Can't understand this clojure make-adder example

Question

I'm trying to read up a bit on Clojure, but I hit a brick wall with the following basic example:

(defn make-adder [x]
  (let [y x]
    (fn [z] (+ y z))))
(def add2 (make-adder 2))
(add2 4)
-> 6

What I don't understand is how is add2 passing the number 4 to the make-adder function, and how does that function turn assigns that number to z.

Thanks in advance!

Answer 1

make-adder returns a function that takes one parameter (z), the parameter passed in to make-adder is used to assign a value to y. add2 is set equal to the result of evaluating make-adder with a parameter of 2. So add2 is set equal to the function returned from make-adder, which (since y has been assigned to the parameter from make-adder) looks like

(fn [z] (+ 2 z))

So (add2 4) calls this function which evaluates to 6. Does that help?

Answer 2

To me, it seems you are working on an interesting problem.

Your example make-adder function (rewritten without let bindings)

(defn make-adder [x]
  "Returns a function that returns the sum of x and yet to be supplied z."
  (fn [z] (+ z x))))

returns a function that sums x and z, where make-adder has to have already been called with a value. I believe your example is implementing the Clojure partial function, which is pretty cool.

Here is your make-adder function rewritten to sum its two parameters, and not return a function (so we can use partial in add2),

(defn make-adder
  "Returns sum of x and y." 
  [x y] 
  (+ x y))

and here is add2 rewritten using partial with 2 as the x parameter:

(def add2 (partial make-adder 2))

If I call (add2 2) the answer is 4, (add2 3), the answer is 5 and so on.