Rust CLI with Clap
72 comments
·July 1, 2025csnover
sshine
> I got a nickel every time I spent 15 minutes replacing a trivial use of clap with pico-args and thus reduced the binary size and compile time of some project by at least 80%, I would have at least three nickels.
Hahaha, awesome. Thanks for the pico-args recommendation.
It supports the bare minimum.
I sure would like deriving-style parsing and --help auto-generation.
I think deriving-style unavoidably causes build time and complexity.
But it could be done without the dependency explosion.
There's a list of options here:
https://github.com/rosetta-rs/argparse-rosetta-rs#rust-arg-p...
Among the ones you recommend, argh supports deriving, auto-generates --help and optimizes for code size. And its syntax is very comparable to clap, so migrating is very easy. gumdrop seems very similar in its feature set (specifying help strings a little differently), but I can't find a defining feature for it.
johncolanduoni
You’re right that there are only trade-offs in engineering. But the key to evaluating trade-offs is evaluating impact, and how long my dependencies take to compile when I first check out a repo or whether it takes 1ms or 2ms to parse my command line (if we’re even talking about something above microseconds) have no discernible impact for approximately all use-cases. If you’re making some odd CLI tool that has to run on an microcontroller with 1MB of RAM or something, fine, agonize about whether your command line parser is parsimonious enough. Otherwise you’ve abjectly failed to evaluate one of the most important trade-offs in engineering: whether something is even worth your time to think about.
csnover
> Otherwise you’ve abjectly failed to evaluate one of the most important trade-offs in engineering: whether something is even worth your time to think about.
Phew, several folks have replied to me about how it’s not worth the time thinking about these impacts at all, thus creating a paradox whereby more time has been spent thinking about writing about whether to think about it than has been spent in not thinking about it and just accepting that I wrote a reply on HN about how I feel there are more suitable command-line parsers than clap for most Rust projects! :-)
I agree that much of high-level engineering is knowing whether something is worth thinking about; in this case, I did the thinking already, and I am sharing what I know so others can benefit from (or ignore) my thinking and not have to do so much of their own. If my own personal anecdote of significantly reducing compile times (and binary sizes) by taking a few minutes to replace clap is insufficient, and if the aggregate of other problems I identified don’t matter to others, that’s alright. If reading my comment doesn’t make someone go “huh, I didn’t know {argh|gumdrop|pico-args} existed, clap does seem a little excessive now that you mention it, I will try one of these instead on my next project and see how it goes”, then I suppose they were not the target audience.
I don’t really want to keep engaging on this—as almost everyone (including me) seems to agree, command-line parser minutiae just aren’t that important—but I guess I will just conclude by saying that I believe that anchoring effects have led many programmers to consider any dependency smaller than, say, Electron to be not a big deal (and many think Electron’s fine too, obviously), whereas my experience has been that the difference between good and bad products usually hinges on many such ‘insignificant’ choices combining in aggregate.
Assuming whichever command-line parser one uses operates above a certain baseline—and I believe all of the argparse libraries in that benchmark do—it seems particularly silly to make wasteful choices here because this is such a small part of an application. Choosing wastefulness because it’s technically possible, then rationalising the waste by claiming it increases velocity/usability/scalability/whatever without actually verifying that claim because it’s ‘not worth thinking about’, seems more problematic to me than any spectre of premature or ‘unnecessary’ optimisation. I hope to find better ways to communicate this in future.
qaq
Hmm isn't optimizing to save 690KiB for an optimised release binary and getting incremental builds to be significantly less than 400ms actually much closer to the after-mentioned "every web site must be able to scale like Facebook” type logic" ?
csnover
No, it is the following the principle of YAGNI.
qaq
The “every website must scale like Facebook” mindset is premature optimization driven by hypothetical future needs exactly what YAGNI advises against. But in your case, you’re investing time upfront to avoid a heavier dependency that already works and has no clear downside for the majority of users.
