Question

I have a simple little grammar which keeps giving a multiple alternatives error when I try to generate Xtext artefacts.
The grammar is:

grammar org.xtext.example.hyrule.HyRule with org.eclipse.xtext.xbase.Xbase

generate hyRule (You can only use links to eclipse.org sites while you have fewer than 25 messages )

Start:
    rules+=Rule+
    ;

Rule:
    'FOR''PAYLOAD'payload=PAYLOAD'ELEMENTS' elements+=JvmFormalParameter+'CONSTRAINED' 'BY' expressions+= XExpression*;

PAYLOAD:
    "Stacons"|"PFResults"|"any"
;

And the exact error I get is:

![warning(200): ../org.xtext.example.hyrule/src-gen/org/xtext/example/hyrule/parser/antlr/internal/InternalHyRule.g:3197:2: Decision can match input such as "{RULE_ID, '=>', '('}" using multiple alternatives: 1, 2
As a result, alternative(s) 2 were disabled for that input
error(201): ../org.xtext.example.hyrule/src-gen/org/xtext/example/hyrule/parser/antlr/internal/InternalHyRule.g:3197:2: The following alternatives can never be matched: 2][1]

I have attached the Syntax diagram for the generated antlr grammar in antlrworks, and can clearly see the multiple alternatives(JvmFormalParameter can match RULE_ID via the JvmTypeReference or the ValidID rule).

So it looks as if JvmFormalParameter is ambiguous…Apologies for my stupidity but could someone point out what it is I’m missing? Is there some way of overcoming this ambiguity when using the JvmFormalParameter rule in my grammar?

Answer 1

The rule JvmFormalParameter is defined as

JvmFormalParameter returns types::JvmFormalParameter:
    (parameterType=JvmTypeReference)? name=ValidID;

so the type of the parameter is optional. If you use elements+=JvmFormalParameter+, you allow multiple parameters without a delimiter thus the parser cannot decide about the input sequence

String s

since both String and s could be names of two parameters or String s could be a single parameter with a type String and the name s. You should use a delimiter like

elements+=JvmFormalParameter (',' elements+=JvmFormalParameter)*

or use the rule FullJvmFormalParameter which is defined with a mandatory type reference:

FullJvmFormalParameter returns types::JvmFormalParameter:
    parameterType=JvmTypeReference name=ValidID;