Debugging in Clojure

#1

Post by DaveWM » Tue, Jul 06, 2021, 4:52 PM UTC

Re: Debugging in Clojure

#2

Post by fulafel » Wed, Jul 07, 2021, 5:04 AM UTC

The first example about spyscope potentially confuses the reader a bit by using it in context of println debugging. The point of it is you can stick it in front of any form. For a better example, see https://github.com/dgrnbrg/spyscope#spyp

Otherwise great article, I hadn't heard about scope-capture.

Re: Debugging in Clojure

#3

Post by NightMKoder » Wed, Jul 07, 2021, 5:42 AM UTC

This isn’t mentioned in the article, but I found debug-repl [1] to be amazing when debugging in the repl. For simple prints, wrapping expressions in (doto prn) has always been enough for me without custom readers. But when debugging something super odd, just stopping somewhere and evaluating a bunch of code (with locals in scope) to understand what you’re dealing with is invaluable. Clojure’s immutability makes it especially nice since you can just rerun (most) expressions to “see what happens.”

[1] https://github.com/gfredericks/debug-repl

Re: Debugging in Clojure

#4

Post by zurn » Wed, Jul 07, 2021, 6:31 AM UTC

For those of us using CIDER don't forget #break / #dbg!

https://docs.cider.mx/cider/debugging/debugger.html#using-th...

Re: Debugging in Clojure

#5

Post by geokon » Wed, Jul 07, 2021, 6:46 AM UTC

zurn wrote: post #4

For those of us using CIDER don't forget #break / #dbg! https://docs.cider.mx/cider/debugging/debugger.html#using-th...

That allows you to step through code. but it doesn't break when you get a crash (like gdb). At least when I just tested:

     #dbg(defn get-first [input] (nth input 0))
     (get-first [4 2 3]) ;; 4
     (get-first 5) ;; gives me a stack trace and no program state

So it's nice to have, but I think this is maybe only useful if you're working through someone else's code?

Re: Debugging in Clojure

#6

Post by geokon » Wed, Jul 07, 2021, 7:02 AM UTC

Could `scope-capture` be used to capture state before a crash?

What would be nice is a GDB-like state along with stacktraces when you get a crash. Clojure stack traces are notoriously long and spooky. You learn to read the tea leaves, but even if you manage to identify where the crash happened and what triggered it (not always obvious..) you then need to pepper things with `println` to figure out the last local state before the whole thing blew up.

It looks like once you've found the problem area you can use `sc.api/spy` ..? It'd at least solve half the problem

Re: Debugging in Clojure

#7

Post by slifin » Wed, Jul 07, 2021, 7:30 AM UTC

Some additional conversation on this blog post here:

https://www.reddit.com/r/Clojure/comments/oe40af/debugging_i...

Re: Debugging in Clojure

#8

Post by phoe-krk » Wed, Jul 07, 2021, 7:37 AM UTC

For people who want to get a more CL-like debugging setup on Clojure, there is a CL-style condition system available as a library.

https://github.com/IGJoshua/farolero/

Re: Debugging in Clojure

#9

Post by brokenkebab » Wed, Jul 07, 2021, 8:03 AM UTC

Elephant in the room: Sayid. It's made for Emacs, and it's so powerful, you may want to switch to Emacs just because of it.

Re: Debugging in Clojure

#10

Post by billfruit » Wed, Jul 07, 2021, 8:50 AM UTC

Does the Clojure debugger give a live Repl, where one can examine values of locals and restart execution if required, likes the elisp debugger in Emacs?