Custom User Defined Syntax Errors

[bd82]

Example

$.RULE("formalParameterList", () => {
    let foundRestArg = false
    $.MANY_SEP({
        SEP: tokens.Comma,
        DEF: () => {
            $.MANY2(() => {
                $.SUBRULE($.variableModifier)
            })
            $.SUBRULE($.typeType)
            $.OPTION(() => {
                $.CONSUME(tokens.DotDotDot)
                foundRestArg = true
            })
            $.SUBRULE($.variableDeclaratorId)
	    if (foundRestArg && $.LA(1).tokenType === tokens.Comma) {
                // This will fail at runtime now!
                throw this.SAVE_ERROR(new Error("oops, can't have additional params after rest param"))
            }
        }
    })
})

Requires a slight modification to the SAVE_ERROR code to avoid throwing a new error if the error is not a pre-known "RecognizerError"

[bd82]

Initiated due to discussion in: https://github.com/SAP/chevrotain/issues/645

[matthew-dean]

Just a note on this: I'm not sure a throw call is the best here.

Namely, if you're in some kind of "linting" scenario, you won't necessarily produce a single error. You might want to recover, but add an error to the stack. I guess this will work as long as throw is not a requirement, and you can just do

if (foundRestArg && $.LA(1).tokenType === tokens.Comma) {
  this.SAVE_ERROR(new Error("oops, can't have additional params after rest param"))
}

[bd82]

If it is a syntax error it needs to throw and the Parser will auto recover if errorRecovery is enabled, That is what happens for example in MisMatchTokenError

• https://github.com/SAP/chevrotain/blob/master/packages/chevrotain/src/parse/parser/traits/recognizer_engine.ts#L936-L938

If it is a linting error which does not affect parsing flow it could be saved in a separate data structure, preferably during a post parsing step (e.g CST Visitor).

Although there could be some kind of mish-mash that seems partially syntax and partially semantics related. in that case you could still use SAVE_ERROR without throwing the result.

• https://github.com/SAP/chevrotain/blob/master/packages/chevrotain/src/parse/parser/traits/error_handler.ts#L35-L51

The (minor) problem that this issue needs to resolve is that the SAVE_ERROR API:

• Is not public
• Does not work when provided with a new Custom Error type that it does not recognize.