Marie-Hélène Burle

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


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

Parsing and evaluating

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>()

Stylistic conventions

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