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378 :: Questions on the pegs module

• h3rald (orginal) [2014-02-22T10:26:21+01:00] view original

  I've been experimenting with Nimrod for a while and I came across the pegs module, which is definitely a great replacement for Regexp (and it is also a pure library!), but I have two questions:

  1. The replace proc (and others) allow you to access captures with $n, like this:

  "var1=key; var2=key2".replace(peg"{\ident}'='{\ident}", "$1<-$2$2")

  Which does the job, but what if I want to pass $1 and $2 to a proc and do something with them? I guess what I am missing is Ruby's block syntax to process matches, like for the gsub method:

  http://robots.thoughtbot.com/rubys-gsub-with-a-block

  Is there any way to achieve this in Nimrod?

  2. I understand that that the pegs module allows a maximum of 10 captures. This is great for most of the things, and it is as much as you need if you are using pegs like you would use regexp. But potentially you can define a whole language with a peg, and it would make the creation of a parser relatively easy.

  However, with the current limits I couldn't find any way to create a parser easily straight from a peg in Nimrod, due to the limits in the number of captures among other things. Am I overlooking something really obvious here?

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• Araq (orginal) [2014-02-22T11:01:25+01:00] view original

  Yes we need another overload of 'replace' taking a callback.

  Well that's outside of the scope of the pegs module. Feel free to write a pegparsers module that supports parser generation. You can either provide a DSL via nimrod's macros to attach actions to rules or have a generic AST that is derived from the grammar and return this AST after a successful parse.

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• h3rald (orginal) [2014-02-22T15:35:09+01:00] view original

  Thanks Araq, good ideas! I'll look into it when I get a chance!

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• OderWat (orginal) [2016-05-07T17:38:13+02:00] view original

  I am currently experimenting with the pegs module and implemented the callback replace which was proposed here. If you are interested you may want to checkout my PR for that: https://github.com/nim-lang/Nim/pull/4136

  BTW: I know that this is a zombie topic but peg did just came up in a different topic and I think there is much to few coverage for this module anyway!

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• andrea (orginal) [2016-05-08T21:53:32+02:00] view original

  Yeah, I think it is pretty useful. I would really like to see a module to implement parsing via pegs, ideally through some internal DSL instead of strings, a little like Parboiled or PetitParser

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