if and unless

These two constructs can be used with the following syntax:

if <expression> do
  # expression was truthy
else
  # expression was falsy
end

unless <expression> do
  # expression was falsy
else
  # expression was truthy
end

As with the def construct, they can be inlined. For if, you'd do this:

if <expression>, do: # expression was truthy, else: # expression was falsy 

For both constructs, the else clause is optional. They will return nil if the main clause doesn't match and no else clause was provided.