If you don’t actually need ultra-small binaries or sub-200ms compile times, then replacing Clap just in case seems like a textbook case of violating YAGNI rather than applying it.
theknarf
Convenience always win. If we want smaller more purposefully built dependencies then we need better tooling that makes those choices convenient.
1vuio0pswjnm7
Rust invites serious disregard for resources and time. Sadly, many will accept that invitation. But some won't.
unrealhoang
And you disregard user experience and other developer experience with your own custom parsing code. Acts as if there's no trade-off whatsoever in your own decision and my way is holier than thou in engineering is beyond sad.
WiSaGaN
I am wondering how much of this can be mitigated by carefully designing feature flags, and make default feature set small.
null
kstrauser
That 690KB savings is 1/97000th of the RAM on the machine I develop and run most of my Rust software on.
If I ever encounter a single howto or blog post or Stack Overflow answer that tells me how to use Clap to do something 5 minutes more quickly than with an alternative, it’s paid for itself.
Amdahl’s Law says you can’t optimize a system by tweaking a single component and get more than that component’s total usage back. If Clap takes 1% of a program’s resources, optimizing that down to 0 will still use 99% of the original resources.
It’s just not worth it.
never_inline
At this point you're just flexing that you have 96GiB machine. (Average developer machines are more like 16GiB)
But that's not the point. If every dependency follows same philosophy, costs (compiler time, binary size, dependency supply chain) will add up very quickly.
Not to mention, in big organizations, you have to track each 3rd party and transitive dependency you add to the codebase (for very good reasons).
kstrauser
I can write and have written hand-tuned assembly when every byte is sacred. That’s valuable in the right context. But that’s not the common case. In most situations, I’d rather spend those resources on code ergonomics, a flexible and heavily documented command line, and a widely used standard that other devs know how to use and contribute to.
And by proportion, that library would add an extra .7 bytes to a Commodore 64 program. I would have cheerfully “wasted” that much space for something 100th as nice as Clap.
I’ve worked in big organizations and been the one responsible for tracking dependencies, their licenses, and their vulnerable versions. No one does that by hand after a certain size. Snyk is as happy to track 1000 dependencies as 10.
saghm
96? It sounds more like 64 to me, which is probably above average but not exactly crazy. I've had 64 GB in my personal desktop for years, and most laptops I've used in the past 5 years or so for work have had 32 GB. If it takes up 1/4700 of memory, I don't think it changes things much. Plus, argument parsing tends to be done right at the beginning of the program and completely unused again by the time anything else happens, so even if the parsing itself is inefficient, it seems like maybe the least worrisome place I could imagine to optimize for developer efficiency over performance.
ben0x539
structopt/Clap's derive magic is one of the first things I miss when I go to write some more-or-less trivial program in a non-Rust language these days. Being able to define all the data for a command line argument in one place (how/where to store it, what the type/valid input is, the association between the name and a variable/field, the documentation for --help...) seems like table stakes but afaict almost every other argument parsing library makes me repeat myself to the point where it takes all the joy out of writing a simple program.
(Python's docopt is also amazing, fwiw)
ameliaquining
I want to like docopt, but that the only data types it supports are boolean and string—if you want anything else, you have to do another round of parsing and error checking—destroys a lot of the advantage of using a high-level library for handling command-line arguments.
iloveyoudearly
Not sure if it's on par with Clap, but for Python I don't see enough people talk about SimpleParsing: https://github.com/lebrice/SimpleParsing
It has quirks once you try to do something more complex/advanced, but for most of the simple stuff it's very nice to use.
woile
In Python you can use pydantic to create a cli:
https://docs.pydantic.dev/latest/concepts/pydantic_settings/...
never_inline
People are used to the `click` way, where you can define args as function parameters. It's little more verbose but it helps click is a very established library which also provides many other things needed by CLI tools.
There's also `typer` from the creator of `fastapi` which relies on type annotations. I have not had the opportunity to use it.
porridgeraisin
Nice. I had made something similar (but less featureful), funnily enough also for an ML training script usecase. Here it is in a gist:
https://gist.github.com/porridgewithraisins/313a26ee3b827f73...
