Exceptions¶

An error in a form arrives as an exception, and try decides which ones you catch. Errors is the message you see when nothing does.

catch names a class, and there are three:

(try
  (throw (ex-info "arm too far" {:angle 95}))
  (catch ExceptionInfo e (ex-data e))      ; => {:angle 95.0}
  (catch Exception e (ex-message e)))      ; any other error

The first matching catch wins, so ExceptionInfo comes before Exception: Exception would catch the ex-info error too.

ex-info takes a message and a map, ex-message returns the message, and ex-data returns the map:

user=> (try (throw (ex-info "boom" {:code 7}))
         (catch ExceptionInfo e [(ex-message e) (ex-data e)]))
["boom" {:code 7.0}]

finally¶

finally runs whichever way the body leaves: with a value, with a caught error, or with one that nothing catches:

user=> (try (throw (ex-info "boom" {:code 7})) (finally (println "cleanup")))
cleanup
Execution error: boom

Nothing cuts a finally short, so it is the place to undo what the body did. In a loop it runs when the loop stops, by (control/stop), by STOP or by an error; see Control Loops.

recur and try¶

recur cannot be wrapped in a try, so a loop that catches must keep the try inside one round:

user=> (loop [i 0] (when (< i 3) (try (println i)) (recur (inc i))))
0.0
1.0
2.0
nil

The other way round is a syntax error:

user=> (loop [] (try (recur)))
Syntax error: Can only recur from tail position