Parsing command line arguments using a finite state machine and backtracking | Hacker News Reader

Parsing command line arguments using a finite state machine and backtracking

jawher.me

43 pointselasticdog10 comments

That was interesting to read and certainly a different approach to solving a problem, but the biggest question I had in my mind while reading the article was "is it really necessary to create all this complexity --- and how does it compare to the traditional getopt()?"

I've used getopt() many times, even implemented its functionality for fun, and parsing commandline arguments has never been something I thought was particularly difficult or complex.

I would think the biggest benefit is the fuller understanding of what you built. Probably some efficiency benefits.

Of course, this is also the biggest fault, I would think. You rarely have the luxury of understanding to this level all of the parts of your program. And something like command line parsing probably is not the bottleneck in any way of your program.

docopt() does few things and does them well:

* parse the call arguments and split them into options and arguments

* perform a very basic validation, i.e. handle the options which take a value and those who don't

Granted, mow.cli is much more complex. But it also does much more:

* Generate contextual help message

* Enforce arbitrarily complex validation rules (see the cp example) in your stead, meaning less code (and bugs) for you to write

* Does the routing, i.e. you don't need to have multi-level switch/case and if/else blocks to select and execute the correct code path based on the input arguments.

I talked about this in a previous article [1].

And finally, the library user doesn't have to deal with all this complexity: this is an implementation detail which I thought would interest technical readers, hence blogging about it.

You as a user only use the much easier to grasp exposed API [2]

[1] https://jawher.me/2015/01/13/parsing-command-line-arguments-...

[2] http://godoc.org/github.com/jawher/mow.cli

You could use this technique to examine the man page for a program and infer from that what valid command line arguments would be: some terminal shells (fish, for one, possibly zsh as well?) do something like this already to support autocomplete.

I ended up writing a blog post on how to handle CP arguments a while back. I started coding before really thinking about it -- I mean, it's CP, how hard could it be (famous last words)?

http://blog.typeobject.com/thinking-out-loud-file-copy-tool-...

It turns out the arguments to CP are really tough to reason about if you just naively dive into it. The approach in this article is much better than mine. It feels like I was fumbling around in the dark.

If you're interested in seeing my implementation (it's buggy, but handles most cases), it can be found at:

https://github.com/hpc/dcp/blob/master/src/handle_args.c

The entry point to handle_args.c is DCOPY_parse_path_args, which is called by the main() in:

https://github.com/hpc/dcp/blob/master/src/dcp.c

I thought about the same thing in regards to cp. I've been using OpenBSD a lot lately and having it's source installed wanted to take a look at their coding style and such. I picked cp as my first glance as I thought that should be simple enough to understand quickly.

As you say, it's option parsing makes up the bulk of the cp.c code, with utils.c doing most of the actual file copying. It turns out it's pretty tough to offer a lot of options with some of them being mutually exclusive. Obvious in hindsight but interesting to me at the time I was reading through it.

http://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/bin/...

Why write an FSM and backtracking explicitly? Use parser combinators, they basically do the same thing, just express it in a clearer, grammar-friendly way: http://theorangeduck.com/page/you-could-have-invented-parser...

Parser combinators would certainly have been easier for me, the library author but not necessarily so for the end user.

Contrast how it is done now:

  [-R [-H | -L | -P]] [-fi | -n] [-apvX] SRC... DST

With a parser combinator based approach:

  and(
    optional(and('-R', or('-H', '-L', '-P'))),
    optional(or(
                '-fi', 
                '-n'
             )
    ),
    optional('-apvX'),
    repeatable('SRC'),
    'DST')

And this didn't even handle the fact that options order is not important.

For that I think you can do something similar to what mpc for C (https://github.com/orangeduck/mpc) and LPeg for Lua (http://www.inf.puc-rio.br/~roberto/lpeg/), which is to provide the parsing machinery and write a small DSL with the same machinery for the users.

How does this avoid the exponential-time worst-case complexity of backtracking approaches to walking NFAs?

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