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- Thu, 18 Nov 1993 09:59:57 +0000
-To: Robert.Corbett@Eng.Sun.COM
-cc: partain@dcs.gla.ac.uk
-Subject: Re: [Robert.Corbett@Eng.Sun.COM: Re: possible bug, byacc 1.9]
-In-reply-to: Your message from 9:46 AM GMT
-Date: Thu, 18 Nov 93 09:59:53 +0000
-From: Jim Mattson <mattson@dcs.gla.ac.uk>
-
-It's clear that this feature improves error detection, but it's not
-clear to me how it improves the scope of possible error recoveries.
-
-If I understand your explanation, it sounds like the only alternative
-(short of changing the byacc source) is to add tens or hundreds of
-error productions sprinkled throughout the code anywhere that an
-unexpected symbol may appear, since no intervening reductions are
-allowed.
-
-Although the addition of all of these error productions increases the
-scope of possible error recoveries, the same functionality (with, in fact,
-the same approach) is provided by other versions of yacc. The apparent
-advantage of other versions of yacc is that they provide a facility by
-which a single _default_ error production can handle a number of
-possibilities (after some possibly illegal reductions have been performed).
-
-Am I missing something?
-
---jim
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-In reply to the following message:
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-
-------- Forwarded Message
-
-Date: Wed, 17 Nov 93 22:33:44 PST
-From: Robert.Corbett@Eng.Sun.COM (Robert Corbett)
-Message-Id: <9311180633.AA07545@lupa.Eng.Sun.COM>
-To: partain@dcs.gla.ac.uk
-Subject: Re: possible bug, byacc 1.9
-
-It is a feature. One difference between Berkeley Yacc and its
-predecessors is that the parsers Berkeley Yacc produces detect
-errors as soon as possible. That will lead to different behavior.
-
-In this particular case, the token "IN" is not a permitted
-lookahead symbol in state 390. AT&T Yacc parsers will not detect
-the error until after doing more reductions than Berkeley Yacc
-parsers. Doing reductions in illegal contexts limits the scope of
-recoveries that are possible (unless backtracking is possible).
-
-I am sorry that my attempt to provide better error detection is
-causing you trouble. You can get the AT&T Yacc behavior by
-replacing the routine sole_reduction in mkpar.c with a routine
-that returns the most frequently occurring reduction.
-
- Yours truly,
- Bob Corbett
-
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-
->From partain@dcs.gla.ac.uk Wed Nov 17 05:03:44 1993
-To: robert.corbett@Eng
-Subject: possible bug, byacc 1.9
-Date: Wed, 17 Nov 93 12:33:42 +0000
-From: Will Partain <partain@dcs.gla.ac.uk>
-
-Sadly, it's in a *HUGE* grammar, which I will send you if you have the
-stomach for it.
-
-The problem occurs where {Sun's /usr/lang/yacc, bison} say:
-
- state 390
-
- aexp -> var . (rule 356)
- aexp -> var . AT aexp (rule 366)
-
- AT shift, and go to state 508
- $default reduce using rule 356 (aexp)
-
-but byacc says
-
- state 396
- aexp : var . (356)
- aexp : var . AT aexp (366)
-
- AT shift 511
- error reduce 356
- VARID reduce 356
- CONID reduce 356
- VARSYM reduce 356
- CONSYM reduce 356
- MINUS reduce 356
- INTEGER reduce 356
- FLOAT reduce 356
- CHAR reduce 356
- STRING reduce 356
- CHARPRIM reduce 356
- INTPRIM reduce 356
- FLOATPRIM reduce 356
- DOUBLEPRIM reduce 356
- CLITLIT reduce 356
- VOIDPRIM reduce 356
- CCURLY reduce 356
- VCCURLY reduce 356
- SEMI reduce 356
- OBRACK reduce 356
- CBRACK reduce 356
- OPAREN reduce 356
- CPAREN reduce 356
- COMMA reduce 356
- BQUOTE reduce 356
- RARROW reduce 356
- VBAR reduce 356
- EQUAL reduce 356
- DOTDOT reduce 356
- DCOLON reduce 356
- LARROW reduce 356
- WILDCARD reduce 356
- LAZY reduce 356
- WHERE reduce 356
- OF reduce 356
- THEN reduce 356
- ELSE reduce 356
- PLUS reduce 356
-
-The token that comes in is "IN"; bison/sun-yacc-generated parser
-tickles the default, reduces to "aexp", but byacc-generated tickles
-"error" and the rest is history.
-
-Maybe this is enough for you to exclaim, "Oh yes, that's a feature."
-
-As I say, more info if you want it.
-
-Will Partain
-
-
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-
-
-
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