Intel XENIX 286 Programmers Guide (86) - Tenox.tc

XENIX Programming yacc: Compiler-Compiler
Once again the parser can do two valid things; there is a shift/reduce conflict. The
application of disambiguating Rule 1 tells the parser to shift in this case, which leads to
the desired grouping.
This shift/reduce conflict arises only when there is a particular current input symbol,
ELSE, and particular inputs already seen, such as
I F ( C 1 ) I F ( C2 ) S 1
In general, there may be many conflicts, and each one will be associated with an input
symbol and a set of previously read inputs. The previously read inputs are characterized
by the state of the parser.
The conflict messages of yacc are best understood by examining the verbose (-v) option
output file. For example, the output corresponding to the above conflict state might be
23: shift/reduce conflict (shift 45, reduce 18) on ELSE
state 23
stat : IF ( cond ) stat ( 18)
stat : IF ( cond ) stat-ELSE stat
ELSE shift 45
reduce 18
The first line describes the conflict, giving the state and the input symbol. The ordinary
state description follows, giving the grammar rules active in the state, and the parser
actions. Recall that the underline marks the portion of the gram mar rules that has been
seen. Thus in the example, in state 23 the parser has seen input corresponding to
IF ( cond ) stat
and the two grammar rules shown are active at this time. The parser can do two
possible things. If the input symbol is ELSE, it is possible to shift into state 45. State
45 will have, as part of its description, the line
stat : IF ( cond ) stat ELSE stat
since the ELSE will have been shifted in this state. Back in state 23, the alternative
action, described by "·" , is to be done if the input symbol is not mentioned explicitly in
the above actions; thus in this case, if the input symbol is not ELSE, the parser reduces
by grammar rule 18:
stat : IF '(' cond ')' stat
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