Derive Reference

1. Overview
2. Attributes
   1. Terminology
   2. Command Attributes
   3. Arg Attributes
   4. Arg Enum Attributes
   5. Possible Value Attributes
3. Arg Types
4. Doc Comments
5. Tips
6. Mixing Builder and Derive APIS

Overview

To derive clap types, you need to enable the derive feature flag.

See demo.rs and demo.md for a brief example.

Let's start by breaking down the anatomy of the derive attributes:

use clap::{Parser, Args, Subcommand, ValueEnum};

/// Doc comment
#[derive(Parser)]
#[clap(APP ATTRIBUTE)]
struct Cli {
    /// Doc comment
    #[clap(ARG ATTRIBUTE)]
    field: UserType,

    #[clap(value_enum, ARG ATTRIBUTE...)]
    field: EnumValues,

    #[clap(flatten)]
    delegate: Struct,

    #[clap(subcommand)]
    command: Command,
}

/// Doc comment
#[derive(Args)]
#[clap(PARENT APP ATTRIBUTE)]
struct Struct { 
    /// Doc comment
    #[clap(ARG ATTRIBUTE)]
    field: UserType,
}

/// Doc comment
#[derive(Subcommand)]
#[clap(PARENT APP ATTRIBUTE)]
enum Command {
    /// Doc comment
    #[clap(APP ATTRIBUTE)]
    Variant1(Struct),

    /// Doc comment
    #[clap(APP ATTRIBUTE)]
    Variant2 {
        /// Doc comment
        #[clap(ARG ATTRIBUTE)]
        field: UserType,
    }
}

/// Doc comment
#[derive(ValueEnum)]
#[clap(ARG ENUM ATTRIBUTE)]
enum EnumValues {
    /// Doc comment
    #[clap(POSSIBLE VALUE ATTRIBUTE)]
    Variant1,
}

fn main() {
    let cli = Cli::parse();
}

• Parser parses arguments into a struct (arguments) or enum (subcommands).
• Args allows defining a set of re-usable arguments that get merged into their parent container.
• Subcommand defines available subcommands.
  • Subcommand arguments can be defined in a struct-variant or automatically flattened with a tuple-variant.
• ValueEnum allows parsing a value directly into an enum, erroring on unsupported values.
  • The derive doesn't work on enums that contain non-unit variants, unless they are skipped

See also the tutorial and examples.

Attributes

Terminology

Raw attributes are forwarded directly to the underlying clap builder. Any Command, Arg, or PossibleValue method can be used as an attribute.

Raw attributes come in two different syntaxes:

#[clap(
    global = true, // name = arg form, neat for one-arg methods
    required_if_eq("out", "file") // name(arg1, arg2, ...) form.
)]

• method = arg can only be used for methods which take only one argument.
• method(arg1, arg2) can be used with any method.

As long as method_name is not one of the magical methods - it will be translated into a mere method call.

Magic attributes have post-processing done to them, whether that is

• Providing of defaults
• Special behavior is triggered off of it

Magic attributes are more constrained in the syntax they support, usually just <attr> = <value> though some use <attr>(<value>) instead. See the specific magic attributes documentation for details. This allows users to access the raw behavior of an attribute via <attr>(<value>) syntax.

NOTE: Some attributes are inferred from Arg Types and Doc Comments. Explicit attributes take precedence over inferred attributes.

Command Attributes

These correspond to a clap::Command which is used for both top-level parsers and when defining subcommands.

Raw attributes: Any Command method can also be used as an attribute, see Terminology for syntax.

