Add nix language parser

[bsima]

~~This is a work in progress.~~ Okay I think this is in good shape now, seems to work well in my usage and passes unit tests.

[masatake]

To docs/news/HEAD.rst, please add:

* Nix *optlib* by YOURNAME 

[masatake]

Thank you.

[bsima]

I updated the commit with your comments. Next I'll work on expanding the test case to actually test real-world nix code: I'll make a small library of functions and a couple builders with different formatting.

[bsima]

I added some better test nix code, I tried to look at some of my personal nix code and copy a few formatting patterns that I use, which is similar to the style you see in upstream nixpkgs afaik. So I would expect this optlib to work reasonably well everywhere.

I thought I would need a multiline regex for the multilineFunc and multiline_attrset but I don't? It just worked? Maybe I don't understand regex as well as I thought?

Anyway sorry for taking so long to finish this. But I've been using this optlib in my personal code, which runs over nixpkgs and indeed finds most definitions I need, and my work code, which has thousands of lines of custom nix and it seems to work more often than not.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.

Project coverage is 85.39%. Comparing base (08e07dc) to head (8dfde48). Report is 37 commits behind head on master.

Files	Patch %	Lines
optlib/nix.c	0.00%	21 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4020      +/-   ##
==========================================
- Coverage   85.43%   85.39%   -0.04%     
==========================================
  Files         235      236       +1     
  Lines       56727    56750      +23     
==========================================
  Hits        48462    48462              
- Misses       8265     8288      +23     

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[masatake]

Could you rebase your chnages on the latest code?

[masatake]

The multiline regex meta parser is a toy in many situations. The multi-table byte-oriented meta parser (--_tabledef-<LANG>, and --_mtable-regex-<LANG>) is the way to go.

I don't know the Nix language at all. However, I guess you may want to fill out the scope fields for tag entries. Without it, a client tool like Vim cannot draw a tree, which is useful for navigation.

struct point {
   int x;
   int y;
};

For the input of C language, ctags can emit the following tags:

point	input.c	/^struct point {$/;"	s	file:
x	input.c	/^  int x;$/;"	m	struct:point	typeref:typename:int	file:
y	input.c	/^  int y;$/;"	m	struct:point	typeref:typename:int	file:

From the tags output, x is a part of point.

  attrset = {
    foo = "bar";
  };

The current nix parser doesn't extract foo because the name is too short. I guess an ideal Nix parser may emit:

attrest	input.nix	/^  attrset = {$/;"	a
foo	input.nix		;"	a	attr:attrest

Too many tags may not be a problem if the scope fields are filled out well.

However, these improvements can be made after merging this pull request.

[bsima]

Ah, scoped tags is a very good idea. It's something I would need if I want to continue using tags in nixpkgs. Currently I just scan through the list of duplicated tags for the correct one, but scopes is a much better solution. I will read about scopes tomorrow.