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From: rajan@rti.UUCP (Sundar Varadarajan)
Newsgroups: comp.bugs.4bsd
Subject: Bugs in yacc error handling and types on value stack.
Message-ID: <1600@rti.UUCP>
Date: Mon, 3-Aug-87 13:23:26 EDT
Article-I.D.: rti.1600
Posted: Mon Aug  3 13:23:26 1987
Date-Received: Tue, 4-Aug-87 03:51:25 EDT
Organization: Research Triangle Institute, RTP, NC
Lines: 43
Keywords: yacc error types

Hi,

     There seem to be two problems with the way yacc handles errors.

(1)  If, in a state the action on error is to shift, but the  default  action
     is  to  reduce, then yacc will reduce, rather than shift, on error. (See
     "Introduction to Compiler Construction with UNIX" by Axel  T.  Schreiner
     and H. George Friedman Jr. Chapter 4, page 71 for a full explanation).

(2)  If, in a state the action on error is  to  reduce  by  a  rule  but  the
     default  action  is  to reduce by another rule, then yacc will reduce by
     the latter rather than by the former(on error).

     The fixes for the two problems are as follows. If in a state there is  a
shift on the special terminal "error", then make the default action the error
action.  That is, yacc recognizes the input as being in error by default. For
the second problem, if there is a reduce on error, then make that the default
action.

     This is done by modifying the code in y3.c in the yacc  source  as  fol-
lows.

    288c296
    <       if( temp1[chfind(2,"error")] > 0 ) lastred = 0;
    ---
    >       if( temp1[1] > 0 ) lastred = 0;
    290,293d297
    <       /* for error recovery, arrange that, if there is a reduce on the */
    <       /* error recovery token, `error', the default be that reduction */
    <       if( temp1[chfind(2,"error")] < 0 ) lastred = -temp1[chfind(2,"error")];
    <

(This is extracted by "diff newy3.c y3.c". Furthermore, the line numbers  are
likely to be slightly off).

     Another yacc bug. Yacc sometimes gets confused with  the  type  (of  the
value  on  the value stack) of a terminal. If you have more than 64 different
types attached to non-terminals and if you  declare  the  type  of  the  non-
terminals  before  the  type  of  the  terminals  (with the "%type <...>" and
"%token <...>" declarations), yacc will get confused about the  type  of  the
terminals.  Always  declare the type of the terminals first and then the non-
terminals.
Sundar