Bison parser expanding rule instead of reducing rule - parsing

Question

Is it possible for a Bison rule to expand instead of reducing so that it turns into more tokens? Asked a different way: is it possible to insert extra tokens to be parsed before the next token in the parser input?
Here is an example where I might want this:
Suppose I want a parser that understands three token types. Numbers (just positive integers for the sake of simplicity - INT), words (any number of letters, upper or lower case STRING) and some kind of other symbol (lets use an exclamation mark for no good reason - EXC)
Suppose I have a rule that reduces a word followed by a number followed by an exclamation mark. This rule results in an integer type, let's say for now that it simply doubles its input. This rule also allows itself to be the integer that it parses.
I also have a rule to accept any number of these in a row (the start rule).
The Bison parser look like this: (quicktest.y)
%{
#include <stdio.h>
%}
%union {
int INT_VAL;
}
%token STRING EXC
%token <INT_VAL> INT
%type <INT_VAL> somenumber
%%
start: somenumber {printf ("Result: %d\n", $1);}
| start somenumber {printf ("Result: %d\n", $2);}
;
somenumber: STRING INT EXC {$$ = $2 *2;}
| STRING somenumber EXC {$$ = $2 *2;}
;
%%
main(int argc, char ** argv){
yyparse();
}
yyerror(char* s){
fprintf(stderr, "%s\n", s);
}
The tokens can be generated with a flex lexer like so: (quicktest.l)
%{
#include "quicktest.tab.h"
%}
%%
[A-Za-z]+ {return STRING;}
[1-9]+ {yylval.INT_VAL = atoi(yytext); return INT;}
"!" {return EXC;}
. {}
This can be built with the following commands:
bison -d quicktest.y
flex quicktest.l
gcc -o quicktest quicktest.tab.c lex.yy.c -lfl -ggdb
I can now input something like this:
double double 2 ! !
and get the result 8
Now if I want the user to be able to avoid having lots of exclamation marks on one line, like this:
a b c d e f 2 ! ! ! ! ! !
I'd like to be able to allow them to input something like this:
a b c d e f 2 !*6
So I can add a flex expression for such a token that simply extracts the number of exclamations needed:
!\*[1-9]+ {
char *number = malloc(sizeof(char) * (strlen(yytext)-1));
strcpy(number, yytext+2);
yylval.INT_VAL = atoi(number);
free(number);
printf("Multiple exclamations: %d\n", yylval.INT_VAL);
return REPEAT_EXC;
}
But how would I implement the bison side of things?
I can add the token type like so:
%token <INT_VAL> REPEAT_EXC
And then a rule of some kind perhaps?
repeat_exc: REPEAT_EXC {/*expand into n exclamation marks (EXC tokens)*/}
;
Does Bison support this in any way?
If not how should I implement this?
Should I somehow have the lexer return the EXC token n times when it receives the repeat exc expression? (I'd rather avoid this if possible as this requires the flex code to keep record of some kind of state, it could be in the repeat exclamation state or in a normal state. The lexer is then not as simple to maintain.)

Answer 1

That's really not possible in a context-free grammar.
It's not that difficult to do in a traditional lexer, but as you say it requires that the lexer maintain state. An easier approach is to use a push parser, where the parser is called from the lexer rather than the other way around. [Note 1]
The bison manual doesn't explain the API very well; if you declare a pure push parser, the interface you get is:
int yypush_parse(yypstate*, int, const YYSTYPE*);
or, if position-tracking is enabled:
int yypush_parse(yypstate*, int, const YYSTYPE*, YYLTYPE*);
I made fairly minimal changes to your example, in order to show the push_parser interface. First, the parser; the only differences are the %define directives to declare a push parser; the elimination of main (the lexer is now top-level), and the declaration of yyerror with an explicit void return type. [Note 2]
%{
#include <stdio.h>
void yyerror(char* msg);
%}
%define api.pure full
%define api.push-pull push
%union {
int INT_VAL;
}
%token STRING EXC
%token <INT_VAL> INT
%type <INT_VAL> somenumber
%%
start: somenumber {printf ("Result: %d\n", $1);}
| start somenumber {printf ("Result: %d\n", $2);}
;
somenumber: STRING INT EXC {$$ = $2 *2;}
| STRING somenumber EXC {$$ = $2 *2;}
;
%%
void yyerror(char* s){
fprintf(stderr, "%s\n", s);
}
The lexer has some more substantial changes, but I don't think the end result is any harder to read or maintain. It might even be easier.
The macro PARSE sends a token with a specified type tag and value to yyparse; the macro PARSE_TOKEN sends a token without a semantic value.
The %options line removes several warnings from the compile step
The initialization of the parser state was added. (Indented lines after the %% and before any rule are inserted at the top of the lexer function, in this case yypush_parse, so they can be used to declare and initialize local variables.)
The INT rule was changed to allow 10 to be a valid integer.
The !*<int> rule was added.
The <<EOF>> rule was added. (It's pretty well boiler-plate for lexer-driven push-parsing.)
A main function was added, which calls yylex.
(Oh, and I changed a rule to avoid echoing new lines.)
%{
#include "push.tab.h"
#define PARSE(tok,tag,val) do { \
YYSTYPE yylval = {.tag=val}; \
int status = yypush_parse(ps, tok, &yylval); \
if (status != YYPUSH_MORE) return status; \
} while(0)
#define PARSE_TOKEN(tok) do { \
int status = yypush_parse(ps, tok, 0); \
if (status != YYPUSH_MORE) return status; \
} while(0)
%}
%option noyywrap nounput noinput
%%
yypstate *ps = yypstate_new ();
[A-Za-z]+ {PARSE_TOKEN(STRING);}
[1-9][0-9]* {PARSE(INT,INT_VAL,atoi(yytext));}
"!*"[1-9][0-9]* {int r = atoi(yytext+2);
while (r--) PARSE_TOKEN(EXC);
}
"!" {PARSE_TOKEN(EXC);}
.|\n {}
<<EOF>> {int status = yypush_parse(ps, 0, 0);
yypstate_delete(ps);
return status;
}
%%
int main(int argc, char** argv) {
return yylex();
}
Notes
This is the style of the lemon parser generator. lemon was originally written to create the sqlite SQL parser but is used in various projects precisely for the convenience of the "push" interface. bison's push-parser support is more recent, and very welcome.
I'm not crazy about INT_VAL; I prefer lower-case for union tags, but I was trying to minimize the diff.