#+title: macros

Macros are a feature in certain [[file:20210307152738-programming_languages.org][programming languages]] that allow for editing
source code at compile or runtime. Similar to functions, macros are a means of
code reuse, but rather than rewrite functionality they rewrite code.

Macros first appeared, to my knowledge, in [[file:20200604222651-lisp.org][Lisp]].

In Lisp (specifically [[file:20200713103540-emacs_lisp.org][Emacs Lisp]]), a macro looks like this:

#+begin_src emacs-lisp
(defmacro ++ (var)
  "Incrementing operator like in C."
  (list 'setq var (list '+ 1 var)))

(let ((my-var 1))
  (++ my-var))
#+end_src

#+RESULTS:
: 2

The above, at runtime, is expanded in the following manner:

#+begin_src emacs-lisp :results value raw
(macroexpand '(++ foo))
#+end_src

#+RESULTS:
(setq foo (+ 1 foo))

Consider also the following example in [[file:20200705154112-rust.org][Rust]]:

#+begin_src rust :eval never
macro_rules! inc {
    ($name:ident) => {
        $name = $name + 1
    }
}

fn main () {
    let mut foo = 1;
    inc!(foo); // macros in rust end with exclamation points
    println!("foo: {}", foo); // println is also a macro
    // => 2
}

#+end_src

Macros allow for [[file:20210708161728-programming_language_extension.org][programming language extension]].