Clapper 👏

Yes there is clap, but this is clapper (pun intended).

A more complete command line parser with optionals, tagged-defaults and auto-help.

Differences to clap

A lot. Like pointer receivers (optionals), defaults, auto-help, etc. I know that clap originally deals with defaults by assigning them inside the struct on init. It did not like it. However, I liked clap as a command line parser but was missing some of these features and the pre-defined order so I always struggled with it.

Maturity

NOTE with version 1.0.0 this package must be included like

import "github.com/mittwald/clapper"

the pkg/clapper-Path has been removed.

Expectations could be assured from the argument-parser- and interpreter tests which are hopefully exhaustive enough.

If you find a bug feel free to add a test and file a PR.

Usage

Define your clapper-tag like this:

type Foo struct {
    // evaluates --some-defaulted-value - or -s. If not given, default applies.
    SomeDefaultedValue int     `clapper:"short=s,long,default=42,help='Set this to have some value here instead of 42'"`
    // evaluates only --optional-value as `short` is not given. If not given, the value is nil.
    OptionalValue      *string `clapper:"long"`
    // this values has to be given or the `Parse()` will fail.
    MandatoryValue     string  `clapper:"long`
    // bool value will be `false` if not given, or `true` if -F is given.
    FlagValueOptional  bool    `clapper:"short`
    // unevaluated value as there is no `clapper`-Tag.
    IgnoredValue       bool
}

// Parse all os.Args and try to set then to `foo`
var foo Foo
trailing, err := clapper.Parse(&foo)

the inner order of the tag does not matter at all. The given example gives command line options -s and --some-value, defaulting to value 42 if not given. OptionalValue is optional. FlagValueOptional will become true if -F is given, false if not.

Changes

1.1.0

Added command tag to support a struct embedded command from the command line. Usage:

type Foo struct {
    Help    bool   `clapper:"long"`
    Command string `clapper:"command,help=show|hide"`
}

Given an invocation like someProgram foo bar, foo will be assigned to Command in the Foo-struct.

command-tags can have any supported type. This means if it has a slice type, all given trailing inputs will be assigned to the command-tag-value. If you have a single type, only the first trailing input will be assigned and the rest still remains as trailing from the Parse() invocation.

If a required command is not given, the resulting error will respect any help inside the tag and enrich the error message accordingly.

Guarantees and rules

• If no arguments than the target have been given to Parse(), the os.Arguemnts will be taken.
• All value properties except bool are mandatory unless a default ist given.
• All pointer properties are optional. default applies.
• A bool proerty not given will remain untouched.
• At least short or long name must be provided.
• • If both are given, the long-name provided value has higher priority. --some 1 -s 2 -> 1.
• If a flag is provided several times with different values and the property is not a slice, only the first value will be taken. --foo 1 --foo 2 -> foo=1
• If a flag is provided several times and the property is a slice, the values are appended in the given order. --foo 1 --foo 2 -> foo=[1,2]
• Slice properties can also be filled like --foo a b c -d. -> foo=[a,b,c]
• Short flags -s -a -d can be combined as -sad and will be interpreted as -s -a -d.
• If combined short-flags are provided with a value -sad 123, the value will be bound to the last short-flag. d=123
• Unknown but given flags are silently discared.
• Short flags are always -[char] - one dash, one character
• Long flags have to be always --[some-string] two dahes, longer than 1 char.
• Boolean properties can be set to true if the flag is given by command line. -f.
• • Values for bools are not accepted. Defined means true.
• Command line input like --foo=bar or --foo bar are interpreted as the same.
• If the last command line parameters are assigned to a slice --foo a b c then all these parameters will be appended to the slice. There are no trailing parameters then.
• • In opposite if there is a slice --foo a b c --bar baz 1 2 3, then the trailing parameters 1 2 3 will be returned from the Parse.
• Only one command-tag can be defined. If it is defined more than once, the Parse() will fail.
• If a command-tag is defined, the input is mandatory.
• A single command-tag target will be set with the first trailing argument.
• A slice type command-tag will be set to all trailing arguments.

Tag-Options

short

Given only as short, assumes that the first lowercased-letter of the property is taken as the short parameter. For exmaple Something becomes -s.

someprogram -s "hello world"

To override the default assumption, you can write short=X where X would be the overriden short parameter.

someprogram -X "hello world"

long

Given as long, assumes that the Kebab-Case-converted name of the property is the paramter name. For example FooBar becomes --foo-bar

someprogram --foo-bar "Hello World"

Also here, the name assumption can be overriden by paramter. long=not-so-foo

someprogram --not-so-foo "Hello World"

default

The default value taken if the parameter is not provided via command line. Set this to have all mandatory flags to be optional with this default.

If default is defined for a pointer-value, the default will also be applied then.

help

Clapper has a auto-help feature and this optional tag-option can be set to let your users have some extra idea of the meaning of your flag.

Exmaple:

type Foo struct {
    WeirdParameter int `clappper:"short,help='Set to the value you whish to have on your bank account.'"`
}

help := clapper.HelpDefault()
fmt.Println(help)

Call clapper.Help() to print out the avaiable options with their help text.

FYI cllapper HelpDefault() will use the originally designed help format and prints out the parameters only. You may want to have a

type Foo struct {
    // ...
    ShowHelp bool `clapper:"long=help"`
}

in order to check that and display the help. But it is up to you.

command

Up from version 1.1.0 clapper supports a command-tag which will be filled with the trailing arguments given. Only one field with command can be specified.

If the struct field type is a slice, all trailing arguments will be assigned to it. If the type is a single value property, only the first trailing argument will be assigned and the rest will still be returned as trailing arguments from the Parse() function as before.

If a command is specified, it becomes mandatory and the Parse() will fail if no command was found (i.e. no trailing arguments found). If such a required command is not given, the resulting error will respect any optionally given help inside the tag and enrich the error message accordingly.

Example:

type Foo struct {
    Help    bool   `clapper:"long"`
    Command string `clapper:"command,help=show|hide"`
}

Trailing?

Clapper works different from clap and does not include trailing as a struct property. Trailing parameters are returned from the Parse() command. It is up to you to do whatever you like with them. Please also see the command-tag above.