Macros have some clear differences from functions, but there is one more subtle difference that isn’t quite obvious at first.

I read some more on Mastering Clojure Macros, and it pointed out something very important about macros: Macros are not values.

In Clojure, you can treat functions equally with other values types: numbers, strings, collections, etc. But you cannot always treat macros as functions.

Filtering Numbers

I want a function that will only take numbers divisible by 5. One way I might implement this is to create a function that checks for 5’s divisibility, then pass that function into a filter.

user=> (defn divisible-by-5? [n] (= 0 (rem n 5)))
#'user/divisible-by-5?

user=> (filter divisible-by-5? '(1 5 2 7 10 4 15 19))
(5 10 15)

Now what if I decided to use a macro for the divisibility function instead? How would that work? Let’s try it out.

user=> (defmacro divisible-by-5? [n] `(= 0 (rem ~n 5)))
#'user/divisible-by-5?

user=> (filter divisible-by-5? '(1 5 2 7 10 4 15 19))
Syntax error compiling at (REPL:1:1).
Can't take value of a macro: #'user/divisible-by-5?

What happened here? “Can’t take value of a macro”? Hold on… maybe I missed something. Let me try something simpler.

(defn do-nothing [_])
#'user/do-nothing

(defmacro nothing-macro [_])
#'user/nothing-macro

user=> (do-nothing nothing-macro)
Syntax error compiling at (REPL:1:1).
Can't take value of a macro: #'user/nothing-macro

user=> (do-nothing ->)
Syntax error compiling at (REPL:1:1).
Can't take value of a macro: #'clojure.core/->

(> functions macros)

It seems that no matter what we try, we cannot pass macros to other functions. This isn’t exclusive to function parameters, however. Macros cannot be used as any data value.

user=> [->]
Syntax error compiling at (REPL:0:0).
Can't take value of a macro: #'clojure.core/->

user=> (defn get-thread-first [] ->)
Syntax error compiling at (REPL:1:1).
Can't take value of a macro: #'clojure.core/->

Functions, on the other hand, can be used just about anywhere.

user=> [do-nothing]
[#object[user$do_nothing 0x78ed7141 "user$do_nothing@78ed7141"]]

user=> (defn get-my-func [] do-nothing)
#'user/get-my-func

In Mastering Clojure Macros, the author stresses the use of functions in place of macros, whenever possible. This is because functions are simpler, easier to debug, and there are fewer surprises to people who use the function.

Imagine if you wanted to map a plus-ten function over a collection, and it turned out to be a macro! You’d have to rewrite the macro as a function, and then hope everyone’s code still works. This is all after you’ve spent time looking for the problem in your map.

Macros aren’t all bad though. There are plenty of cases where macros are the best tool for the job.