CODEWITHSUNDEEP
functionclojure
Boris Zagoruiko
Boris Zagoruiko

Reputation: 13174

Wrong arity of simple function in clojure

I've started to learn clojure. In my book there is following exercise:

Write a function, mapset, that works like map except the return value is a set:

(mapset inc [1 1 2 2])
; => #{2 3}

I've started with something like this:

(defn mapset
  [vect]
  (set vect))

The result is error

"Wrong number of args (2) passed to: core/mapset"

I tried [& args] as well.

So, the question is: how can I solve such problem?

Upvotes: 1

Views: 754

Answers (2)

Sam Estep
Sam Estep

Reputation: 13354

Take a closer look at your call to mapset:

(mapset inc [1 1 2 2])

Since code is data, this "call" is just a list of three elements:

  1. The symbol mapset
  2. The symbol inc
  3. The vector [1 1 2 2]

When you evaluate this code, Clojure will see that it is a list and proceed to evaluate each of the items in that list (once it determines that it isn't a special form or macro), so it will then have a new list of three elements:

  1. The function to which the symbol core/mapset was bound
  2. The function to which the symbol clojure.core/inc was bound
  3. The vector [1 1 2 2]

Finally, Clojure will call the first element of the list with the rest of the elements as arguments. In this case, there are two arguments in the rest of the list, but in your function definition, you only accounted for one:

(defn mapset
  [vect]
  (set vect))

To remedy this, you could implement mapset as follows:

(defn mapset
  [f vect]
  (set (map f vect)))

Now, when you call (mapset inc [1 1 2 2]), the argument f will be found to the function clojure.core/inc, and the argument vect will be bound to the vector [1 1 2 2].

Upvotes: 4

Timothy Pratley
Timothy Pratley

Reputation: 10682

Your definition of mapset takes a single argument vect

At a minimum you need to take 2 arguments, a function and a sequence

(defn mapset [f xs] (set (map f xs)))`

But it is interesting to think about this as the composition of 2 functions also:

(def mapset (comp set map))

Upvotes: 4

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