Be sparing when using unless

However, given the flexibility of Ruby’s syntax, it is easy to make code harder to understand than it needs to be.

Instead of…

…using unless in ways that makes your brain hurt.

# Example 1
unless something?
  # do something
else
  # do other thing
end

# Example 2
unless something? || another_thing?
  # do something
end

Use…

…if where it makes the code clearer.

# Example 1
if !something?
  # do other thing
else
  # do something
end

# Example 2
if !something? && !another_thing?
  # do something
end

But why?

Avoid using an else block with an unless. This structure is hard to reason about because the else block is a double negative: it is “not not” the specified condition. Instead, replace the unless with an if, taking care to refactor the condition statement as needed, then swap around the blocks of code in the main and else branches.

It is much harder to understand the code when using an unless conditional with boolean algebra (&& or ||). It is an easy way to confuse yourself! The conditional unless one && two is equivalent to if !one || !two. Although the logic looks messier the positive version is easier to reason about.

A complex if condition might well be an opportunity to extract the conditional into a well-named method so it can be more easily understood.

Why not?

I’m definitely not saying don’t use unless, but it’s important to remember it is sugar and thus important only to use it where it makes the resulting code clearer.

Translated into Japanese here