Macros

Author

Marie-Hélène Burle

Julia code is itself data and can be manipulated by the language while it is running.

Metaprogramming

Large influence from Lisp.
Since Julia is entirely written in Julia, it is particularly well suited for metaprogramming.

Let’s start with something simple:

2 + 3

How is this run internally?

The string "2 + 3" gets parsed into an expression:

Meta.parse("2 + 3")

:(2 + 3)

Then that expression gets evaluated:

eval(Meta.parse("2 + 3"))

They resemble functions and just like functions, they accept as input a tuple of arguments.

BUT macros return an expression which is compiled directly rather than requiring a runtime eval call.

So they execute before the rest of the code is run.

Macro’s names are preceded by @ (e.g. @time).

Julia comes with many macros and you can create your own with:

macro <name>()
    <body>
end

As with functions, Julia suggests to use lower case, without underscores, as macro names.