Re: LR(1) Parsing : Error Handling & Recovery

On 7/17/2014 10:15 AM, Evangelos Drikos wrote:


> But, I guess they could say the same thing if they replaced the hand
> coded Fortran parser with a LALR parser. I've compared recently an
> enhanced LALR parser with gfortran; the former has better error
> recovery and in many cases more accurate error messages whereas the
> latter sometimes just prints the cliche "Unclassifiable Statement".


It is my understanding (admittedly not based on personal experience)
that adding good error handling to LALR parser generator rules leads to
rule explosion and difficulties in producing a consistent rule set
without odd corner cases. Can you comment? If possible, can you give a
sense of the difference in rule-set size and parser performance between
your improved error set and one for the same language with no error
productions at all?


BTW: the Univac 1108 Fortran II compiler had, as many batch compilers
did, a cut-off so that if the compiler went completely off the rails it
would stop printing errors and append "Not all errors listed" as a final
message.


You guessed it: at SINTEF (NTH, Trondheim) circa 1974 we had a Fortran
program whose *only* diagnostic was: "Not all errors listed".


Ivan
[The problem with LALR error recovery is that the generated parser
combines states that are the same if your program is valid, but they
are from different grammar rules, so you'd want to produce different
error messages. The size of LR(1) parser tables was an issue on a 64K
PDP-11, but they're trivial now, and I don't know of any reason other
than inertia to use LALR rather than LR(1). -John]

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