Defining macros

In previous chapters, we have already used macros, such as @printf, in Chapter 2, Variables, Types, and Operations, and @time in Chapter 3, Functions. Macros are like functions, but instead of values they take expressions (which can also be symbols or literals) as input arguments. When a macro is evaluated, the input expression is expanded, that is, the macro returns a modified expression. This expansion occurs at parse time when the syntax tree is being built, not when the code is actually executed.

 

 

The following descriptions highlight the difference between macros and functions when they are called or invoked:

• Function: It takes the input values and returns the computed values at runtime
• Macro: It takes the input expressions and returns the modified expressions at parse time

In other words, a macro is a custom program transformation. Macros are defined with the keyword as follows:

macro mname 
# code returning expression  
end 

It is invoked as @mname exp1 exp2 or @mname(exp1...