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

• Grajdeanu Alex: <joke>I find this piece of code offensive: def main([_|_] = args) do</joke>
• 200_success: The question looks a bit sketchy. What arguments do you expect, and what do they mean?
• G3z: args should be a list of command line arguments (es if you call cmd --foo bar, args will be ["foo", "bar"] if you call cmd args will be [])
• G3z: @Dex'ter my question is: how would you rewrite that?
• mdfst13: As stands, if this question were open, I think it would be closed as stub code from ` # do stuff with args`. There may not be much happening in your trivial application, but there is more than you show here.

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(_)).