如何有效地捕获不同功能的相同异常？

Question

The following works in REPL, in case the database is down I get the map back.

(try
 (insert-table "db" "table" {:id 1 :text "text"}) 
 (catch Exception e {:err "can't connect to database"}))

I though I could write a function that takes a db operation and wrap it with a (try).

(defn catch-db-connection-errors
  [db_operation]
  (try
    (db_operation)
    (catch Exception e {:err "can't connect to database"})))

It does not catch the exception though. I might just overlook something very simple...

Answer 1

The problem is that the form you pass in as an argument (eg (insert-table "db" "table" {:id 1 :text "text"}) ) is evaluated before the function is called, and the resulting value is passed into the function. To use another example, if you write (println (+ 1 1)) , Clojure will first evaluate (+ 1 1) to get 2 , and then will call (println 2) . So if an exception is thrown in your DB code, it's before the catch-db-connection-errors function is called and thus outside the try form.

What you want is a macro, which thankfully is something Clojure is great at.

(defmacro catch-db-connection-errors [& db-operations]
  `(try
    ~@db-operations
    (catch Exception e {:err "can't connect to database"})))

This allows you to pass in however many database-handling forms you want and wraps them in your try-catch pair.

(Incidentally, you might want to catch something a little more specific than Exception, or else you could end up catching exceptions you don't intend do. But of course that's beside the point here.)