Question

我想包装系统或用户抛出的异常（无关紧要）并强制它返回一些值。

我为它写了宏，但它不起作用。

宏：

(defmacro safe-fn
  [form]
  (try
    `(do ~form)
    (catch Throwable e
      1)))

用法：(safe-fn (throw (RuntimeException. "Try me!")))

实际输出：RuntimeException Try me! clojure-brave-and-true.core/eval2219 (form-init6122238559239237921.clj:1)

所需的输出：1

Answer 1

宏只是返回要评估的代码的函数，因此您可以像这样编写E/AndroidRuntime: FATAL EXCEPTION: main Process: com.plnech.example, PID: 14269 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setTag(java.lang.Object)' on a null object reference at com.plnech.library.databinding.HitsItemBinding.<init>(HitsItemBinding.java:31) at com.plnech.library.databinding.HitsItemBinding.bind(HitsItemBinding.java:118) at android.databinding.DataBinderMapper.getDataBinder(DataBinderMapper.java:11) at android.databinding.DataBindingUtil.bind(DataBindingUtil.java:185) at android.databinding.DataBindingUtil.inflate(DataBindingUtil.java:120) at android.databinding.DataBindingUtil.inflate(DataBindingUtil.java:88) at com.plnech.library.helper.databinding.ResultsAdapter.onCreateViewHolder(ResultsAdapter.java:36) at com.plnech.library.helper.databinding.ResultsAdapter.onCreateViewHolder(ResultsAdapter.java:21) at android.support.v7.widget.RecyclerView$Adapter.createViewHolder(RecyclerView.java:5482) at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:4707) at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:4617) at android.support.v7.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:1994) at android.support.v7.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1390) at android.support.v7.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1353) at android.support.v7.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:574) at android.support.v7.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:3028) at android.support.v7.widget.RecyclerView.onMeasure(RecyclerView.java:2625) at android.view.View.measure(View.java:18788) at android.widget.RelativeLayout.measureChild(RelativeLayout.java:676) at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:479) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.support.v7.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:391) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465) at android.widget.LinearLayout.measureVertical(LinearLayout.java:748) at android.widget.LinearLayout.onMeasure(LinearLayout.java:630) at android.view.View.measure(View.java:18788) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951) at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643) at android.view.View.measure(View.java:18788) at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2100) at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1216) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1452) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107) at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858) at android.view.Choreographer.doCallbacks(Choreographer.java:670) at android.view.Choreographer.doFrame(Choreographer.java:606) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:148)：

safe-fn

示例：

(defmacro safe-fn
  [form]
  `(try
     ~form
     (catch Throwable ~'_
       1)))

有关宏的详细信息，请参阅我对this question的回答，特别是使用它们来捕获异常。

Answer 2

来自the Tupelo library的宏with-exception-default完全符合您的要求：

异常情况下的默认值

有时你知道一个操作可能会导致异常，而你   希望将Exception转换为默认值。那   是你需要的时候：

(with-exception-default default-val & body)
  Evaluates body & returns its result.  In the event of an exception the
  specified default value is returned instead of the exception."

(with-exception-default 0
  (Long/parseLong "12xy3"))
;=> 0

在tupelo.parse中可以很好地使用此功能，您可以在其中找到功能如下：

(parse-long "123")                  ; throws if parse error
;=> 123
(parse-long "1xy23" :default 666)   ; returns default val if parse error
;=> 666

如何在Clojure宏中包装Exception？

2 个答案: