Question

Why does this function not work as expected?

(defn my-juxt
  [& fns]
  (if (= 1 (count fns))
    (fn [& a] (list (apply (first fns) a)))
    (fn [& a]
      (cons (apply (first fns) a) ((my-juxt (rest fns)) a)))))

Note: This works –

(defn new-juxt
  [& fns]
  (fn [& a]
    (map #(apply % a) fns)))

Answer 1

The problem is in how varargs are used. my-juxt has params [& fns] while it’s given [fns] in the last string. The same is with the function it returns as a result: it expects [& a] while provided [a].

The code below will work (please note two extra apply‘s there)

(defn my-juxt
  [& fns]
  (if (= 1 (count fns))
    (fn [& a] (list (apply (first fns) a)))
    (fn [& a]
      (cons (apply (first fns) a) 
            (apply (apply my-juxt (rest fns)) a)))))