Clojure/ClojureScript try...catch

l3nz · November 18, 2018, 9:26am

I am just coming to ClojureScript trying to have CLI-matic work on it. My biggest pain point, at the moment, is try/catch blocks and throw calls. I would like to have as much logic as possible in a common .cljc file, and defer to functions help in .clj/.cljs the calling of platform-specific features.

I mean, as exceptions are platform-specific, how do you handle this case for code that is supposed to be platform-agnostic? write your own try/catch and throws macros, that resolve differently for each platform? or is there something ready? try/catch block are control, so I’d hate to port a full function just because there is a try/catch block in it, or it needs to blow up on errors…

borkdude · November 18, 2018, 10:40am

I am not 100% sure what you mean, but I wrote a couple of .cljc macros here that run a body in a scope with a try/catch construction:

github.com

slipset/speculative/blob/master/src/speculative/test.cljc#L81




(defmacro try-return
  "Executes body and returns exception as value"
  [& body]
  `(try ~@body
        (catch ~(? :clj 'Exception :cljs ':default) e#
          e#)))


;; with-(i/u)nstrumentation avoids using finally as a workaround for
;; https://dev.clojure.org/jira/browse/CLJS-2949
(defmacro with-instrumentation
  "Executes body while instrumenting symbol."
  [symbol & body]
  `(let [was-instrumented?#
         (boolean
          (seq (unstrument ~symbol)))
         ret# (try-return
               (instrument ~symbol)
               ~@body)]
     (when-not was-instrumented?#
       (unstrument ~symbol))

mfikes · November 18, 2018, 1:43pm

I’d suggest using portable ex-info and related functions (ex-data, ex-message, ex-cause) for creating and pulling apart exceptions when possible.

For catching exceptions I think something like https://dev.clojure.org/jira/browse/CLJ-1293 would ultimately help in writing portable code, but in the meanwhile reader conditionals seem to be the way to go.

l3nz · November 19, 2018, 8:16pm

I personally hate the reader conditionals in shared code, but I think there is no other way.

l3nz · November 19, 2018, 8:17pm

Thanks for the suggestion. I did not know about ex-... functions (like - I fear - I don’t know much about 90% of core…)

seancorfield · November 19, 2018, 8:42pm

It’s probably worth mentioning that ex-message and ex-cause are new in Clojure in the 1.10 prerelease builds, although they’ve been in ClojureScript for a while. So they’re not portable for pre-1.10 Clojure versions.

system · May 21, 2019, 8:42am

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