Arguably — Command Line Interface


An option can have an unlimited number of long-form aliases and single-character shortcuts: --option, -o.

Option values can be separated by either a space, --opt 123, or an equals symbol, --opt=123. Either syntax can be used with shortcuts: -o 123, -o=123.

Multiple shortcuts can be condensed into a single block, e.g. -abc foo bar. Trailing arguments are consumed in sequence as required by the options.

Options are registered with default values which are used if the option is not found. If an option is found multiple times its value is the final value encountered.

Multivalued Options

Options can be treated as singular or multivalued as circumstances require. Each option maintains an internal list to which newly parsed values are appended; the (singular) value of the option is the final value in the list or the default value if the list is empty.

For example, in the command below:

$ myapp --foo abc --foo def

the value of the option foo is "def" but the list ["abc", "def"] is also available for use if required.


Flags are valueless options — they're either present or absent, but take no arguments. Like options, flags can have an unlimited number of long-form aliases and single-character shortcuts: --flag, -f.

Positional Arguments

Options and flags can be preceded by, followed by, or interspaced with positional arguments which are assembled by the parser into a vector of strings.

Arguably supports the standard -- switch for turning off option-parsing. All arguments following a -- will be treated as positional arguments, even if they begin with a single or double dash.


Arguably supports git-style command interfaces with arbitrarily-nested commands. Commands have builtin support for an automatic --help flag and an automatic help <cmd> command, i.e. the commands

$ myapp <cmd> --help


$ myapp help <cmd>

are functionally identical and will both print the help text registered with the command.

Non-Unicode Arguments

To keep its API simple, this library only works with command line arguments which are valid unicode strings. If the parser finds an argument which is not a valid unicode string the .parse() method will return an Error::InvalidUnicode result.

Negative Numbers

Some argument-parsing libraries struggle with negative numbers — for example, they will try to parse -3 as a flag or option named 3. This library always treats arguments beginning with a dash and a digit as positional arguments or option values, never as flag or option names.