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

yacc: Compiler-Compiler XENIX Programming
Preparing Specifications
This section contains miscellaneous hints on preparing clear, efficient, and easy-tochange
specifications.
Input Style
It is difficult to provide rules with substantial actions and still have a readable
specification file. These hints will help:
• Use uppercase letters for token names and lowercase letters for nonterminal
names. This rule helps you to know who to blame when things go wrong.
• Put grammar rules and actions on separate lines. This allows either to be changed
without an automatic need to change the other.
• Put all rules with the same left-hand side together. Put the left-hand side in only
once, and let all following rules begin with a vertical bar.
• Put a semicolon only after the last rule with a given left-hand side, and put the
semicolon on a separate line. This allows new rules to be easily added.
• Indent rule bodies by two tab stops and action bodies by three tab stops.
The examples in the text of this section follow this style (where space permits). The
user must make up his own mind about these stylistic questions; the central problem,
however, is to make the rules visible through the morass of action code.
Left Recu rsion
The algorithm used by the yacc parser encourages so-called left recursive grammar
rules, rules of the form
name : name rest-of-rule
These rules frequently arise when writing specifications of sequences and lists:
and
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list item
I list ',' item
seq item
I seq item