• e.g. #[clap(arg_required_else_help(true))] would translate to cmd.arg_required_else_help(true)

Magic attributes:

• name = <expr>: clap::Command::name
  • When not present: crate name (Parser container), variant name (Subcommand variant)
• version [= <expr>]: clap::Command::version
  • When not present: no version set
  • Without <expr>: defaults to crate version
• author [= <expr>]: clap::Command::author
  • When not present: no author set
  • Without <expr>: defaults to crate authors
• about [= <expr>]: clap::Command::about
  • When not present: Doc comment summary
  • Without <expr>: crate description (Parser container)
    • TIP: When a doc comment is also present, you most likely want to add #[clap(long_about = None)] to clear the doc comment so only about gets shown with both -h and --help.
• long_about = <expr>: clap::Command::long_about
  • When not present: Doc comment if there is a blank line, else nothing
• verbatim_doc_comment: Minimizes pre-processing when converting doc comments to about / long_about
• next_display_order: clap::Command::next_display_order
• next_help_heading: clap::Command::next_help_heading
  • When flattening Args, this is scoped to just the args in this struct and any struct flattened into it
• rename_all = <expr>: Override default field / variant name case conversion for Command::name / Arg::name
  • When not present: kebab-case
  • Available values: camelCase, kebab-case, PascalCase, SCREAMING_SNAKE_CASE, snake_case, lower, UPPER, verbatim
• rename_all_env = <expr>: Override default field name case conversion for env variables for clap::Arg::env
  • When not present: SCREAMING_SNAKE_CASE
  • Available values: camelCase, kebab-case, PascalCase, SCREAMING_SNAKE_CASE, snake_case, lower, UPPER, verbatim

And for Subcommand variants:

• skip: Ignore this variant
• flatten: Delegates to the variant for more subcommands (must implement Subcommand)
• subcommand: Nest subcommands under the current set of subcommands (must implement Subcommand)
• external_subcommand: clap::Command::allow_external_subcommand(true)
  • Variant must be either Variant(Vec<String>) or Variant(Vec<OsString>)

Arg Attributes

These correspond to a clap::Arg.

Raw attributes: Any Arg method can also be used as an attribute, see Terminology for syntax.

• e.g. #[clap(max_values(3))] would translate to arg.max_values(3)

Magic attributes:

• name = <expr>: clap::Arg::new
  • When not present: case-converted field name is used
• value_parser [= <expr>]: clap::Arg::value_parser
  • When not present: will auto-select an implementation based on the field type
  • To register a custom type's ValueParser, implement ValueParserFactory
  • When present, implies #[clap(action)]
• action [= <expr>]: clap::Arg::action
  • When not present: will auto-select an action based on the field type
  • When present, implies #[clap(value_parser)]
• help = <expr>: clap::Arg::help
  • When not present: Doc comment summary
• long_help = <expr>: clap::Arg::long_help
  • When not present: Doc comment if there is a blank line, else nothing
• verbatim_doc_comment: Minimizes pre-processing when converting doc comments to help / long_help
• short [= <char>]: clap::Arg::short
  • When not present: no short set
  • Without <char>: defaults to first character in the case-converted field name
• long [= <str>]: clap::Arg::long
  • When not present: no long set
  • Without <str>: defaults to the case-converted field name
• env [= <str>]: clap::Arg::env (needs env feature enabled)
  • When not present: no env set
  • Without <str>: defaults to the case-converted field name
• flatten: Delegates to the field for more arguments (must implement Args)
  • Only help_heading can be used with flatten. See clap-rs/clap#3269 for why arg attributes are not generally supported.
  • Tip: Though we do apply a flattened Args's Parent Command Attributes, this makes reuse harder. Generally prefer putting the cmd attributes on the Parser or on the flattened field.
• subcommand: Delegates definition of subcommands to the field (must implement Subcommand)
  • When Option<T>, the subcommand becomes optional
• from_global: Read a clap::Arg::global argument (raw attribute), regardless of what subcommand you are in
• parse(<kind> [= <function>]): clap::Arg::validator and clap::ArgMatches::values_of_t
  • Deprecated:
    • Use value_parser(...) for from_str, try_from_str, from_os_str, and try_from_os_str
    • Use action(ArgAction::Count for from_occurrences
    • Use action(ArgAction::SetTrue for from_flag
  • Default: try_from_str
  • Warning: for Path / OsString, be sure to use try_from_os_str
  • See Arg Types for more details
• value_enum: Parse the value using the ValueEnum trait
• skip [= <expr>]: Ignore this field, filling in with <expr>
  • Without <expr>: fills the field with Default::default()
• default_value = <str>: clap::Arg::default_value and clap::Arg::required(false)
• default_value_t [= <expr>]: clap::Arg::default_value and clap::Arg::required(false)
  • Requires std::fmt::Display or #[clap(value_enum)]
  • Without <expr>, relies on Default::default()
• default_value_os_t [= <expr>]: clap::Arg::default_value_os and clap::Arg::required(false)
  • Requires std::convert::Into<OsString> or #[clap(value_enum)]
  • Without <expr>, relies on Default::default()

