Fetching environment variables — can I use pattern matching?

Question

I've got the following Elixir code:

defmodule Envvar do
    def exists?(env) do :os.getenv(env) != false end

    def get(env) do 
        if exists?(env) do
            {env, :os.getenv(env)}
        else 
            {env, nil}
        end 
    end
end

Two things:

1. Am I using the ? correctly here? I'm assuming that any predicate function should have the ? on the end of the name by convention.
2. Is there a way to code the get function as a pattern match rather than a if/else? I'm asking here because I'm guessing that a pattern match would be more idiomatic Elixir code.

Answer 1

Using the ? for a boolean predicate is fine. At least it is what I would do in Ruby and Elixir.

You won't be able to call exists?(env) in a guard clause so pattern matching does not win you much here.

You are calling :os.getenv() twice unneccessarily. I'm not sure why you just want to convert 'false' to 'nil', here is a way:

defmodule Envvar do
  def get(env)
    do_get(:os.getenv(env))
  end

  defp do_get(false)
    {:env, nil}
  end

  defp do_get(result)
    {:env,result}
  end
end

Answer 2

Is it necessary to return nil or is false enough? They both evaluate to false when using boolean operators.

If false is good enough:

defmodule Envvar do
  def get(env) do
    {:env, :os.getenv(env)}
  end
end

If the return value has to be nil, I would just convert the second element and leave the first one intact.

defmodule Envvar do
  def get(env) do
    {:env, false_to_nil(:os.getenv(env)) }
  end

  defp false_to_nil(false) do
    nil
  end

  defp false_to_nil(v) do
    v
  end
end