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9: Hints for Preparing Specifications
This section contains miscellaneous hints on preparing efficient, easy to change, and clear specifications. The individual subsections are more or less independent.
Input StyleIt is difficult to provide rules with substantial actions and still have a readable specification file. The following style hints owe much to Brian Kernighan.
The example in Appendix A is written following this style, as are the examples in the text of this paper (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 RecursionThe 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: list : item | list \',\' item ; and seq : item | seq item ; 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 | 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.
It is worth considering whether a sequence with zero elements has any meaning, and if so, consider writing the sequence specification with an empty rule: seq : /* empty */ | 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!
Lexical Tie-insSome 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 statements. Consider: %{ int dflag; %} ... other declarations ... %% prog : decls stats ; decls : /* empty */ { dflag = 1; } | decls declaration ; stats : /* empty */ { dflag = 0; } | stats statement ; ... other rules ... The flag dflag is now 0 when reading statements, and 1 when reading declarations, .ul except for the first token in the first statement. This token must be seen by the parser before it can tell that the declaration section has ended and the statements have begun. In many cases, this single token exception does not affect the lexical scan.
This kind of ``backdoor'' approach can be elaborated to a noxious degree. Nevertheless, it represents a way of doing some things that are difficult, if not impossible, to do otherwise.
Reserved WordsSome programming languages permit the user to use words like ``if'', which are normally reserved, as label or variable names, provided that such use does not conflict with the legal use of these names in the programming language. This is extremely hard to do in the framework of Yacc; it is difficult to pass information to the lexical analyzer telling it ``this instance of `if' is a keyword, and that instance is a variable''. The user can make a stab at it, using the mechanism described in the last subsection, but it is difficult.
A number of ways of making this easier are under advisement. Until then, it is better that the keywords be reserved \|; that is, be forbidden for use as variable names. There are powerful stylistic reasons for preferring this, anyway.