How can I trace code execution in Clojure?

Question

Why learning Clojure, I sometimes need to see what a function does at each step. For example:

(defn kadane [coll]
   (let [pos+ (fn [sum x] (if (neg? sum) x (+ sum x)))
         ending-heres (reductions pos+ 0 coll)]
     (reduce max ending-heres)))

Should I insert println here and there (where, how); or is there a suggested workflow/tool?

Answer 1

This may not be what you're after at the level of a single function (see Charles Duffy's comment below), but if you wanted to do get an overview of what's going on at the level of a namespace (or several), you could use tools.trace (disclosure: I'm a contributor):

(ns foo.core)

(defn foo [x] x)
(defn bar [x] (foo x))

(in-ns 'user) ; standard REPL namespace

(require '[clojure.tools.trace :as trace])
(trace/trace-ns 'foo.core)

(foo.core/bar 123)
TRACE t20387: (foo.core/bar 123)
TRACE t20388: | (foo.core/foo 123)
TRACE t20388: | => 123
TRACE t20387: => 123

It won't catch inner functions and such (as pointed out by Charles), and might be overwhelming with large code graphs, but when exploring small-ish code graphs it can be quite convenient.

(It's also possible to trace individually selected Vars if the groups of interest aren't perfectly aligned with namespaces.)

Answer 2

If you use Emacs with CIDER as most Clojurians do, you already have a built-in debugger:

https://docs.cider.mx/cider/debugging/debugger.html

Chances are your favorite IDE/Editor has something built-in or a plugin already.

There is also (in no particular order):

• spyscope
• timbre/spy
• tupelo/spyx
• sayid
• tools.trace
• good old println

I would look at the above first. However there were/are other possibilities:

• https://gist.github.com/ato/252421
• https://github.com/philoskim/debux
• https://github.com/pallet/ritz/tree/develop/nrepl-core
• https://github.com/hozumi/eyewrap
• probably many more

Also, if the function is simple enough you can add defs at development-time to peek inside the bindings at a given time inside your function.