[SimpleParse] Re: SimpleParse debugging question

[Mike C. F. <mcf...@ro...>]

bru...@mi... wrote:

> Mike,
>
> I'm trying out SimpleParse 2.0. The samples worked fine. I'm now 
> working on a very simple grammar and input file. Function 
> parser.parse() returns (0,[],N). No error message was output. How does 
> one debug in this system? Are there any error messages that can be 
> turned on in some way? I have no idea whether the problem is in my 
> grammar or input file, or both.
>
This is a question that's been coming up more often lately, so I'll 
respond on the simpleparse-users list as well as directly. 

The most reliable way to *build* a grammar for SimpleParse is to use 
test-driven development (aka TDD), so that you build, and test parsers 
for each production against the expected results for a given piece of 
text.  You can see examples of this approach in the tests subdirectory.

The easiest way to *debug* a completed grammar is to use the 
cut/ErrorOnFail directive (spelled "!") liberally throughout your 
grammar.  This directive tells the parser "if you start parsing this 
production, and wind up not finishing, then raise a SyntaxError, rather 
than continuing to parse".  For instance:

    ifStatement := 'if', !, ts, ':', suite

says, if the parser encounters an 'if' (when looking for an ifStatement) 
and then fails to find some space, a colon, and then a suite of lines, 
this is an error condition which should be signaled to the user via a 
syntax error.  You need to be careful that you add them only where there 
is a true SyntaxError on failure, i.e.:

    ifStatement := if1 / if2
    if1 := 'if', !, ts, ':', suite
    if2 := 'if', !, ts, ':', statement

Would likely not have the effect you want, as if2 could never match (if1 
would always raise a SyntaxError in any case where if2 would match).

You can read a little more about the cut directive here:

    http://simpleparse.sourceforge.net/simpleparse_grammars.html

and see a real-world example of it's use in the 
examples/vrml_erronfail.py module, which is an error-on-fail-annotated 
version of the vrml.py module in the same directory.

HTH,
Mike

_______________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://members.rogers.com/mcfletch/