Clojure PN (java.lang.ClassCastException)

Question

I'm using Clojure for a code golf challenge, in which the aim is to write a function that evaluates and returns the value from a Polish Notation expression. Below is my code.

(require '[clojure.string :as s])
(defn p[e](
    (load-string (reduce (fn[x c] 
        (s/replace x #"([\+\-\*\/] (\(.*\) \d+|\d+ \d+))" "($0)")
    ) e (subvec (s/split e #"\d") 1)))))

Remember this is golfed code (not meant to be readable).

In short, the function takes a string, the string is then split into a vector. Reduce iterates over the vector and applies the expression for the n-1 elements in the vector. What comes out is a string that Clojure should be able to evaluate with load-string.

If I declare a var called e with the value "+ 3 5" then run just the reduce from the function, "(+ 3 5)" is expected as returned. But when I call the function as (p "+ 3 5") I get the following error.

java.lang.ClassCastException: java.lang.String cannot be cast to clojure.lang.IFn (NO_SOURCE_FILE:0)

No idea what's going on here.

Actually what I get is: ClassCastException java.lang.Long cannot be cast to clojure.lang.IFn — Chiron
– Chiron, Commented Sep 1, 2013 at 22:44
1. I think I'd choose Forth for this challenge -- done. 2. Are you forgetting the reverse in RPN? 3. (defn rpn [& e](reduce #(if (fn? %2)(let [[l r & m]%](cons (%2 r l) m))(cons %2 %))[]e)) @learnclojure — A. Webb
– A. Webb, Commented Sep 2, 2013 at 2:40
@A.Webb 1. I shall look into Forth. 2. Sorry, I meant PN. It's been a while since I learned about it. Got the two slightly muddled in my head. 3. Tasty! I'm going to pick that apart. Thanks. — Dan Prince
– Dan Prince, Commented Sep 2, 2013 at 3:54

Chiron · Accepted Answer · 2013-09-01 22:46:54Z

1

You are using extra parentheses after defn args vector. It should be:

(defn r[e]
  (load-string 
    (reduce 
      (fn[x c] 
        (s/replace x #"([\+\-\*\/] (\(.*\) \d+|\d+ \d+))" "($0)")) 
      e 
      (subvec (s/split e #"\d") 1))))

user=> (r "+ 3 5")
8

answered Sep 1, 2013 at 22:46

Chiron

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