Why am I getting "mismatched input 'addr' expecting {<EOF>, 'addr'}" - antlr

Question

Given this g4 grammar:
grammar smaller;
root
: ( componentDefinition )* EOF;
componentDefinition
: Addr
Id?
Lbrace
Rbrace
Semi
;
ExprElem
: Num
| Id
;
Addr : 'addr' {System.out.println("addr");};
Lbrace : '{' ;
Rbrace : '}' ;
Semi : ';' ;
Id : [a-zA-z0-9_]+ {System.out.println("id");};
Num : [0-9]+;
//------------------------------------------------
// Whitespace and Comments
//------------------------------------------------
Wspace : [ \t]+ -> skip;
Newline : ('\r' '\n'?
| '\n'
) -> skip;
and this file to parse
addr basic {
};
this cmdline:
rm *.class *.java ; java -Xmx500M org.antlr.v4.Tool smaller.g4 ; javac *.java ; cat basic | java org.antlr.v4.runtime.misc.TestRig smaller root -tree
I get this error:
line 2:0 mismatched input 'addr' expecting {<EOF>, 'addr'}
(root addr basic { } ;)
If I remove the ExprElem (which is not used anywhere else in the grammar), the parser works:
addr
id
(root (componentDefinition addr basic { } ;) <EOF>)
Why? Note that this is a greatly reduced version of the grammar. Normally, the ExprElem does have a purpose.
Addr is a literal, so it shouldn't conflict with Id in the way that other questions like this usually do.

Answer 1

Your rule ExprElem is a lexer rule, not a parser rule (it begins with an upercase) and is masking the Addr rule, so, no Addr :(
Also, as ExprElem is a lexer rule and it relies on Id or Num rule. Consequently, when an Id is found, ANTLR lexer gives it the ExprElem token type and not the Id token type.
So, two things, you can either rewrite your ExprElem rule to exprElem (assuming you want a parser rule):
exprElem : Num | Id;
or you can use Id token in your ExprElem as part of the rule but you need something that can differentiate ExprElem from Id (example below, but I really think you want a parser rule):
Addr : 'addr' {System.out.println("addr");};
ExprElem
: Sharp Num // This token use others but defines its own 'pattern'
| Sharp Id
;
Lbrace : '{' ;
Rbrace : '}' ;
Semi : ';' ;
Id : [a-zA-z0-9_]+ {System.out.println("id");};
Num : [0-9]+;
Sharp : '#';
From what I suppose, this is definitely not what you want, but I just put it here to illustrate how lexer rule can reuse others.
When you have doubt about what your token do, do not hesitate to display the recognize tokens. Here is the Java code fragment I often use (I named your grammar test in this case):
public class Main {
public static void main(String[] args) throws InterruptedException {
String txt =
"addr Basic {\n"
+ "\n"
+ "};";
TestLexer lexer = new TestLexer(new ANTLRInputStream(txt));
CommonTokenStream tokens = new CommonTokenStream(lexer);
TestParser parser = new TestParser(tokens);
parser.root();
for (Token t : tokens.getTokens()) {
System.out.println(t);
}
}
}
NOTE: by the way, Num will never be recognized as Id rule can match the same thing. Try this instead:
Id : Letter (Letter | [0-9])*;
Num : [0-9]+;
fragment Letter : [a-zA-z_];