Surprises

surprises.clj

;; This is a collection of code that surprised me.

;; 
;; Example 1
;;
(let [counter (atom 0)
      coll [(swap! counter inc)]]
  [(take 1 coll) @counter])
;;=> [(1) 1]

(let [counter (atom 0)
      coll (lazy-cat [(swap! counter inc)])]
  [(take 1 coll) @counter])
;;=> [(1) 0]

;; Explanation -> `take` preserves laziness, so we aren't actually realizing anything
;; in `coll` until printing the output. The `deref` of `counter` happens before that.

;; 
;; Example 2
;;
(def lazy-boys
  (let [lazy-boy (lazy-seq (cons (throw (Exception. "oh no")) '()))]
    (repeatedly
      #(try
         (doall lazy-boy)
         (catch Exception e
           {:message (.getMessage e)})))))

(take 2 lazy-boys)
;;=> ({:message "oh no"} {:message "oh no"})

(def lazy-boyz
  (let [lazy-boy (lazy-cat [(throw (Exception. "oh no"))])]
    (repeatedly
      #(try
         (doall lazy-boy)
         (catch Exception e
           {:message (.getMessage e)})))))

(take 2 lazy-boyz)
;;=> ({:message "oh no"} ())

;; Explanation -> idk yet