Improve debugging: peek show next data in errors #46

Activity

1. deliciouslytyped commented on Sep 25, 2020

   

   deliciouslytyped

   on Sep 25, 2020

   Author

   More actions

   I don't know if I'm missing something but I've been messing with parsy for a couple days and it's really hard to debug?
2. spookylukey commented on Sep 25, 2020

   

   spookylukey

   on Sep 25, 2020

   Member

   More actions

   I'm a bit hesitant to change the behaviour regarding error messages here, because it could affect so many users, and I don't know how to do it in a way that is generically useful. Do you have any ideas @jneen ?

   Parsy certainly doesn't have advanced features for syntax error reporting. If you are needing to pass on detailed parsing error message to end users (e.g. for a programming language), then it probably requires you to build this yourself, or use a different more heavyweight tool (e.g. lark perhaps?)

   For building parsers for small "languages", I've found it usually pretty easy to debug my code, by ensuring that I'm building up progressively larger parsers from smaller ones and testing piece by piece. I'm not sure what your use case is.
3. jneen commented on Sep 25, 2020

   

   jneen

   on Sep 25, 2020

   More actions

   I don't use combinators myself these days, for the exact reason that other approaches (for me, skeleton trees) result in better error messages. @deliciouslytyped do you mean debugging your parser or providing debugging information for your user? If it's the latter, I'd point you to the index parameter of the Failure object, which you can use to index into the source string and find the "upcoming data".
4. jneen commented on Sep 25, 2020

   

   jneen

   on Sep 25, 2020

   More actions

   For debugging your own parsers, it may also be useful to add something like:

   class Debug(namedtuple('Debug', 'stream index')):
       pass
   
   debug = Parser(lambda stream, index: Result.success(index, Debug(stream, index)))

   or

   def debug_log(p, *args):
       @Parser
       def debug_parser(stream, index):
           result = p(stream, index)
           print(f"debug@{index}: '{stream[index:index+10]}' = {result}")
       return debug_parser
5. deliciouslytyped commented on Sep 25, 2020

   

   deliciouslytyped

   on Sep 25, 2020

   Last edited by deliciouslytyped

   Author

   More actions

   I'm talking about debugging my own parsers. Maybe it's just lot's of PEBCAK.
   I just wrote almost the exact thing you just did 5 minutes ago for debug(), so that's a step in the right direction at least.
   A lot of the time I can't tell which part of my code is responsible for the error messages I'm getting.

   Edit: rest of post moved to #48.

   Progressively building up parsers in small steps certainly does help.