How to show element only once but any of them?

[Serhioromano]

I have code that can be

VAR_GLOBAL
END_VAR_GLOBAL

VAR_GLOBAL RETAIN
END_VAR_GLOBAL

VAR_GLOBAL CONSTANT
END_VAR_GLOBAL

VAR_GLOBAL CONSTANT PERSISTENT
END_VAR_GLOBAL

So it might have one or number for keywords but every keywork only once, here is what I do.

VarGlobal:
    'VAR_GLOBAL' (decconstant+=DeclarationConstant)?
    'END_VAR_GLOBAL'
;

DeclarationConstant returns string:
    'CONSTANT' | 'RETAIN' | 'PERSISTENT'

But in this case I can only place one keyword, How is it done correctly?

[msujew]

Hey @Serhioromano,

so in theory you could use the unordered groups feature:

VarGlobal:
    'VAR_GLOBAL' (decconstant+='CONSTANT' & decconstant+='RETAIN' & decconstant+='PERSISTENT')
    'END_VAR_GLOBAL'
;

However, I would recommend a more flexible approach with better error messages:

VarGlobal:
    'VAR_GLOBAL' (decconstant+=DeclarationConstant)* // Allow an unlimited amount of modifiers
    'END_VAR_GLOBAL'
;

DeclarationConstant returns string:
    'CONSTANT' | 'RETAIN' | 'PERSISTENT'

And then simply write a custom validator to determine whether modifiers are duplicated. That allows you to write a specific message that helps users identify issues (i.e. something like Duplicate declaration constant modifier). If you want to purely address this feature by using parser features, users would encounter a cryptic parser error instead (i.e. Expected X but found Y).

[Serhioromano]

Works perfectly. Thank you.