Question
Answer and Explanation
In Yacc (or Bison), it's not typically possible to perform actions before a token is matched, because Yacc's core functionality relies on matching sequences of tokens according to your grammar rules. Actions in Yacc are tied to the reduction of grammar rules. However, you can achieve similar effects by strategically placing your actions or by manipulating the lexer's behavior.
Here are several approaches to handle situations where you need to "do something" before Yacc actually uses a token:
1. Using Lexical Analysis (Lex or Flex):
- The most common and effective method is to perform pre-processing actions within the lexical analyzer (Lex or Flex). The lexer is responsible for tokenizing the input stream. You can modify the lexer to perform actions before returning the token to the parser.
- Example: Suppose you want to normalize all identifiers to lowercase before they're used by the parser. You can do this in the Lex file. If you have an identifier such as 'VariableName', you can convert it to 'variablename' before sending it to Yacc. Here’s a sample Lex snippet:
%{
#include "y.tab.h"
#include <string.h>
#include <ctype.h>
%}
%%
[a-zA-Z_][a-zA-Z0-9_] {
char p = yytext;
while(p){
p = tolower(p);
p++;
}
yylval.str = strdup(yytext);
return ID;
}
...
- In this example, `yytext` holds the matched token, and the example converts all chars to lowercase before it sends the token back to the parser (Yacc).
2. Actions in Yacc Before a Reduction:
- Though you can't act before matching a token, you can act before reducing a rule. If your "something" doesn't have to happen precisely before token matching but before rule reduction, this approach is suitable. - Example: If you need to process a declaration before its use, structure your grammar rules to act before reduction.
3. Using Global Variables and Flags:
- You can set flags in your lexical analyzer to indicate specific conditions. Then, in your Yacc rules, use these flags to control the actions taken when a specific rule is being reduced, thus simulating a pre-match behavior.
4. Custom Lexer Interaction:
- For more complex scenarios, you can write your own custom token-handling logic in the lexer and manage the token stream. This lets you “peek” ahead before fully committing to return a token for a Yacc match. This can be particularly useful if you need to look at context before the match.
5. Pre-Processing Phase:
- You might do a very first pass on the raw input before you call yacc/lex. This is helpful for complicated processing like macro expansion or code sanitization before any parsing actually starts.
Summary: While you can't directly perform actions before a token is matched by the Yacc parser, combining lexical analysis with strategic placement of actions and flags can effectively handle situations where some "preprocessing" is required. Choose the method that best suits the particular pre-processing you need to do. The Lex-level manipulation is usually the most straightforward solution for most needs.