Patten matching to check for non-empty argument list

Question

I'm learning Elixir. while building a trivial cli application (as seen here http://asquera.de/blog/2015-04-10/writing-a-commandline-app-in-elixir/) I'm defining a module that implements a main/1 function that accepts a List as an argument.

My question is: What is the best way to pattern match a method against a non-empty list ?

This is what i did and it seems to work but i was wondering if the elixir community has better suggestions ( maybe def main(args) when is_list(args) and length(args) > 0 do is considered better ? )

defmodule Cli do
    def main([]) do
        IO.puts "arguments are needed"
    end

    def main([_|_] = args) do
        options = parse_args(args)
        input = options[:name]
        size = options[:size]
        output(input, size)
    end

    def parse_args(args) do
        {options, _, _} = OptionParser.parse args,
            switches: [name: :string, size: :integer]
        options
    end

    def output() do
        IO.puts "Missing required --name parameter"
    end

    def output(input) do
        # defaulting size to 50
        output(input, 50)
    end

    def output(input, block_size) do
        IO.puts "you entered #{input} and #{block_size}"
    end
end

Answer 1

You've implemented it by pattern matching against empty lists with def main([]). If an empty list is passed to main, it will be caught here. In the second def main(args), args should never be an empty list.

If you want to make sure that args is a list, you could use a guard clause: def main(args) when is_list(args). (You should then write a catch-all third definition: def main(_)).