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title: "claptrap: a Go flags library. Very small, much features! No dependencies & getoptish." date: 2022-09-28T21:34:00Z updated: 2022-10-17T18:41:00-05:00 tags: ["golang", "library", "flags"]

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Turn on dark mode: upper right, little sun-looking thing. Code blocks in this theme look hideous in light mode, and I simply haven't been able to work up the enthusiasm to learn enough scss to figure out how to fix it. I could change themes, I guess, but that affects the whole site, and... just use the dark theme for now, ok? Thanks.

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claptrap opts lib: very small, much features, getoptish

https://sr.ht/~ser/claptrap/

Every few months, when I start a new CLI, I spend a good amount of time looking around for a flags library. For trivial programs, the stdlib `flag` library is OK, but it breaks `getopt` arguments style, the undisputed most popular arguments standard for POSIXy OSes, in the world. And there are a wealth of alternative flags libraries available, some quite good, but I am inevitably unsatisfied with them for usually one of three reasons:

They're huge. Lots of code, lots of dependencies. This is especially a problem when you're trying to validate all of your dependencies for a sensitive program, like something that interacts with credentials.
They have an API that feels more like a framework than a library. These are flags libraries you find yourself building your application around. Any flags library that forces you to restructure your program falls in this category.
They're *too* bare-bones. Having to re-write input type validation code over and over gets tedious. This is where I shy away from most of the `getopts` clones. `getopts` is pretty bare bones, but conforming to the usage patterns doesn't force you to copy the skeletal feature set.

Over time, I'd built up some tooling to evaluate lib tools; I've posted that, and the analysis, here[1], and I focused this time on clapper[2], which seemed to fit the bill. There were a couple of small features I wanted, and I sent in a couple of patches, but after one particularly engaging hacking session I ended up with a drastic rewrite that maintained and expanded the feature set while preserving the small size and general `getopts` compatability. At this point, it was a different library, so I rebranded it and Claptrap was born.

1: https://hg.sr.ht/~ser/flags-libs-analysis

2: https://github.com/thatisuday/clapper

Claptrap is a small but powerful Go package to parse command-line arguments

getopt(3) style*[3]. Designed to not burden your application down while providing a rich feature set and validation. for making CLI based libraries with ease. It has support for:

3: http://man7.org/linux/man-pages/man3/getopt.3.html

getopt long & short flags (--bool, -b)
combined short (-b -e == -be)
inverted bools (--bool => --no-bool)
typed params (int, bool, Duration, string, float)
mandatory flags and arguments
positional arguments
global flags
subcommands
variadic flags (-a 1 -a 2 -a 3)
partial command matching (--list == --ls == --st)
Usage()

And it's 455 lines of code, and **0** dependencies.