Hands-On Design Patterns and Best Practices with Julia

Example 1 – Making a new expression

Let's start with a simple one. Suppose that we want to create a macro called @squared that takes an expression and just squares it. In other words, if we run @squared(x), then it should be translated into x * x:

macro squared(ex)    return :($(ex) * $(ex))end

At first glance, it seems to work fine when we run it from the REPL:

But this macro has a problem with the execution context. The best way to illustrate the problem is by defining a function that uses the macro. So let's define a foo function, as follows:

function foo()    x = 2    return @squared xend

Now, when we call the function, we get the following error: ...