I love the ergonomics of this method, and I was going to improve it to support subcommands, etc, but now I think I will use the library you posted.
hamandcheese
I like clap a lot, however I find that clap derive is not very easily discoverable. I always have to google for the right macro incantation to get what I want. Whereas editor completions from rust analyzer get me quite far without needing to leave my editor when I'm just using an ordinary library.
I think this is more a criticism of rust-analyzer than clap itself, any macro-heavy library I have similar issues with.
(Yes I know clap can be used without derive, but I'm willing to deal with the pain to parse directly into a struct)
rendaw
I hope you don't mind me plugging my thing here, but I had the 100% same problem and made aargvark (https://docs.rs/aargvark/latest/aargvark/). When I was using clap, every time I'd need to look up how to do X, or what combination of things I needed to put an enum here, or find out that this nesting of data types wasn't supported, etc.
It's still derive macro-based, but there's only one derive (`Aargvark`) rather than `Parser`, `Subcommand`, etc, and it can handle any data structure composition orthogonally (although crazy structures may result in fairly awkward command lines).
jrimbault
FYI, maybe for you and other readers. My key to really understanding clap's derive macro was to understand that the macros takes as argument every methods of clap's Command struct https://docs.rs/clap/latest/clap/struct.Command.html
Looking at this resolved most of my issues about discoverability.
tucson-josh
I feel like there's a sweet spot for complexity that the derive macro hits pretty well. When things get more complex it can feel like a maze, but below that complexity it's pretty nice.
woodruffw
Nice write up. “Good” CLI semantics are pretty devilish, and overall I think clap does a pretty great job of picking the right (or at least most intuitive) behavior.
(One edge case that consistently trips me up, which other argument parsers similarly struggle with: an environment variable fallback has the same “weight” as its option counterpart, so any CLI that makes use of grouping/exclusivity will eventually hit user confusions where the user passes `--exclusive` and gets a failure because of an unrelated environment variable.)
tucson-josh
The argument / environment variable duality is a challenging one, especially when developing server software that should take security into account where you don't want to encourage users to put secrets into scripts. Do you end up with some items that can only be entered via one mechanism or another? Maybe that's where the fun of being a developer comes in is making those choices.
b0a04gl
10kloc for command line parsing. TEN THOUSAND LINES. pico-args does it in 700 lines and probably handles 99% of real world use cases. compile times go to shit binary size bloats and for some edge case you'll never hit.most CLI tools need what three four flags max, maybe a subcommand or two. you don't need the swiss army knife of argument parsing for that. tried replacing clap with pico-args on three different projects last month. 80% reduction in compile time every single time. binary went from 8mb to 2mb on one of them.the "disk space is cheap" argument's acceptable partially but compile time isn't. developer experience isn't. startup time isn't. memory usage isn't
ModernMech
No help generation
Only flags, options, free arguments and subcommands are supported
A properer parser would knew that --arg2 is a key and will return an error, since the value is missing.
If such behavior is unacceptable to your application, then you have to use a more high-level arguments parsing library.
So 700 LOC gets us a mediocre argument parser with no features. What do you get for an additional 9300 LOC? A "properer" parser (dev and user experience+). Help generation (dev experience+). Multiple interfaces (dev experience+). Color terminal output (user experience+). Suggested completions (user experience+).
Is it worth it? I dunno that's a per-project choice. If you absolutely need the smallest footprint and compile times possible, probably you don't want to go with clap. You also probably don't want to go with Rust.
vikrantrathore
I have used clap to build perspt (https://github.com/eonseed/perspt). The project has extensive documentation on how it was built, as we did it as a learning exercise.
porphyra
The macro magic in rust is amazing. It's so nice to just define a struct, slap a `#[derive(Parser)]` on it, and call it a day.
mootoday
That was a great intro to clap, thanks for writing it up!
I've been building clap CLIs for a while and started to put together a template: https://github.com/mootoday/cli-template.
