version 4.9.1 and everything works fine now.

Thanks a lot!