Like `if`, but operates on the level of functions.

iffn.clj

(defmacro iffn
  "Like `if`, but operates on the level of functions. Returns a function which applies its arguments to `test-fn`.
If the return value of `test-fn` is truthy, `then-fn` is called with the same arguments to get the return value.
Otherwise, `else-fn` is used. In other words, it lets you rewrite this:

    #(if (pos? %) (inc %) (dec %))

as this:

    (iffn pos? inc dec)"
  [test-fn then-fn else-fn]
  `(fn [& args#]
    (if (apply ~test-fn args#)
      (apply ~then-fn args#)
      (apply ~else-fn args#))))

; some examples:
(let [magnify (iffn pos? inc dec)]
  [(magnify -2)
   (magnify 5)])
; [-3 6]

(let [toggle-membership (iffn contains? disj conj)]
  [(toggle-membership #{:a :b} :a)
   (toggle-membership #{:c :d} :z)])
; [#{:b} #{:z :c :d}]