It also includes a crate I developed to reduce the boilerplate code for nested commands: https://crates.io/crates/clap-nested-commands
tucson-josh
Thanks for the feedback. Nested commands are definitely full of boilerplate and your crate looks interesting.
ameliaquining
Worth noting that you can do this in languages other than Rust. Python, for example, has https://github.com/fastapi/typer. https://github.com/shadawck/awesome-cli-frameworks lists relevant libraries in many languages, though not all of them support clap-style structured parsing.
FujiApple
Something I’ve been working on recently is a command line tool [1] to bring clap declarative command line parsing to shell scripts. Unfinished WIP but largely functional.
msgodel
It really bothers me how much people use crates in Rust. "Minimalist" crates have tens of dependencies.
It's like the node.js of systems languages. Touching it feels gross.
deathanatos
… I don't want every program to attempt to implement argument parsing; bespoke implementations will be lower quality than clap. Not reinventing an argument parser is an extremely reasonably dependency to take.
Clap, on the non-derive side, has approximately two dependencies: anstream/anstyle (for terminal coloring, another thing that sounds deceptively simple at first pass, if you think all the world is a VT100, but really isn't; this is a reasonable dep. for a CLI arg parser) and strsim (string similarity, again, a reasonable dep for a CLI arg parser…). And that's it¹ for clap's direct deps.
(¹I'm omitting clap_lex, as an "internal" dep.)
On the derive side, there's the usual proc-macro2/quote/syn trio, but those come up frequently in derive crates due to what they do, and other than that, there's just `heck`, which is again an obvious dependency in context.
… what is so quizzical to me about the "so many dependencies!" complaint is that when we do get examples like this, they're almost always on crates that bottle up genuinely tricky functionality (like what we see here) — exactly the sort of thing that a.) is hard to get right and b.) isn't relevant to the problem I want to solve. That's like "absolutely this is a dependency" central, to me…
msgodel
This shouldn't even need terminal coloring, in fact that sounds annoying because it's going to have to behave differently if you pipe it to less (or it's going to do something dumb like the rust compiler itself and just reopen the tty.)
This actually reminds me of my other issue with this kind of "oh we just get it for free" attitude that tends to result in overbuilding things that I also dislike in rust.
No I think people would be better off with a bespoke option parser actually.
pksunkara
Actually,
1. `color` feature and thus the `anstream` dep is optional.
2. Even if you use it, it handles all the behaviour correctly regarding the piping and no color support, which is why it is a dependency in the first place.
Source: I am clap maintainer
rendaw
Why is needing to behave differently when you pipe annoying? Are you saying it doesn't work? But also FWIW I don't think piping command help output is a common use case.
hamandcheese
I agree with you that this is a ridiculous criticism to level against clap specifically.
But I also share the same overall sentiment. Every moderately sized rust project I've worked on has quite a lot of transitive deps, and that makes me a little bit nervous.
inferiorhuman
On the derive side, there's the usual proc-macro2/quote/syn trio, but
those come up frequently in derive crates due to what they do, and other
than that, there's just `heck`, which is again an obvious dependency in
context.
sebtron
Completely agree. I find it crazy that this is encouraged in the very first example in the official tutorial [1].
[1] https://doc.rust-lang.org/book/ch02-00-guessing-game-tutoria...
dietr1ch
If all the code was crammed into the std library it'd be fine?
Functions need to build on top of simpler functions to be able to abstract problems and tackle them one at a time. There's innate complexity around and without trying to tame it into smaller functions/packages it seems you'll end up in a worse spot.
hamandcheese
Not OP, but more code in stdlib does indeed sound better.
I'm not against abstraction and re-use. What I don't like is that for every given thing I want to do, there are multiple crates that offer the same functionality, and it can be really fatiguing trying to vet them. And it is truly a rarity to find a crate that is past the 1.0 version milestone.
Compare to golang for example. You can get quite far in go without needing to pull in any libraries. But in rust you need a library for even a basic http request.
dietr1ch
Trying to get everything into std requires a lot of work that I think gets harder when considering Rust's goals of being low level and paying only for what you use. Not having finalised some aspects of the language itself would also slow this down even more.
