Hey Thierry, Thank you for your input!

On 2 April 2015 at 16:59, Thierry Goubier &lt;thierry.goubier(a)gmail.com <mailto:thierry.goubier@gmail.com>> wrote: Hi Jan, 2015-04-02 16:36 GMT+02:00 Jan Kurš &lt;kurs(a)iam.unibe.ch <mailto:kurs@iam.unibe.ch>>: Hi All, I have pushed a new version for indentation sensitivity into the PetitParser. If I am not mistaken, with the new changes, you can parse languages such as Python, F#, Haskell, YAML or Markdown. There are proof of concept implementations in the PetitParser repository. YAML and Markdwon are almost complete, for Python only the indentation-sensitive rules. There is a small introduction to the indentation parsing with PetitParser (http://scg.unibe.ch/research/indentParsing), let me know, if you are missing something or if you want to know something more. There are also many examples of the indentation sensitive grammars in the repository. This is interesting. Thanks for the description and the link. One note in the end: isn't it that in fact what you are eating with the indentation stack is more than whitespace (i.e. bullet points)? See last paragraph, because your approach benefit is that you can use more than whitespace as indent, anything arbitrary would do, if I have understood correctly. Oh, you are absolutely right, you can consume any parsing expression with the indentation stack (even though I would prefer nothing more complicated than regex there :).

I did not mentioned it in the link above. But for example in Markdown, one use the indentation stack to recognize nesting of quoted blocks and lists, that starts with '>' and with whitespaces. In markdown, each quoted block or nested list pushes to the indentation stack and all the expressions on the indentation stack are evaluated in a sequence at the beginning of each line. So IMO you understand that correctly.

If you wonder about performance, the indentation sensitive grammar is approximately five times slower, because the position and an extra indentation stack is remembered instead of the simple position. If you don't use indentation, there is no slowdown. I am working on improvements, any ideas are welcome :) Totally out of the blue: your typical indentation parsing expression is a regex type of thing. What about compiling it as a SmaCC scanner? Such a PP parser would be equivalent to a Lexing automaton, and good compilers exist for such automatons. Good point, I do plan to do something like this for any parsing expression to get some performance. And it is a good idea to use SmaCC infrastructure for this.

Yet, the main performance problem with indentation expressions is to manage the indentation stack. That stack needs to be memoized (before choices, sequences, predicates) and restored if necessary, which takes most of the time. I hope some copy-on-demand will lower the overhead.