Manual

Chapter 5: The Bison Parser Algorithm 79
%token BOGUS
...
%%
...
return_spec:
type
| name ’:’ type
/* This rule is never used. */
| ID BOGUS
;
This corrects the problem because it introduces the possibility of an additional active
rule in the context after the ID at the beginning of return_spec. This rule is not active
in the corresponding context in a param_spec, so the two contexts receive distinct parser
states. As long as the token BOGUS is never generated by yylex, the added rule cannot alter
the way actual input is parsed.
In this particular example, there is another way to solve the problem: rewrite the rule
for return_spec to use ID directly instead of via name. This also causes the two confusing
contexts to have different sets of active rules, because the one for return_spec activates
the altered rule for return_spec rather than the one for name.
param_spec:
type
| name_list ’:’ type
;
return_spec:
type
| ID ’:’ type
;
For a more detailed exposition of LALR(1) parsers and parser generators, please see:
Frank DeRemer and Thomas Pennello, Efficient Computation of LALR(1) Look-Ahead Sets,
ACM Transactions on Programming Languages and Systems, Vol. 4, No. 4 (October 1982),
pp. 615–649 http://doi.acm.org/10.1145/69622.357187.
5.8 Generalized LR (GLR) Parsing
Bison produces deterministic parsers that choose uniquely when to reduce and which reduction
to apply based on a summary of the preceding input and on one extra token of
look-ahead. As a result, normal Bison handles a proper subset of the family of context-free
languages. Ambiguous grammars, since they have strings with more than one possible sequence
of reductions cannot have deterministic parsers in this sense. The same is true of
languages that require more than one symbol of look-ahead, since the parser lacks the information
necessary to make a decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (see Section 5.7 [Mystery Conflicts], page 77), there are
languages where Bison’s particular choice of how to summarize the input seen so far loses
necessary information.
When you use the ‘%glr-parser’ declaration in your grammar file, Bison generates
a parser that uses a different algorithm, called Generalized LR (or GLR). A Bison GLR