Customizable error messages.

[srinivasmk]

No description provided.

[bd82]

Can you provide a code snippet demonstrating what you are trying to achieve?
I.E a little rule grammar to show the current error message and separately describe the custom
message you want to use instead?

[srinivasmk]

Thanks for the quick reply. I am trying to implement a parser using Chevrotain. I like to add my own custom error message for each rule.
For example,

$.RULE("statement", function () {
      $.CONSUME(variable);
      $.CONSUME(equals);
      $.CONSUME(StringLiteral);
      $.CONSUME(terminal);
    });

This is the sample rule for defining a variable (without space)
Input :
variable="testStr";

If semicolon gets missed at the end the statement, then it is throwing error as
Expecting token of type --> terminal <-- but found --> '' <--

Instead of using above message, can i implement my own custom message ?
Example : Missing ";" at the end of the statement

Thanks in advance.

[bd82]

Current Status

There is some ability to customize error messages.

• Using labels for tokens

• In alternations (OR) you can supply a custom error message if none of the alternatives fits.

Requested Feature

The capability you are requesting is not currently supported.
However It is something I would very much like Chevrotain to have.

A naive implementation would just support providing the desired string by the end users.
However that would quickly become verbose.

$.RULE("statement", function () {
      $.CONSUME(variable, "missing variable name in assignment statement");
      $.CONSUME(equals, "missing operator of assignment statement");
      $.CONSUME(StringLiteral, "missing RHS of assignment statement");
      $.CONSUME(terminal, "Missing ";" at the end of the statement");
    });

A better solution would probably be to provide the parser with an "ErrorMessageCustomizer"
function during parser construction.

Given an Error type (MismatchedTokenException/NoViableAltException/...) and the position in the grammar this customize should produce the appropriate error message.

• A user's messageCustomizer should be able to fallback to the default implementation.

[bd82]

Basically the existing hardcoded error message building need to be refactored to use some sort of
customizer (strategy) pattern and thus allow any end-user to build his/hers own error messages.

[srinivasmk]

Thank you for considering this.

[bd82]

I'm playing around with implementing this.
So far it seems pretty simple...

If you don't want to wait for the official API
You could just override the existing method that produce the error message in that specific example you provided.

• https://github.com/SAP/chevrotain/blob/8042b2a220a87b546b5594e0e9fb6b94c3632592/src/parse/parser_public.ts#L1710

Unfortunately this seems like the only easy one to override.