- From: Dave Beckett <dave.beckett@bristol.ac.uk>
- Date: Mon, 25 Oct 2004 16:17:20 +0100
- To: public-rdf-dawg@w3.org
In going through the WD grammar
http://www.w3.org/TR/2004/WD-rdf-sparql-query-20041012/#grammar
I was implementing with lex and yacc, based on earlier things derived
from BRQL, derived from RDQL.
I found the following shift/reduce conflicts:
1) 1 shift/reduce conflict at '.' in the grammar:
PatternElement: TriplePatternList .
TriplePatternList: TriplePatternList . TriplePattern
i.e.:
( (
^here
which can be nesting
( ( ...
PatternElement
OR
( ( s p o)
TriplePattern
This seems to be known and you need to do at least 2-token lookahead
or possibly arbitrary length, since you can have many nested ((()))s
That means extra work in the lexer since lookahead isn't available by
default in the standard lex/flex + yacc/bison/byacc tools that I
have. This parallels RDF/XML which needed 2 token lookahead near
property elements. I'd like this fixed.
Possible solutions could be:
Changing the block syntax to be {}s
Removing nested optionals and thus the need for blocks
2) (as in RDQL), 2 shift/reduce conflicts at '.' in the grammar:
UnaryExpression: UnaryExpressionNotPlusMinus . PLUS UnaryExpression
| UnaryExpressionNotPlusMinus . MINUS UnaryExpression
| UnaryExpressionNotPlusMinus .
This one isn't serious, it's just the usual thing with expression
grammars and unary operators. I think! Could still be my error.
Dave
Received on Monday, 25 October 2004 15:20:40 UTC