Re: gnubol: [rpragana@acm.org: Some comments about a cobol parser]

[RKRayhawk@aol.com]

In a message dated 11/23/99 12:21:59 PM EST,
a hard working  Tim Josling, TIMJOSLING@prodigy.net writes:

<< 
 My test parser just parses. No error production, though I have an idea how 
to do
 good enough error recovery..
  >>


Permit a suggested terminology. We could have positive logic rules 
(productions) and error productions. These look the same to the parser tool.  
Error recovery is sometimes used to refer to the engagement of mechanisms 
built into the code generated by the parser tool.

I suggest that restricted use of the word. Users make coding mistakes, by 
compensating for it in the rules we are in a pedestrian sense recovering from 
the coders error; that is a somewhat confusing use of the word in the 
technical work context, because if we have coded some rule that actually 
traps the error, then the generated compiler is not in error recovery mode; 
it thinks it is just reducing or shifting.

Backtracking is a kind of error recover mode. But that is bending the word in 
a way because the exact purpose of backtracking is to prevent the other 
categories of error recovery.

Some tools generate an algorithm for us that can actually keep going after 
real scan interuption, though staggered this.  Some parsers will try to skip 
as many as three tokens in order to get to something it can recognize. This 
count (3) is usually mediated by a parm for the grammar rule interpretation 
when we are trying to generate the compiler. This type of recovery can 
_sometimes_ leave you in bad shape.

A regular positive logic case might be
  IF cond THEN stmt_block ELSE stmt_block END-IF.
    { life is fine };
It would not be too surprising if we saw someone code an error production to 
trap excessive END-IFs
  IF cond THEN stmt_block ELSE stmt_block END-IF  END-IF.
    { life is not so fine, extra END-IF ignored (call it unbalanced) };

That would be an error production, but it does not actually involve 'error 
recovery', in the constrained sense of the phrase.

Yet another form such as this

  IF cond THEN stmt_block ELSE stmt_block END-IF  error EOS
    { life is real bad };

uses the special error symbol. This is a production that engages the error 
recovery mechanism of the grammar tool generated compiler algorithm and gives 
the free fall a place to land.  Thus it involves hunches about where the 
coder makes mistakes.

Yet another rule might be 
  IF cond THEN stmt_block error ELSE stmt_block END-IF  
    { life is real bad down deep in the nest };

This too engages the error recovery mode of the algorithm in the compiler. 

You can not actually code the error recovery mode. That is the problem, once 
the compiler algorithm goes into error 'recovery' mode we are atleast 
somewhat out of control. The error symbol helps minimize this problem, and 
error productions (which just look like normal productions to the tool) help 
head things off at the pass.  Error productions, in cases they trap, actually 
prevent error recovery mode. :-)

Best Wishes,
Bob Rayhawk
RKRayhawk@aol.com

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