Path: utzoo!attcan!uunet!husc6!purdue!decwrl!megatest!djones
From: djones@megatest.UUCP (Dave Jones)
Newsgroups: comp.lang.misc
Subject: Re: Algol-style semicolons
Message-ID: <1132@goofy.megatest.UUCP>
Date: 28 Dec 88 00:00:49 GMT
References: <600@tuck.nott-cs.UUCP>
Organization: Megatest Corporation, San Jose, Ca
Lines: 48


> In article <4396@tekgvs.GVS.TEK.COM> toma@tekgvs.GVS.TEK.COM (Tom Almy) writes:
>
>[...] "Why do we need semicolons at all?".
> 


They are "gobble-stoppers" -- symbols which clue the compiler as to
how to recover from a syntax error. A typical LR technique is the following:
On an error, the compiler discards states from the parse stack until a
state which can shift "error" is uncovered. It shifts the error. 
It then makes reductions any necessary before it discards (gobbles)
tokens which can not be shifted. Success is declared when some fixed
number of consecutive tokens have been shifted since the last error. The
usual number is three.  But when a gobble stopper is found, you
can say, "That's enough."

Here's a short yacc example from a production Pascal compiler currently
under construction.  The nonterminal "semicolon" is used all over the place. 
When a semicolon is actually found, yyerrok tells the parser that no 
more tokens need be shifted in order to consider that an error has been 
"recovered from".  If the semicolon is missing, we could be silent, but 
for the sake of portability, we issue a warning message.

(Note to gurus: The version of yacc being used is a repaired version
which does not have the bogus default reduction bug, which would
invalidate the error-recovery.)

semicolon:  ';' { yyerrok; }
         |  '.' { Mpc_warning("Period replaced with semicolon."); }
         |      { Mpc_warning("Semicolon inserted."); }

;


label_declaration
                : LABEL integer_list semicolon
                        { Mpc_add_labels($2); }
;

integer_list    : INTEGER
                     { $$ = List_new(); List_append($$, $1); }
                | integer_list ',' INTEGER
                      { List_append($1, $3);
                | error
                      { $$ = List_new(); }
                | integer_list ',' error
                      { $$ = $1; }