Handling distinct keys with CstParser and gates

[tidmush]

I am struggling to find an efficient way to enforce distinct (unique) keys when parsing my configuration file and I was hoping to see an example somewhere.
in chevrotain playground, the json parser accept objects with the same name (it is not invalide)

my initial solution suggests a simple gate that looks somewhat like this:

function isKeyExist(type) {
            let max = $.input.length * -1;
            for (let i = 0; i >= max; i--) {
                const current = $.LA(i);
                if (current.tokenType === EOF || current.tokenType === LCurly) return false;
                if (current.tokenType === type) return true;
            }
        }

and combined with this rule, it works well enough:

$.RULE("settingsOptions", () => {
            $.MANY_SEP({
                SEP: Comma,
                DEF: () => {
                    $.OR([
                        { GATE: !isKeyExist(Id), ALT: () => { $.SUBRULE($.id) } },
                        { GATE: !isKeyExist(Version), ALT: () => { $.SUBRULE($.version) } },
                        { GATE: !isKeyExist(Env), ALT: () => { $.SUBRULE($.env) } }
                    ]);
                }
            })

it is still a work in progress and I haven't built the visitor yet, but I was wondering if I got it wrong and it is not the parser responsibility to enforce distinct keys but the visitor's. or maybe there is a standard way of doing it.

[NaridaL]

If you are parsing a CST, it is simpler to check this in a second pass. This allows you to output much more helpful errors.

Use the https://chevrotain.io/docs/tutorial/step3a_adding_actions_visitor.html#the-cst-visitor to add a check to the relevant node.

[tidmush]

So you suggest enforcing distinct keys in the visitor itself.
OK, that makes sense, thanks

[bd82]

@NaridaL is correct.

Don't try to push too many "laws" into the Parser...
Instead keep it focused on the syntax and add semantic validations in a separate phase.