Bug I read a few posts recently on r/clojure asking about Clojure debugging tools. It seemed that those asking the questions were looking for the kind of tools we have come to expect from IDEs such as IntelliJ, Eclipse, Visual Studio and their ilk. It’s a familiar question since I asked it myself when I started learning Clojure.

I think I have come to the same conclusion as other Clojure programmers in that my short answer to the question is “there isn’t one”, or “it’s the REPL”, and that my long answer is, well, long.

This cryptic answer needs expanding upon, because it’s not really true. The real answer is that, like in any other language, the best Clojure debugger is your brain.

This is no less cryptic.

In coming up with the long answer, I found it fits best into two parts. This first part covers what I’m defining as debugging, and my overall approach – my philosophy, perhaps. The second part will cover building debuggable systems, and finally get into the debugging tools themselves.



Hi-tech coding At work, I am constrained to write Clojure in a terminal (we use remote tmux) and really, using Emacs. This is a fine set of constraints, but my debugging solutions haven’t considered tools like Nightcode, Light Table, La Clojure or other IDE-like tools. Looking at the various product websites, point-and-click debugging has different levels of support, and if that’s what you really want, perhaps La Clojure would be a good start.

I also have not yet tried out the Ritz toolchain (this article has lots of tidbits that are on my to-try list), nor have I tried Schmetterling (which makes exception-triggered or breakpoint-triggered stacktrace inspection and interactive REPLs available through a browser connected to your REPL), or Mycroft.

This article is concerned with debugging Clojure, in Emacs, using simple Clojure code, tools and libraries to make life a little easier. Much of it applies equally to non-Clojure, and non-functional programming. Solving bugs in 100k+ LOC Java codebases isn’t magically made tractable by having IntelliJ’s debugger.

What is “debugging” anyway?

I will define a bug as a deviation between your expectation of, and your observation of the behavior of a system.

The “system” here comprises your code, the libraries your code uses, the JVM, OS and all the way on down to the physical hardware (though you can likely limit your concern to the first two – see “Understand your assumptions” below).

Debugging is the process of investigating, verifying, isolating the cause for, and closing the gap between your expectations and your observations. You are allowed to change the system; you are also allowed to change your expectations of the system.

Chop it up Debugging is a reductionist process: understanding of the whole is achieved through understanding of the composite parts. The systems we build are frequently too complex to really comprehensively understand all at once, but by considering a bug from how it manifests at the highest level of abstraction, and digging down into the layers of the system, we can understand enough of the system to understand a bug, then make informed decisions about how to address it.

Given this fairly straightforward reductionist approach, it is fairly straightforward to sketch out an algorithm for debugging.


Above I laid out the case for writing “code that is easy to exercise in the REPL, whose behavior is easily observed, and with unit test coverage”. Here is my “algorithm” for debugging:

  1. Identify the area of your system expressing the bug. You should be able to clearly state the desired behavior.
  2. Run that subset of your code and observe the behavior; the outcome will be different to your expectation. Ensure you can repeatably observe that outcome is different to your expectation. If you can’t reproduce it, you aren’t in a position to fix it.
  3. Read the code. Consider the inputs, and try to understand how it generates the output. Add temporary instrumentation to the code as necessary.
  4. If the code is too large or complex to understand, divide and conquer. Refactoring may be helpful, even as a temporary measure. You must somehow break the problem down into smaller, understandable pieces. Go back to step 2 and repeat for each of the smaller pieces. Proceed when you fully understand how the code works.
  5. As long as you are always mindful of what your expectation of the code is, the problem will at this point become clear, and you need to devise a way to fix it. Remember, the problem might be your expectation.

So, the debugging process can – in general – be outlined. There are architectural decisions you can make, and tools you can use, in order to better pose and answer questions about the runtime behavior of your code. The second part of this article will cover covers what a debuggable Clojure-based codebase would be, and then digs into some Emacs and Clojure tools I have found useful.

Image credits: