Question

I’m trying to display the items in the menu-items variable using the for-macro:

(defrecord MenuItem
  [select-char description])

(def menu-items [(MenuItem. "1" "add an expense")
         (MenuItem. "2" "add an income")
         (MenuItem. "0" "exit")])

(defn display-menu [items]
  (for [item items]
    (println (:select-char item))))

(defn menu-prompt [items]
  (display-menu items)
  (read-val ">>>"))

(println menu-items)
(menu-prompt menu-items)

However, nothing but the >>> prompt is displayed. Could someone explain why that is, and how to display the items?

Answer 1

this is a case of “the lazy bug”

for produces a lazy sequence that is only evaluated as it is read.
the call to display-menu just returns a reference to the list and then
goes on it’s marry way having done nothing.

wrap it in a call to doall

user> (def a (for [x (range 10)] (println "doing work " x)))
#'user/a
user> a
(doing work  0
doing work  1
doing work  2
doing work  3
doing work  4
doing work  5
doing work  6
doing work  7
doing work  8
doing work  9
nil nil nil nil nil nil nil nil nil nil)

if you use doall or dorun then it will do the work immediately.

user> (dorun (for [x (range 10)] (println "doing work " x)))
doing work  0
doing work  1
doing work  2
doing work  3
doing work  4
doing work  5
doing work  6
doing work  7
doing work  8
doing work  9
nil
user> (doall (for [x (range 10)] (println "doing work " x)))
doing work  0
doing work  1
doing work  2
doing work  3
doing work  4
doing work  5
doing work  6
doing work  7
doing work  8
doing work  9
(nil nil nil nil nil nil nil nil nil nil)