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bisonyacc

Do some versions of Yacc use '=' between rules and actions?

I am compiling a very old program (wu-ftpd 2.6.1 circa 2000) and it was failing on this code:

cmd_list:   /* empty */
    | cmd_list cmd
    =   {
        fromname = (char *) NULL;
        restart_point = 0;
    }
    | cmd_list rcmd
    ;

Bison says the problem is the unexpected '=' which makes sense, as every other Yacc program I've seen (admittedly not very many) does not use an '=' sign there. I've replaced the =\t{ pattern with \t{ and the software builds (with many warnings of course) but I'm wondering how it was built in the past. Has Yacc syntax changed?

Solution

  • Yes, very old versions of yacc allowed the semantic action to be signaled with an =. They also allowed actions consisting of single statements without enclosing braces.

    This syntax was noted as obsolete in Stephen Johnson's original Yacc paper, which dates from the 1970s. See Appendix D: Old Features Supported but not Encouraged:

    1. Actions may also have the form

       =     { . . . }

      and the curly braces can be dropped if the action is a single C statement.

    I did a quick check on my hard drive and found that the ={...} syntax was accepted by bison 1.2.5 (1996), but it doesn't seem to be present in v1.875 (2003). Somewhere between those two versions, bison's original hand-built lexer was replaced with a lexer generated by (f)lex (and the hand-built parser was replaced with a bootstrapped parser); I suppose the obsolete syntax never made it into the new parser.