爲什麼不在這裏生效？

Question

下面是代碼：

(def city-nodes (atom nil)) 
(def city-edges (atom nil)) 

(def player-pos (ref nil)) 
(def visited-nodes (ref #{})) 
(def player-status (atom :in-progress)) 

(def ^:dynamic *node-num* 8) 
(def ^:dynamic *edge-num* 5) 
(def ^:dynamic *worm-num* 3) 
(def ^:dynamic *cop-odds* 4) 

(defn filterb 
    [pred coll] 
    (let [rt (filter pred coll)] 
    (if (empty? rt) 
     nil 
     rt))) 

(defn all-nodes [] 
    (range 1 (inc *node-num*))) 

(all-nodes) 

(defn rand-node [] 
    (inc (rand-int *node-num*))) 

(defn edge-pair [x y] 
    (when-not (= x y) 
    [[x y] [y x]])) 

(defn sample-2diff-nodes [] 
    (let [[x y] [(rand-node) (rand-node)]] 
    (if (= x y) 
     (sample-2diff-nodes) 
     [x y]))) 

(edge-pair 1 1) 
(take 4 (repeatedly #(sample-2diff-nodes))) 

(def ^:dynamic t-edges (take 4 (repeatedly #(sample-2diff-nodes)))) 

(defn make-edge-vec [] 
    (->> (repeatedly #(sample-2diff-nodes)) 
     (take *edge-num*) 
     (apply concat) 
     set 
     vec)) 

(binding [t-edges (take 4 (repeatedly #(sample-2diff-nodes)))]) 
(prn t-edges) 
(binding [t-edges (make-edge-vec)]) 
(prn t-edges) 
(binding [t-edges (apply concat t-edges)]) 
(prn t-edges) 

結果：

(binding [t-edges (take 4 (repeatedly #(sample-2diff-nodes)))]) 
(prn t-edges) 
(binding [t-edges (make-edge-vec)]) 
(prn t-edges) 
(binding [t-edges (apply concat t-edges)]) 
(prn t-edges) 

([6 1] [1 2] [5 2] [3 1]) 
([6 1] [1 2] [5 2] [3 1]) 
([6 1] [1 2] [5 2] [3 1]) 
nil 

我最後PRN命令，我預期（6 1 1 2 5 2 3 1）。

Answer 1

結合呼叫的收)是放錯了地方：

(binding [t-edges (apply concat t-edges)]) 
(prn t-edges) 

應該是：

(binding [t-edges (apply concat t-edges)] 
    (prn t-edges)) 

雖然在這種情況下，你幾乎可以肯定要使用let表達，而不是