COCO/R bug fix

John Gough has reported a bug in Coco/R (the LL(1) parser generator that
originated from Hanspeter Mossenbock in Zurich). It exists in my MS-DOS
port (version 1.27), which I know several readers of this group have used,
and I suspect exists in the Oberon version too. A simple input that will
set it off is the following


          COMPILER E


          PRODUCTIONS
              E = % | % .
          END E.


The problem occurs when there are unrecognisable symbols (like %) in the
alternatives for a production. It does not always cause trouble; when it
does, the system loops infinitely.


The fix is as follows:


In CRP.MOD the code for PROCEDURE Term currently allows one to leave the
procedure without assigning proper values to the parameters gL and gR if
an unrecognisable terminal is encountered. A simple extra line sorts that
out


        PROCEDURE Term (VAR gL, gR: INTEGER);
            VAR
                gL2, gR2: INTEGER;
            BEGIN
                gL := 0; gR := 0; (* <= =============== add line here *)
                IF In(symSet[2], sym) THEN (* This is the DOS version; Oberon
                                                                                one is slightly different here *)
                    Factor(gL, gR);


The simplest fix is to alter CR.ATG. If you do this as below you can
generate a new CRP.MOD file by a bootstrap process:


      CR.ATG : we need to get the extra line generated ======
                          so the attribute grammar needs fixing |
                                                                                                                          |
Term<VAR gL, gR: INTEGER> (.VAR |
                                                                  gL2, gR2: INTEGER;.) |
= (.gL := 0; gR := 0.) <----- add here
( Factor <gL, gR>
        { Factor <gL2, gR2> (.CRT.ConcatSeq(gL, gR, gL2, gR2).)
        }
    | (.gL := CRT.NewNode(CRT.eps, 0, 0); gR := gL.)
    ).


Pat Terry, Computer Science, Rhodes University, GRAHAMSTOWN 6140, RSA
cspt@alpha.ru.ac.za or cspt@giraffe.ru.ac.za or pdterry@psg.com
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