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

XENIX Programming yacc: Compiler-Compiler
In each of these cases, the first rule will be reduced for the first item only, and the
second rule will be reduced for the second and all succeeding items.
With right recursive rules, such as
seq item
I item seq
the parser would be a bit bigger, and the items would be seen, and reduced, from right
to left. More seriously, an internal stack in the parser would be in danger of
overflowing if a very long sequence were read. Thus, the user should use left recursion
wherever reasonable.
Consider whether a sequence with zero elements has any meaning, and if so, consider
writing the sequence specification with an empty rule:
seq /* empty *I
I seq item
Once again, the first rule would always be reduced exactly once, before the first item
was read, and then the second rule would be reduced once for each item read.
Permitting empty sequences often leads to increased generality. However, conflicts
might arise if yacc is asked to decide which empty sequence it has seen, when it hasn't
seen enough to know.
lexica l Tie-ins
Some lexical decisions depend on context. For example, the lexical analyzer might want
to delete blanks normally, but not within quoted strings. Or names might be entered
into a symbol table in declarations, but not in expressions.
One way of handling this situation is to create a global flag that is examined by the
lexical analyzer and set by actions. For example, suppose a program consists of 0 or
more declarations, followed by 0 or more statem ents.
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