异常处理

Clojure代码里面抛出来的异常都是运行时异常。当然从Clojure代码里面调用的java代码还是可能抛出那种需要检查的异常的。 try , catch , finally 以及 throw 提供了和java里面类似的功能:

(defn collection? [obj]
  (println "obj is a" (class obj))
  ; Clojure collections implement clojure.lang.IPersistentCollection.
  (or (coll? obj) ; Clojure collection?
      (instance? java.util.Collection obj))) ; Java collection?

(defn average [coll]
  (when-not (collection? coll)
    (throw (IllegalArgumentException. "expected a collection")))
  (when (empty? coll)
    (throw (IllegalArgumentException. "collection is empty")))
  ; Apply the + function to all the items in coll,
  ; then divide by the number of items in it.
  (let [sum (apply + coll)]
    (/ sum (count coll))))

(try
  (println "list average =" (average '(2 3))) ; result is a clojure.lang.Ratio object
  (println "vector average =" (average [2 3])) ; same
  (println "set average =" (average #{2 3})) ; same
  (let [al (java.util.ArrayList.)]
    (doto al (.add 2) (.add 3))
    (println "ArrayList average =" (average al))) ; same
  (println "string average =" (average "1 2 3 4")) ; illegal argument
  (catch IllegalArgumentException e
    (println e)
    ;(.printStackTrace e) ; if a stack trace is desired
  )
  (finally
    (println "in finally")))

上面代码的输出是这样的：

obj is a clojure.lang.PersistentList
list average = 5/2
obj is a clojure.lang.LazilyPersistentVector
vector average = 5/2
obj is a clojure.lang.PersistentHashSet
set average = 5/2
obj is a java.util.ArrayList
ArrayList average = 5/2
obj is a java.lang.String
#<IllegalArgumentException java.lang.IllegalArgumentException:
expected a collection>
in finally