== Tutorial

=== Why OptionParser?

When a Ruby program executes, it captures its command-line arguments
and options into variable ARGV.
This simple program just prints its \ARGV:

  :include: argv.rb

Execution, with arguments and options:

  $ ruby argv.rb foo --bar --baz bat bam
  ["foo", "--bar", "--baz", "bat", "bam"]

The executing program is responsible for parsing and handling
the command-line options.

OptionParser offers methods for parsing and handling those options.

With \OptionParser, you can define options so that for each option:

- The code that defines the option and code that handles that option
  are in the same place.
- The option may take no argument, a required argument, or an optional argument.
- The argument may be automatically converted to a specified class.
- The argument may be restricted to specified _forms_.
- The argument may be restricted to specified _values_.

The class also has:

- Method #summarize: returns a text summary of the options.
- Method #help: displays automatically-generated help text.

=== Defining Options

A common way to define an option in \OptionParser
is with instance method OptionParser#on.

The method may be called with any number of arguments
(whose order does not matter),
and may also have a trailing optional keyword argument +into+.

The given arguments determine the characteristics of the new option.
These may include:

- One or more short option names.
- One or more long option names.
- Whether the option takes no argument, an optional argument, or a required argument.
- Acceptable _forms_ for the argument.
- Acceptable _values_ for the argument.
- A proc or method to be called when the parser encounters the option.
- String descriptions for the option.

=== Option Names

You can give an option one or more names of two types:

- Short (1-character) name, beginning with one hyphen (<tt>-</tt>).
- Long (multi-character) name, beginning with two hyphens (<tt>--</tt>).

==== Short Option Names

A short option name consists of a hyphen and a single character.

File +short_names.rb+
defines an option with a short name, <tt>-x</tt>,
and an option with two short names (aliases, in effect) <tt>-y</tt> and <tt>-z</tt>.

  :include: short_names.rb

Executions:

  $ ruby short_names.rb -x
  ["x", true]
  $ ruby short_names.rb -1
  ["-1 or -%", true]
  $ ruby short_names.rb -%
  ["-1 or -%", true]

Multiple short names can "share" a hyphen:

  $ ruby short_names.rb -x1%
  ["x", true]
  ["-1 or -%", true]
  ["-1 or -%", true]

This is a good time to note that giving an undefined option raises an exception:

  $ ruby short_names.rb -z
  short_names.rb:9:in `<main>': invalid option: -z (OptionParser::InvalidOption)

==== Long Option Names

A long option name consists of two hyphens and a one or more characters
(usually two or more characters).

File +long_names.rb+
defines an option with a long name, <tt>--xxx</tt>,
and an option with two long names (aliases, in effect) <tt>--y1%</tt> and <tt>--z2#</tt>.

  :include: long_names.rb

Executions:

  $ ruby long_names.rb --xxx
  ["-xxx", true]
  $ ruby long_names.rb --y1%
  ["--y1% or --z2#", true]
  $ ruby long_names.rb --z2#
  ["--y1% or --z2#", true]

==== Mixing Option Names

Many developers like to mix short and long option names,
so that a short name is in effect an abbreviation of a long name.

File +mixed_names.rb+
defines options that each have both a short and a long name.

  :include: mixed_names.rb

Executions:

  $ ruby mixed_names.rb -x
  ["--xxx", true]
  $ ruby mixed_names.rb --xxx
  ["--xxx", true]
  $ ruby mixed_names.rb -y
  ["--y1%", true]
  $ ruby mixed_names.rb --y1%
  ["--y1%", true]

=== Option Arguments

An option may take no argument, a required argument, or an optional argument.

==== Option with No Argument

All the examples above define options with no argument.

==== Option with Required Argument

Specify a required argument for an option by adding a dummy word
to its name definition.

File +required_argument.rb+ defines two options;
each has a required argument because the name definition has a following dummy word.

  :include: required_argument.rb

When an option is found, the given argument is yielded.

Executions:

  $ ruby required_argument.rb -x AAA
  ["--xxx", "AAA"]
  $ ruby required_argument.rb -y BBB
  ["--yyy", "BBB"]

Omitting a required argument raises an error:

  $ ruby required_argument.rb -x
  required_argument.rb:9:in `<main>': missing argument: -x (OptionParser::MissingArgument)

==== Option with Optional Argument

Specify an optional argument for an option by adding a dummy word
enclosed in square brackets to its name definition.

File +optional_argument.rb+ defines two options;
each has an optional argument because the name definition has a following dummy word
in square brackets.

  :include: optional_argument.rb

When an option with an argument is found, the given argument yielded.

Executions:

  $ ruby optional_argument.rb -x AAA
  ["--xxx", "AAA"]
  $ ruby optional_argument.rb -y BBB
  ["--yyy", "BBB"]

Omitting an optional argument does not raise an error.