Path: utzoo!mnetor!uunet!husc6!necntc!ima!johnl
From: harvard!rutgers!cs.rochester.edu!scott@BBN.COM (Michael Scott)
Newsgroups: comp.compilers
Subject: Re: Recursive-descent parser generator wanted
Message-ID: <909@ima.ISC.COM>
Date: 18 Mar 88 13:35:22 GMT
References: <887@ima.ISC.COM>
Sender: johnl@ima.ISC.COM
Reply-To: Michael Scott <harvard!rutgers!cs.rochester.edu!scott@BBN.COM>
Organization: U of Rochester, CS Dept, Rochester, NY
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| [At the risk of much flamage, I'd be interested in comments about why one
| might prefer an R.D. parser generator to an LR one.  We've beaten error
| recovery to death, unless someone has something genuinely new.  -John]

Let's tackle the easy question first: why might one prefer an LL parser to
an LR one.  Answer:
    (1) because the table sizes are a lot smaller
    (2) because many people find the parsing technique a lot more intuitive
    (3) because it lets you put action routines anywhere you want
        to in your grammar
    (4) because it is amenable to very simple, high-quality, automatic
        syntactic error recovery (ala Fischer, Milton, and Quiring)

Now the harder question: why might one prefer recursive descent [John's
note was in response to a request for tools to build recursive descent
parsers automatically].  Possible answers:
    (1) because you can look at the parser and really *see* what it's doing
    (2) because you can modify it even if you don't have the parser
        generator available (this might actually be a concern if you
        want to distribute the parser with source, but without the tools)
    (3) because it might conceivably run a little faster, just as
        "compiled" scanner tables (implemented with gotos) run faster
        than table-driven lex
        
Of course, the recursive descent parser will be larger than the table-driven
LL parser, and it won't support FMQ error recovery.
-- 
Michael L. Scott
University of Rochester    (716) 275-7745
scott@cs.rochester.edu     scott%rochester@CSNET-RELAY
{decvax, allegra, cmcl2}!rochester!scott
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