Article originally posted here : https://bootrails.com/blog/ruby-unless
You are probably familiar with the "if else
" control flow but there are certain situations where there may be a visually better way to write these statements. Here we will discuss how you can achieve this via the unless
keyword in Ruby and the different ways you can use it.
Unless you already know...
When we use an if
statement we want to check whether a condition is true or not. If it is true, you execute a block of code. If it is false, the code does not get executed. Simply put if
checks for a truthy value. Like many other languages.
You can think of unless
as the exact opposite of this. unless
can replace the if
keyword but it will check for a falsy value and execute the code if the condition it is checking is false.
Syntactically this is shown below:
puts "Passed" if score > 50
if score > 50
puts "Passed"
end
Becomes
puts "Passed" unless score < 50
unless score < 50
puts "Passed"
end
As you can see the only difference is that you now need to change the condition. So unless
is just the exact opposite of if
.
You might be wondering why Ruby gives you a keyword for this specifically. Well writing your control flow this way will improve the visual appeal of the code as you will no longer need to flip your conditions around using the not
operator in more complex conditions.
Samples:
# okay, understandable, but your eye may miss the "!"
do_something if !some_condition
# okay, readable, understandable, but "not" is not always liked by Ruby developers
do_something if not some_condition
# okay, readable, understandable
do_something unless some_condition
As a modifier
You may have noticed in the last example that unless
can also be used as a modifier similar to if
.
In this sample:
unless score < 50
puts "Passed"
end
Becomes
puts "Passed" unless score < 50
The left-hand side behaves as a then condition (code that will be executed) and the right-hand side behaves as a test condition.
Notes
Even though it is completely possible to use else
in the unless
statement it is suggested in best practices that you should avoid doing this and prefer to use the if
keyword when you need to have an else condition. This is because using else
with unless
can create confusing control flows to read. For example:
# works, but not advised
unless passed?
puts 'failed'
else
puts 'passed'
end
Personal thoughts
At the beginning, unless
is clearly counter-intuitive. It took me some time before to use it daily without additional headaches. This keyword however is a good example of what Ruby is trying to achieve : increase readability.
Top comments (1)
Nitpick: the negation of
score > 50
is actuallyscore <= 50
.