Arg Enum Attributes

• rename_all = <expr>: Override default field / variant name case conversion for PossibleValue::new
  • When not present: kebab-case
  • Available values: camelCase, kebab-case, PascalCase, SCREAMING_SNAKE_CASE, snake_case, lower, UPPER, verbatim

Possible Value Attributes

These correspond to a clap::PossibleValue.

Raw attributes: Any PossibleValue method can also be used as an attribute, see Terminology for syntax.

• e.g. #[clap(alias("foo"))] would translate to pv.alias("foo")

Magic attributes:

• name = <expr>: clap::PossibleValue::new
  • When not present: case-converted field name is used
• help = <expr>: clap::PossibleValue::help
  • When not present: Doc comment summary

Arg Types

clap assumes some intent based on the type used:

Notes:

• For custom type behavior, you can override the implied attributes/settings and/or set additional ones
  • For example, see custom-bool
• Option<Vec<T>> will be None instead of vec![] if no arguments are provided.
  • This gives the user some flexibility in designing their argument, like with min_values(0)

You can then support your custom type with #[clap(parse(<kind> [= <function>]))]:

Notes:

• from_os_str:
  • Implies arg.takes_value(true).allow_invalid_utf8(true)
• try_from_os_str:
  • Implies arg.takes_value(true).allow_invalid_utf8(true)
• from_occurrences:
  • Implies arg.takes_value(false).multiple_occurrences(true)
  • Reads from clap::ArgMatches::occurrences_of rather than a get_one function
    • Note: operations on values, like default_value, are unlikely to do what you want
• from_flag
  • Implies arg.takes_value(false)
  • Reads from clap::ArgMatches::is_present rather than a get_one function
    • Note: operations on values, like default_value, are unlikely to do what you want

Warning:

• To support non-UTF8 paths, you should use #[clap(value_parser)] otherwise clap will parse it as a String which will fail on some paths.

Doc Comments

In clap, help messages for the whole binary can be specified via [Command::about] and [Command::long_about] while help messages for individual arguments can be specified via [Arg::help] and [Arg::long_help]".

long_* variants are used when user calls the program with --help and “short” variants are used with -h flag.

# use clap::Parser;

#[derive(Parser)]
#[clap(about = "I am a program and I work, just pass `-h`", long_about = None)]
struct Foo {
    #[clap(short, help = "Pass `-h` and you'll see me!")]
    bar: String,
}

For convenience, doc comments can be used instead of raw methods (this example works exactly like the one above):

# use clap::Parser;

#[derive(Parser)]
/// I am a program and I work, just pass `-h`
struct Foo {
    /// Pass `-h` and you'll see me!
    bar: String,
}

NOTE: Attributes have priority over doc comments!

Top level doc comments always generate Command::about/long_about calls! If you really want to use the Command::about/long_about methods (you likely don't), use the about / long_about attributes to override the calls generated from the doc comment. To clear long_about, you can use #[clap(long_about = None)].

TIP: Set #![deny(missing_docs)] to catch missing --help documentation at compile time.

