CustomizingHelp.md 9.9 KB
Customizing Help
Support your users (and yourself) by providing rich help for arguments, options, and flags.
Overview
You can provide help text when declaring any @Argument, @Option, or @Flag by passing a string literal as the help parameter:
struct Example: ParsableCommand {
@Flag(help: "Display extra information while processing.")
var verbose = false
@Option(help: "The number of extra lines to show.")
var extraLines = 0
@Argument(help: "The input file.")
var inputFile: String?
}
Users see these strings in the automatically-generated help screen, which is triggered by the -h or --help flags, by default:
% example --help
USAGE: example [--verbose] [--extra-lines <extra-lines>] <input-file>
ARGUMENTS:
<input-file> The input file.
OPTIONS:
--verbose Display extra information while processing.
--extra-lines <extra-lines>
The number of extra lines to show. (default: 0)
-h, --help Show help information.
Customizing Help for Arguments
For more control over the help text, pass an ArgumentHelp instance instead of a string literal. The ArgumentHelp type can include an abstract (which is what the string literal becomes), a discussion, a value name to use in the usage string, and a visibility level for that argument.
Here's the same command with some extra customization:
struct Example: ParsableCommand {
@Flag(help: "Display extra information while processing.")
var verbose = false
@Option(help: ArgumentHelp(
"The number of extra lines to show.",
valueName: "n"))
var extraLines = 0
@Argument(help: ArgumentHelp(
"The input file.",
discussion: "If no input file is provided, the tool reads from stdin.",
valueName: "file"))
var inputFile: String?
}
...and the help screen:
USAGE: example [--verbose] [--extra-lines <n>] [<file>]
ARGUMENTS:
<file> The input file.
If no input file is provided, the tool reads from stdin.
OPTIONS:
--verbose Display extra information while processing.
--extra-lines <n> The number of extra lines to show. (default: 0)
-h, --help Show help information.
Enumerating Possible Values
When an argument or option has a fixed set of possible values, listing these values in the help screen can simplify use of your tool. You can customize the displayed set of values for custom ExpressibleByArgument types by implementing ExpressibleByArgument/allValueStrings. Despite the name, ExpressibleByArgument/allValueStrings does not need to be an exhaustive list of possible values.
enum Fruit: String, ExpressibleByArgument {
case apple
case banana
case coconut
case dragonFruit = "dragon-fruit"
static var allValueStrings: [String] {
["apple", "banana", "coconut", "dragon-fruit"]
}
}
struct FruitStore: ParsableCommand {
@Argument(help: "The fruit to purchase")
var fruit: Fruit
@Option(help: "The number of fruit to purchase")
var quantity: Int = 1
}
The help screen includes the list of values in the description of the <fruit> argument:
USAGE: fruit-store <fruit> [--quantity <quantity>]
ARGUMENTS:
<fruit> The fruit to purchase (values: apple, banana,
coconut, dragon-fruit)
OPTIONS:
--quantity <quantity> The number of fruit to purchase (default: 1)
-h, --help Show help information.
Deriving Possible Values
ExpressibleByArgument types that conform to CaseIterable do not need to manually specify ExpressibleByArgument/allValueStrings. Instead, a list of possible values is derived from the type's cases, as in this updated example:
enum Fruit: String, CaseIterable, ExpressibleByArgument {
case apple
case banana
case coconut
case dragonFruit = "dragon-fruit"
}
struct FruitStore: ParsableCommand {
@Argument(help: "The fruit to purchase")
var fruit: Fruit
@Option(help: "The number of fruit to purchase")
var quantity: Int = 1
}
The help screen still contains all the possible values.
USAGE: fruit-store <fruit> [--quantity <quantity>]
ARGUMENTS:
<fruit> The fruit to purchase (values: apple, banana,
coconut, dragon-fruit)
OPTIONS:
--quantity <quantity> The number of fruit to purchase (default: 1)
-h, --help Show help information.
For an ExpressibleByArgument and CaseIterable type with many cases, you may still want to implement ExpressibleByArgument/allValueStrings to avoid an overly long list of values appearing in the help screen. For these types it is recommended to include the most common possible values.
