grammar T;
program
: LPAREN program? RPAREN
;
LPAREN : '(';
RPAREN : ')';
WS : (' '|'\t'|'\n'|'\r')+ { skip(); } ;
Jim
From: [email protected]
[mailto:[email protected]] On Behalf Of Hans-Martin Adorf
Sent: Saturday, December 12, 2009 9:14 AM
To: [email protected]
Subject: [antlr-interest] eliminating left-recursion in the PAREN grammar
Folks,
I am working with a toy grammar as a stepping stone for more complicated
grammars. It is a grammar for the well-known PAREN language consisting only of
matched nested parantheses. In other words the language consists of non-empty
strings of the form "()", "(())", "()()", etc.
The grammar I am toying with reads
grammar PAREN;
tokens {
L = '(' ;
R = ')' ;
}
s : L R | L s R | s s;
ANTLR has the foreseeable problem with left-recursion. I was able to fix the
grammar for use with ANTLR by using an EBNF-notation
grammar ParenEBNF;
tokens {
L = '(' ;
R = ')' ;
}
s : L s? R s? ;
However, I would still be interested in a purely recursive grammar that avoids
left-recursion. Theory states that it should be possible to rewrite the grammar
such that it avoids left-recursion (and iterations). But I am lacking the
skills to achieve this, despite the apparent simplicity of the language above.
Any hints are welcome.
Regards
Hans-Martin Adorf
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