Pre-processing

# use clap::Parser;
#[derive(Parser)]
/// Hi there, I'm Robo!
///
/// I like beeping, stumbling, eating your electricity,
/// and making records of you singing in a shower.
/// Pay up, or I'll upload it to youtube!
struct Robo {
    /// Call my brother SkyNet.
    ///
    /// I am artificial superintelligence. I won't rest
    /// until I'll have destroyed humanity. Enjoy your
    /// pathetic existence, you mere mortals.
    #[clap(long, action)]
    kill_all_humans: bool,
}

A doc comment consists of three parts:

• Short summary
• A blank line (whitespace only)
• Detailed description, all the rest

The summary corresponds with Command::about / Arg::help. When a blank line is present, the whole doc comment will be passed to Command::long_about / Arg::long_help. Or in other words, a doc may result in just a Command::about / Arg::help or Command::about / Arg::help and Command::long_about / Arg::long_help

In addition, when verbatim_doc_comment is not present, clap applies some preprocessing, including:

• Strip leading and trailing whitespace from every line, if present.

• Strip leading and trailing blank lines, if present.

• Interpret each group of non-empty lines as a word-wrapped paragraph.

  We replace newlines within paragraphs with spaces to allow the output to be re-wrapped to the terminal width.

• Strip any excess blank lines so that there is exactly one per paragraph break.

• If the first paragraph ends in exactly one period, remove the trailing period (i.e. strip trailing periods but not trailing ellipses).

Sometimes you don't want this preprocessing to apply, for example the comment contains some ASCII art or markdown tables, you would need to preserve LFs along with blank lines and the leading/trailing whitespace. When you pass use the verbatim_doc_comment magic attribute, you preserve them.

Note: Keep in mind that verbatim_doc_comment will still

• Remove one leading space from each line, even if this attribute is present, to allow for a space between /// and the content.
• Remove leading and trailing blank lines

Tips

• To get access to a Command call CommandFactory::command (implemented when deriving Parser)
• Proactively check for bad Command configurations by calling Command::debug_assert in a test (example)

Mixing Builder and Derive APIs

The builder and derive APIs do not live in isolation. They can work together, which is especially helpful if some arguments can be specified at compile-time while others must be specified at runtime.

Using derived arguments in a builder application

Jump to source

When using the derive API, you can #[clap(flatten)] a struct deriving Args into a struct deriving Args or Parser. This example shows how you can augment a Command instance created using the builder API with Args created using the derive API.

It uses the Args::augment_args method to add the arguments to the Command instance.

Crates such as clap-verbosity-flag provide structs that implement Args or Parser. Without the technique shown in this example, it would not be possible to use such crates with the builder API. augment_args to the rescue!

Using derived subcommands in a builder application

Jump to source

When using the derive API, you can use #[clap(subcommand)] inside the struct to add subcommands. The type of the field is usually an enum that derived Parser. However, you can also add the subcommands in that enum to a Command instance created with the builder API.

It uses the Subcommand::augment_subcommands method to add the subcommands to the Command instance.

Adding hand-implemented subcommands to a derived application

Jump to source

When using the derive API, you can use #[clap(subcommand)] inside the struct to add subcommands. The type of the field is usually an enum that derived Parser. However, you can also implement the Subcommand trait manually on this enum (or any other type) and it can still be used inside the struct created with the derive API. The implementation of the Subcommand trait will use the builder API to add the subcommands to the Command instance created behind the scenes for you by the derive API.

Notice how in the previous example we used augment_subcommands on an enum that derived Parser, whereas now we implement augment_subcommands ourselves, but the derive API calls it automatically since we used the #[clap(subcommand)] attribute.

Flattening hand-implemented args into a derived application

Jump to source

When using the derive API, you can use #[clap(flatten)] inside the struct to add arguments as if they were added directly to the containing struct. The type of the field is usually an struct that derived Args. However, you can also implement the Args trait manually on this struct (or any other type) and it can still be used inside the struct created with the derive API. The implementation of the Args trait will use the builder API to add the arguments to the Command instance created behind the scenes for you by the derive API.

Notice how in the example 1 we used augment_args on the struct that derived Parser, whereas now we implement augment_args ourselves, but the derive API calls it automatically since we used the #[clap(flatten)] attribute.