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

yacc: Compiler-Compiler XENIX Programming
In these rules, IF and ELSE are tokens, cond is a nonterminal sy mbol describing
conditional (logical) expressions, and stat is a nonterminal symbol describing statements.
The first rule will be called the simple-if rule, and the second the if-else rule.
These two rules form an ambiguous construction, since input of the form
IF (C1 )IF(C2)51 ELSE 52
can be structured according to these rules in two ways:
or
IF(C1 ){
IF ( C2 ) 51
}
ELSE 52
IF(C1 ) {
IF ( C2 ) 51
ELSE 52
}
The second interpretation is the one given in most programm ing languages having this
construct. Each ELSE is associated with the last IF immediately preceding the ELSE.
In this example, consider the situation where the parser has seen
I F ( C 1 ) I F ( C2 ) 5 1
and is looking at the ELSE. It can immediately reduce by the simple-if rule to get
IF ( C1 ) stat
and then read the remaining input
ELSE 52
and reduce
IF ( C1 ) stat ELSE 52
by the if-else rule. This leads to the first of the above groupings of the input.
On the other hand, the ELSE may be shifted, S2 read, and then the right-hand portion of
IF ( C 1 ) IF ( C2 ) .S 1 ELSE 52
can be reduced by the if-else rule to get
IF ( C1 ) stat
which can be reduced by the simple-if rule. This leads to the second of the above
groupings of the input, which is usually desired.
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