if, unless, and cond

One thing I dislike about most code books is they tell you about syntax rules, but not about how people actually write the code in the wild. I need you to write solid code here at Red:4, so let's take a second to talk about style.

If You Need "if"

Let's revisit our Solar module's power function. When I first wrote it, I did what most programmers would do:

defmodule Solar do
  def power(flare) do
    factor = 1
    if(flare.classification == :M) do
      factor = 10
    end
    if(flare.classification == :X) do
      factor = 1000
    end
    factor * flare.scale
  end
end

This code isn't bad, necessarily, but you'll notice the lack of else if here. Which is OK, as that's basically the same thing as a bunch of if statements.

The interesting thing about if (and unless) in Elixir is that they're not really a part of the language - they're macros:

An interesting note regarding if/2 and unless/2 is that they are implemented as macros in the language; they aren’t special language constructs as they would be in many languages. You can check the documentation and the source of if/2 in the Kernel module docs...

An if (or unless) statement is generally only used when you have a single condition you want to evaluate and they aren't used very often. In fact, I've written a number of Elixir libraries (some quite large) as well as a pretty large ecommerce application. I have, perhaps, two if statements (if that) in all of the Elixir code I've ever written.

You just don't need them when you have pattern matching and smaller, isolated functions.