Clojure Q&A - Recent questions tagged debugging

Useful macro for output redirection (feature request)

Mon, 04 Sep 2023 14:24:24 +0000

I think it would be useful to add a macro which redirects the standard output to *err*:

(with-err-out [& body]
  `(binding [*out* *err*]
     ~@body))

Strategies for diagnosing core.async code that is unexpectedly blocking the main thread?

Sun, 14 Feb 2021 21:07:06 +0000

I'm new to core.async (and CSP more generally) and struggling to find good debugging strategies. In particular, I frequently fail to diagnosis which part of the code is infinitely blocking the main thread. I'll give a particular example:

(deftest passing-test
  (let [c (async/to-chan! [2 1 0 5 4])]
    (is (= [2 1 0 5 4]
           (async/<!! (async/into [] c))))))


(deftest hanging-test
  (let [[e o] (->> (async/to-chan! [2 1 0 5 4])
                   (async/split even?))]
    (is (= [2 0 4]
           (async/<!! (async/into [] e))))
    (is (= [1 5]
           (async/<!! (async/into [] o))))))

The first test passes and does not block the main thread. The second test hangs forever. The only difference is the addition of async/split to route even and odd integers to their respective channels.

My specific questions are as follows:

What explains the difference between the first and second tests with respect to blocking the main thread?
What are some strategies to debugging core.async code that is unexpectedly blocking the main thread?
What is the best way to write unit tests for code that creates and manipulates channels? Is (async/<!! (async/into [] o)) a reasonable way to collect the values flowing through a channel into a collection?

is there a version of pr-str that emit full versions of qualified keywords in maps instead of short forms?

Fri, 10 Apr 2020 07:54:25 +0000

Something like {:foo/bar 1} instead of #:foo{:bar 1}
I need to debug some edn and it's impossible to jump to the desired keywords if they're in the short forms.

How do I get a full stack trace instead of "... 12 more"?

Fri, 26 Jul 2019 19:10:03 +0000

Using lein run. There are no references to my *.clj file anywhere, so I don't know where this java.lang.NullPointerException is coming from??

Exception in thread "main" Syntax error compiling at (/tmp/form-init542794133375754487.clj:1:72).
    at clojure.lang.Compiler.load(Compiler.java:7647)
    at clojure.lang.Compiler.loadFile(Compiler.java:7573)
    at clojure.main$load_script.invokeStatic(main.clj:452)
    at clojure.main$init_opt.invokeStatic(main.clj:454)
    at clojure.main$init_opt.invoke(main.clj:454)
    at clojure.main$initialize.invokeStatic(main.clj:485)
    at clojure.main$null_opt.invokeStatic(main.clj:519)
    at clojure.main$null_opt.invoke(main.clj:516)
    at clojure.main$main.invokeStatic(main.clj:598)
    at clojure.main$main.doInvoke(main.clj:561)
    at clojure.lang.RestFn.applyTo(RestFn.java:137)
    at clojure.lang.Var.applyTo(Var.java:705)
    at clojure.main.main(main.java:37)
Caused by: java.lang.NullPointerException
    at xinova.model$reload_dbs.invokeStatic(model.clj:78)
    at xinova.model$reload_dbs.invoke(model.clj:78)
    at xinova.core$_main.invokeStatic(core.clj:18)
    at xinova.core$_main.doInvoke(core.clj:14)
    at clojure.lang.RestFn.invoke(RestFn.java:408)
    at clojure.lang.Var.invoke(Var.java:384)
    at user$eval140.invokeStatic(form-init542794133375754487.clj:1)
    at user$eval140.invoke(form-init542794133375754487.clj:1)
    at clojure.lang.Compiler.eval(Compiler.java:7176)
    at clojure.lang.Compiler.eval(Compiler.java:7166)
    at clojure.lang.Compiler.load(Compiler.java:7635)
    ... 12 more