Debugging support

It’d be nice to have a way to show what Parsimmon does under the hood. This should help trace bugs in complicated parsers. Maybe using debug or something similar.

For instance:

const parser = P.alt(
  P.regexp(/[0-9]+/i),
  P.regexp(/[a-z]+/i)
).many()

parser.parse('hello10')

I’d like to be able to see something like:

[0:0] "hello10"
  alt(2):
    regexp(/[0-9]+/i): fail
    regexp(/[a-z]+/i): "hello"
[0:5] "10"
  alt(2):
    regexp(/[0-9]+/i): "10"

[mixtur]

At some point I made these helpers for my language’s parser.

It generates output that looks like this when logAnnotations is set to true.

It helped me to find a few bugs that were very hard to find otherwise.

And having this experience, I will probably do it differently next time. Since I was logging everything at the moment of parsing I could only tell if parser succeeded or failed AFTER it was done; so the tree structure is kinda upside down (children first, root last). So maybe it would be better not to log everything at parsing time, But collect data about parsing process, and then provide a way to log it.

There is also sinCounter / breakOnRepeat / MAX_SINS. It helped me to locate cases that looked like

alt(
  seq(a, b),
  seq(a, c)
)

[wavebeem]

I did just have a thought though that using the upcoming createLanguage API we could automatically add a debugging name equal to the object key! On top of that we could just keep track of parse depth and print the debug name with appropriate indentation, and this might actually be useful.