In the previous module, we have introduce important notions about Boolean values. In this module, we will expand upon this knowledge in ways that will enable us to be more expressive with the code we write.

As part of the previous module, we introduced | and & as the or and and operands, respectively. There was a slight omission in this approach, as both of them have a closely related variant, || and &&.

The difference between | and || is particularly confusing, and is related to the fact that one is a short-circuit version of the other. One way to figure out what this means is to experiment with a construct where we will put a Boolean value first, then a Boolean operator, and finally something different, like for example "Hi!".

If you look at the documentation for | and ||, you will see that | is the bit-wise operator; in other words, it will need to be surrounded by Boolean values on both sides. The || is the short-circuit version of the operator, which has the very nice property that its evaluation stops as soon as it can determine the answer with certainty. We can exploit this.s

Let’s try a simple case first:

true || "Hi!"

true

This returns true because the left side is true and the operator is || (or). No matter what the right hand is, there is no need to evaluate it because true | anything is true. The operator serves as a sort of circuit breaker.

But what is the initial element is false? In this case, we need to know the right side, because false | anything is not something we can answer directly; therefore, Julia will have to evaluate the right hand side.

false || "Hi!"

"Hi!"

Interesting..! In this case, the output is not a Boolean, but a String: "Hi!". In the process of evaluating our code, we have managed to execute an operation.s

The short-circuit and works in the exact same way (well, the exact opposite, but still):

false && "Hi!"

false

This returns false directly because there is no right hand side value that could possibly change the result of an operation, and so && serves as a circuit breaker. But if we have a true statement next:

true && "Hi!"

"Hi!"

In this case, we get the right hand side executed!

This is the basis of a common (and very expressive) design pattern – in the same line, we can specify a check and an action. One of our favorite ways to use it is to check if a path exists, and if not create it:

ispath("foo") || mkdir("foo")

"foo"

Let’s be clean and remove this path, but only if it exists:s

ispath("foo") && rm("foo")

Additional uses of this patterns include throwing exceptions, exiting out of loops early, returning early, etc., and it will see a lot of use in this material (because it is concise and elegant).