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

• Onorio Catenacci: That's an interesting take on the pattern matching that I was looking for. Definitely seems a bit more idiomatic to me.
• Onorio Catenacci: By the way, I want the exists? function as a separate function--I would have done a defp if I didn't want to expose it outside of the module. So I'm not really calling :os.getenv() twice unnecessarily.
• Uri Agassi: @OnorioCatenacci - actually in this solution calling get without calling exists? will call :os.getenv() less than with calling exists?. No matter if env exists or not - :os.getenv() is called exactly once.

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