Logic Tests You Can Use on Collections

src/wonderland/chapter_2.clj

@@ -28,4 +28,48 @@
 (= :drinkme :drinkme)
 (= :drinkme 4)
 (= '(:drinkme :bottle) [:drinkme :bottle])
-(not= :drinkme :4)
+(not= :drinkme :4)
+
+; Logic Tests You Can Use on Collections
+(empty? [:table :door :key])
+(empty? [])
+(empty? {})
+(empty? '())
+
+(defn empty?
+  [coll] (not (seq coll)))
+
+(seq [1 2 3])
+(class [1 2 3])
+(class (seq [1 2 3]))
+(seq [])
+(empty? [])
+(seq [])
+
+; Remember, use seq to check for not empty instead of (not (empty? x)).
+; This is because nil is treated as logically false in tests,
+; whereas a non-nil value like [1 3 4] is treated as logically true.
+
+(every? odd? [1 3 5])
+(every? odd? [1 2 3 4 5])
+
+; A predicate is just a function that returns a value used in a logic test.
+
+(defn drinkable? [x]
+  (= x :drinkme))
+
+(every? drinkable? [:drinkme :drinkme])
+(every? drinkable? [:drinkme :poison])
+(every? (fn [x] (= x :drinkme)) [:drinkme :drinkme])
+(every? #(= % :drinkme) [:drinkme :drinkme])
+
+(not-any? #(= % :drinkme) [:drinkme :poison])
+(not-any? #(= % :drinkme) [:poison :poison])
+
+(some #(> % 3) [1 2 3 4 5])
+(#{1 2 3 4 5} 3)
+(some #{3} [1 2 3 4 5])
+(some #{4 5} [1 2 3 4 5])
+(some #{nil} [nil nil nil])
+(some #{false} [false false false])
+(some #{true} [true true true])