Elixir

Parsing command line arguments

Mar 29, 2016

Use OptionParser.parse to parse user-supplied command line arguments into a keyword list:

iex> OptionParser.parse(["--source-path", "lib", "test/enum_test.exs", "--verbose"])
{[source_path: "lib", verbose: true], ["test/enum_test.exs"], []}

The return value is a triple (a 3-size tuple) containing three lists:

  • a keyword list of flags/switches and their values
  • a list of other non-switch arguments
  • a keyword list of invalid flags and their values

You can pattern match the resulting tuple:

parsed = OptionParser.parse(["--source-path", "lib", "test/enum_test.exs", "--verbose"])
case parsed do
	{[verbose: true], [filepath], _} -> :do_verbose_thing
	{_, [filepath], _} -> :do_thing
	_ -> :help
end

Supply the strict parameter for enforcing allowed flags:

iex> OptionParser.parse(["--source-path", "lib", "test/enum_test.exs", "--verbose"], 
			strict: [source_path: :string])
{[source_path: "lib"], ["test/enum_test.exs"], [{"--verbose", nil}]}

Invalid flags such as verbose in the above example are returned in the third list of the OptionParser.parse result.

Elixir converts flags/switches to underscore atoms, so –source-path becomes :source_path, to better suit Elixir conventions. This means that option names on the command line cannot contain underscores; such options will be put in the invalid options list.

strict can also enforce argument types:

iex> OptionParser.parse(["--limit", "xyz"], strict: [limit: :integer])
{[], [], [{"--limit", "xyz"}]}

The above limit flag failed because we expected integer, but received string instead.

Additional reading: