Inlining a function with Clojure macros

Question

More out of curiousity that anything else (but with the expectation that it might occasionally be a useful trick for performance tuning), is it possible to use Clojure macros to "inline" an existing function?

i.e. I would like to be able to do something like:

(defn my-function [a b] (+ a b))

(defn add-3-numbers [a b c] 
  (inline (my-function 
    a 
    (inline (my-function 
      b 
      c)))))

And have it produce (at compile time) exactly the same function as if I had inlined the additions myself, such as:

(defn add-3-numbers [a b c] 
  (+ a (+ b c)))

Answer 1

In case you didn't know, you can define inlined functions using definline

(doc definline)
-------------------------
clojure.core/definline
([name & decl])
Macro
  Experimental - like defmacro, except defines a named function whose
  body is the expansion, calls to which may be expanded inline as if
  it were a macro. Cannot be used with variadic (&) args.
nil

Also checking the source,

(source definline)
-------------------------
(defmacro definline
  [name & decl]
  (let [[pre-args [args expr]] (split-with (comp not vector?) decl)]
    `(do
       (defn ~name ~@pre-args ~args ~(apply (eval (list `fn args expr)) args))
       (alter-meta! (var ~name) assoc :inline (fn ~name ~args ~expr))
       (var ~name))))

definline simply defines a var with meta-data {:inline (fn definition)}. So although its not exactly what you were asking but you can rebind the var with new metadata to get inlined behavior.