I'd rather have libraries built with more freedom and the possibility of having experimental stuff around meanwhile the std worthy solution lands, and if things work fine without them in the standard library then it makes sense to keep them out.
Rust may be lacking an easier way to shop for recommended libraries for common problems. There should be a path to discover all the good and best libraries for each problem. crates.io takes a stab at having this information, but I think more handholding and some sort of community seal of approval is needed.
tucson-josh
I'm not the OP here, but I did write the blog post. I have mixed thoughts on how much should be part of std. I think that what we're seeing is a class of library where we could see a new level introduced to open languages like rust that sits somewhere above std, but is maintained and blessed by a broader group than just the nebulous community. Rust's separation of std from no-std is a good example of tiers that I think we could see evolve. Another example in the same category as clap would be serde and some of the specific serde implementations like serde-yaml that is now in a really painful unmaintained/forked status. Maybe these are things to push for more broadly in the rust community.
msgodel
Yeah I was a little surprised to discover the standard library doesn't even include regex. That's kind of extreme. Even most C environments have that.
eeZah7Ux
Yes, having a good std library would be fine. It would really limit the proliferation of crates.
tcfhgj
NIHS?
jvanderbot
If you got a progress bar, website, and dependency tree for every
#include <argp.h>, <stdio.h>, <sstream>, or <curl.h>
it'd feel pretty crazy too. Imagine if `make` went out and pulled latest upstream changes for `pthreads` every time any one of your dependencies used it. C++ imagine it's pulling and building boost, or abseil.
C#? The entire mono/.net toolchain and system/ FFI libraries.
Imagine if we had "dot-h.io" that tracked how many separate C projects used argp. Laughable! Millions!
Every language has gobs of dependencies. So many dependencies it'd make you sick. Thousands upon thousands of lines of code, just to make something that runs on one target and says "Hello world" to the screen. Hell, some languages require you to run a runtime on your operating system that runs on real hardware _just to launch those thousands of lines of code_. And those are written using curl.h, pthreads.h, etc etc (or similar). Bananas!
At least those with package managers allow you to see it, audit it, update it seamlessly.
If it's too big, use "nostd"
msgodel
I shouldn't have to and will not explain why C's stdio.h and arbitrary 3rd party projects on github are not the same.
gametorch
The compile time guarantees + declarative nature make Clap so amazing and foolproof. This is like heaven compared to imperative, arcane incantations like getopt.
In my opinion, clap is a textbook example of over-engineering for a single metric (UX) at the expense of all other considerations (compilation speed, runtime cost, binary size, auditability, and maintainability). It is an 18kloc command-line parser with an additional 125kloc of dependencies that takes nearly 6 seconds to compile (‘only’ 400ms for an incremental build) and which adds nearly 690KiB to an optimised release binary (‘only’ 430KiB if you strip out most of the sugar that only clap provides).
There are many other command-line parsers to choose from that do all the key things that clap does, with half or less the build cost, and most of them with 30x less binary overhead[0]. argh is under 4kloc. gumdrop is under 2kloc. pico-args is under 700loc. What is the value of that extra 10kloc? A 10% better parser?
I am not saying there is no room for a library like clap—it is, at least, a triumphant clown car of features that can handle practically any edge-case anyone ever thought of—but if I got a nickel every time I spent 15 minutes replacing a trivial use of clap with pico-args and thus reduced the binary size and compile time of some project by at least 80%, I would have at least three nickels.
Just to try to pre-empt arguments like “disk space is cheap”, “compiler time is cheaper than human time”, etc.: there are no golden bullets in engineering, only trade-offs. Why would you default to the biggest, slowest option? This is the “every web site must be able to scale like Facebook” type logic. You don’t even have to do more work to use argh or gumdrop. If clap ends up having some magic feature that no other parser has that you absolutely need, you can switch, but I’ve yet to ever encounter such a thing. Its inertia and popularity carry it forward, but it is perhaps the last choice you should pick for a new project—not the first.
[0] https://github.com/rosetta-rs/argparse-rosetta-rs