Question

; common/math.clj
(defn nths
  "Returns a collection of values for each idx in idxs. Throws an error if any one idx is out of bounds."
  [coll idxs]
  (map #(nth coll %) idxs))

; nrepl
common.math> (try (/ 1 0)
     (catch Exception e (prn "in catch"))
     (finally (prn "in finally")))
"in catch"
"in finally"
nil
common.math> (try (nths '(5 6 7 8 9) '(0 5))
     (catch Exception e (prn "in catch"))
     (finally (prn "in finally")))
"in finally"
IndexOutOfBoundsException   clojure.lang.RT.nthFrom (RT.java:784)
common.math> (nths '(5 6 7 8 9) '(0 1 3))
(5 6 8)
common.math> *clojure-version*
{:major 1, :minor 5, :incremental 0, :qualifier "alpha4"}

我无法弄清楚第二个expr中出了什么问题。我原以为它会再打印出来：

"in catch"
"in finally"

运行unittest时会发生同样的事情：

lein test unittest.common.math

FAIL in (test-nths) (math.clj:87)
expected: (thrown? IndexOutOfBoundsException (nths (quote (5 6 7 8 9)) (quote (0 5))))
  actual: nil

应该通过。

Answer 1

Nths很懒，所以当你的repl尝试打印结果时，函数实际运行：

core> (def result (try (nths '(5 6 7 8 9) '(0 5))
                        (catch Exception e (prn "in catch"))
                        (finally (prn "in finally"))))
"in finally"
#'core/result
core> result
; Evaluation aborted.

您可以在nths中捕获异常，或者在使用它时更有意义捕获它

rsadl.core> (def result (try (nths '(5 6 7 8 9) '(0 5))
                        (catch Exception e (prn "in catch"))
                        (finally (prn "in finally"))))
"in finally"
#'core/result
core> (try (println result) (catch Exception e (prn "in catch")))
("in catch"
nil

或者作为number23_cn指出你可以在创建它时实现结果，只要你不需要因为其他原因而懒惰。

Answer 2

(try (doall (nths '(5 6 7 8 9) '(0 5)))
     (catch Exception e (prn "in catch"))
     (finally (prn "in finally")))
"in catch"
"in finally"
nil
user=>

因为map return lazy-seq？

不同类型的异常没有被抓住？

2 个答案: