Module clap::_faq

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§Documentation: FAQ

  1. Comparisons
    1. How does clap compare to structopt?
    2. What are some reasons to use clap? (The Pitch)
    3. What are some reasons not to use clap? (The Anti Pitch)
    4. Reasons to use clap
  2. How many approaches are there to create a parser?
  3. When should I use the builder vs derive APIs?
  4. Why is there a default subcommand of help?


First, let me say that these comparisons are highly subjective, and not meant in a critical or harsh manner. All the argument parsing libraries out there (to include clap) have their own strengths and weaknesses. Sometimes it just comes down to personal taste when all other factors are equal. When in doubt, try them all and pick one that you enjoy :). There’s plenty of room in the Rust community for multiple implementations!

For less detailed but more broad comparisons, see argparse-benchmarks.

§How does clap compare to structopt?

Simple! clap is structopt. structopt started as a derive API built on top of clap v2. With clap v3, we’ve forked structopt and integrated it directly into clap. structopt is in maintenance mode with the release of clap_derive.

The benefits of integrating structopt and clap are:

  • Easier cross-linking in documentation
  • Documentation parity
  • Tighter design feedback loop, ensuring all new features are designed with derives in mind and easier to change clap in response to structopt bugs.
  • Clearer endorsement of structopt

See also

§What are some reasons to use clap? (The Pitch)

clap is as fast, and as lightweight as possible while still giving all the features you’d expect from a modern argument parser. In fact, for the amount and type of features clap offers it remains about as fast as getopts. If you use clap, when you just need some simple arguments parsed, you’ll find it’s a walk in the park. clap also makes it possible to represent extremely complex and advanced requirements without too much thought. clap aims to be intuitive, easy to use, and fully capable for wide variety use cases and needs.

§What are some reasons not to use clap? (The Anti Pitch)

Depending on the style in which you choose to define the valid arguments, clap can be very verbose. clap also offers so many finetuning knobs and dials, that learning everything can seem overwhelming. I strive to keep the simple cases simple, but when turning all those custom dials it can get complex. clap is also opinionated about parsing. Even though so much can be tweaked and tuned with clap (and I’m adding more all the time), there are still certain features which clap implements in specific ways that may be contrary to some users’ use-cases.

§Reasons to use clap
  • You want all the nice CLI features your users may expect, yet you don’t want to implement them all yourself. You’d like to focus on your application, not argument parsing.
  • In addition to the point above, you don’t want to sacrifice performance to get all those nice features.
  • You have complex requirements/conflicts between your various valid args.
  • You want to use subcommands (although other libraries also support subcommands, they are not nearly as feature rich as those provided by clap).
  • You want some sort of custom validation built into the argument parsing process, instead of as part of your application (which allows for earlier failures, better error messages, more cohesive experience, etc.).

§How many approaches are there to create a parser?

The following APIs are supported:

Previously, we supported:

There are also experiments with other APIs:

  • fncmd: function attribute
  • clap-serde: create an Command from a deserializer

§When should I use the builder vs derive APIs?

Our default answer is to use the Derive API:

  • Easier to read, write, and modify
  • Easier to keep the argument declaration and reading of argument in sync
  • Easier to reuse, e.g. clap-verbosity-flag

The Builder API is a lower-level API that someone might want to use for

You can interop between Derive and Builder APIs.

§Why is there a default subcommand of help?

There is only a default subcommand of help when other subcommands have been defined manually. So it’s opt-in(ish), being that you only get a help subcommand if you’re actually using subcommands.

Also, if the user defined a help subcommand themselves, the auto-generated one wouldn’t be added (meaning it’s only generated if the user hasn’t defined one themselves).