Providing Descriptions for Enum Values
When your argument or option uses an enum type, you can provide detailed descriptions for each enum value that will appear in the help screen. This is especially useful when the enum cases represent complex concepts that benefit from explanation.
To provide descriptions for individual enum values, implement a custom defaultValueDescription property for each case. The ArgumentParser will automatically detect when descriptions differ from the enum's string representation and display them in an enumerated format.
enum OutputFormat: String, CaseIterable, ExpressibleByArgument {
case json
case yaml
case xml
case csv
var defaultValueDescription: String {
switch self {
case .json:
return "JavaScript Object Notation format"
case .yaml:
return "YAML Ain't Markup Language format"
case .xml:
return "eXtensible Markup Language format"
case .csv:
return "Comma-Separated Values format"
}
}
}
struct DataExporter: ParsableCommand {
@Option(help: "Select output format")
var format: OutputFormat = .json
}
This produces help output with detailed descriptions for each enum value:
USAGE: data-exporter [--format <format>]
OPTIONS:
--format <format> Select output format (default: json)
json - JavaScript Object Notation format
yaml - YAML Ain't Markup Language format
xml - eXtensible Markup Language format
csv - Comma-Separated Values format
-h, --help Show help information.
Controlling Argument Visibility
You can specify the visibility of any argument, option, or flag.
struct Example: ParsableCommand {
@Flag(help: ArgumentHelp("Show extra info.", visibility: .hidden))
var verbose: Bool = false
@Flag(help: ArgumentHelp("Use the legacy format.", visibility: .private))
var useLegacyFormat: Bool = false
}
The --verbose flag is only visible in the extended help screen. The --use-legacy-format stays hidden even in the extended help screen, due to its .private visibility.
% example --help
USAGE: example
OPTIONS:
-h, --help Show help information.
% example --help-hidden
USAGE: example [--verbose]
OPTIONS:
--verbose Show extra info.
-h, --help Show help information.
Alternatively, you can group multiple arguments, options, and flags together as part of a ParsableArguments type, and set the visibility when including them as an @OptionGroup property.
struct ExperimentalFlags: ParsableArguments {
@Flag(help: "Use the remote access token. (experimental)")
var experimentalUseRemoteAccessToken: Bool = false
@Flag(help: "Use advanced security. (experimental)")
var experimentalAdvancedSecurity: Bool = false
}
struct Example: ParsableCommand {
@OptionGroup(visibility: .hidden)
var flags: ExperimentalFlags
}
The members of ExperimentalFlags are only shown in the extended help screen:
% example --help
USAGE: example
OPTIONS:
-h, --help Show help information.
% example --help-hidden
USAGE: example [--experimental-use-remote-access-token] [--experimental-advanced-security]
OPTIONS:
--experimental-use-remote-access-token
Use the remote access token. (experimental)
--experimental-advanced-security
Use advanced security. (experimental)
-h, --help Show help information.
Grouping Arguments in the Help Screen
When you provide a title in an @OptionGroup declaration, that type's properties are grouped together under your title in the help screen. For example, this command bundles similar arguments together under a "Build Options" title:
struct BuildOptions: ParsableArguments {
@Option(help: "A setting to pass to the compiler.")
var compilerSetting: [String] = []
@Option(help: "A setting to pass to the linker.")
var linkerSetting: [String] = []
}
struct Example: ParsableCommand {
@Argument(help: "The input file to process.")
var inputFile: String
@Flag(help: "Show extra output.")
var verbose: Bool = false
@Option(help: "The path to a configuration file.")
var configFile: String?
@OptionGroup(title: "Build Options")
var buildOptions: BuildOptions
}
This grouping is reflected in the command's help screen:
% example --help
USAGE: example <input-file> [--verbose] [--config-file <config-file>] [--compiler-setting <compiler-setting> ...] [--linker-setting <linker-setting> ...]
ARGUMENTS:
<input-file> The input file to process.
BUILD OPTIONS:
--compiler-setting <compiler-setting>
A setting to pass to the compiler.
--linker-setting <linker-setting>
A setting to pass to the linker.
OPTIONS:
--verbose Show extra output.
--config-file <config-file>
The path to a configuration file.
-h, --help Show help information.