clojure.lang.PersistentList cannot be cast to clojure.lang.IFn

Question

Hello as part of the 4Clojure problem set I am attempting to return a list of the first N fibonacci numbers. I have successfully implemented this using non-anonymous functions:

(defn nth-fibo [i]
  (cond
    (= i 1) 1
    (= i 2) 1
    :else (+ (nth-fibo (- i 1)) (nth-fibo (- i 2)))))

(defn fibos [i]
  (loop [x i]
    (when (> x 0)
      (cons (nth-fibo x) (fibos (- x 1))))))

(reverse (fibos 5))

However, I must provide a single callable function to the 4Clojure evaluator to pass the tests. My attempt to do so:

#(
  (letfn [
          (nth-fibo [i]
            (cond
              (= i 1) 1
              (= i 2) 1
              :else (+ (nth-fibo (- i 1)) (nth-fibo (- i 2)))))
          (fibos [i]
            (loop [x i]
              (when (> x 0)
                (cons (nth-fibo x) (fibos (- x 1))))))
          ]
    (reverse (fibos %))))

Leads to the error:

java.lang.ClassCastException: clojure.lang.PersistentList cannot be cast to clojure.lang.IFn

Taylor Wood Taylor Wood · Accepted Answer · 2018-01-14T02:57:53

There's an extra set of parentheses around your anonymous function that's causing the issue. See if this works:

#(letfn [(nth-fibo [i]
           (cond
             (= i 1) 1
             (= i 2) 1
             :else (+ (nth-fibo (- i 1)) (nth-fibo (- i 2)))))
         (fibos [i]
           (loop [x i]
             (when (> x 0)
               (cons (nth-fibo x) (fibos (- x 1))))))]
   (reverse (fibos %)))

A smaller reproduction of the same issue:

#(str %)
=> #object[playground.so$eval1751$fn__1752 0x718c0dd5 "playground.so$eval1751$fn__1752@718c0dd5"]
(*1 1)
=> "1"
#((str %))
=> #object[playground.so$eval1767$fn__1768 0x5375a819 "playground.so$eval1767$fn__1768@5375a819"]
(*1 1)
ClassCastException java.lang.String cannot be cast to clojure.lang.IFn  playground.so/eval1767/fn--1768 (so.clj:21)

clojure.lang.PersistentList cannot be cast to clojure.lang.IFn

1 Answers