#100DaysOfClojure (Day [14] flow-control-cont-3)

Today I will be looking at exception handling. Clojure provides the normal try-catch-finally block to catch exceptions and has the following general syntax

(try

   (code)

   catch Exception e1)

(Exception Handler)

(finally

   Cleanup code)

Here is an example

(try

  (/ 2 0)

  (catch ArithmeticException e

    "divide by zero")

  (finally

    (println "cleanup code")))

cleanup code

=> "divide by zero"

You can throw an exception in the following manner

(try

  (throw (ex-info "There was a problem" {:causes "example"}))

  (catch Exception e

    (println ex-data e)))

#object[clojure.core$ex_data 0x5f1df0de clojure.core$ex_data@5f1df0de] #error {

:cause There was a problem

:data {:causes example}

:via

[{:type clojure.lang.ExceptionInfo

   :message There was a problem

   :data {:causes example}

   :at [leinproj.core$eval1739 invokeStatic form-init10068916269700534711.clj 2]}]

:trace

[[leinproj.core$eval1739 invokeStatic form-init10068916269700534711.clj 2]

  [leinproj.core$eval1739 invoke form-init10068916269700534711.clj 1]

  [clojure.lang.Compiler eval Compiler.java 7177]

  [clojure.lang.Compiler eval Compiler.java 7132]

  [clojure.core$eval invokeStatic core.clj 3214]

  [clojure.core$eval invoke core.clj 3210]

  [clojure.main$repl$read_eval_print__9086$fn__9089 invoke main.clj 437]

  [clojure.main$repl$read_eval_print__9086 invoke main.clj 437]

  [clojure.main$repl$fn__9095 invoke main.clj 458]

  [clojure.main$repl invokeStatic main.clj 458]

  [clojure.main$repl doInvoke main.clj 368]

  [clojure.lang.RestFn invoke RestFn.java 1523]

  [nrepl.middleware.interruptible_eval$evaluate invokeStatic interruptible_eval.clj 79]

  [nrepl.middleware.interruptible_eval$evaluate invoke interruptible_eval.clj 55]

  [nrepl.middleware.interruptible_eval$interruptible_eval$fn__971$fn__975 invoke interruptible_eval.clj 142]

  [clojure.lang.AFn run AFn.java 22]

  [nrepl.middleware.session$session_exec$main_loop__1072$fn__1076 invoke session.clj 171]

  [nrepl.middleware.session$session_exec$main_loop__1072 invoke session.clj 170]

  [clojure.lang.AFn run AFn.java 22]

  [java.lang.Thread run Thread.java 832]]}

=> nil

ex-info can contain a message and a map, and ex-data allows you to retrieve the map or return nil if not created with an ex-info.

(println “Bye 4 Now!")