Ever since I started using the Emacs + nrepl combination to play around with Clojure I’ve been getting fairly non descript error messages whenever I pass the wrong parameters to a function.

For example if I try to update a non existent key in a form I get a Null Pointer Exception:

In this case it’s clear that the hash doesn’t have a key ':mark' so the function blows up. However, sometimes the functions are more complicated and this type of reduced stack trace isn’t very helpful for working out where the problem lies.

I eventually came across a thread in the nrepl-el forum where Tim King suggested that adding the following lines to the Emacs configuration file should sort things out:

~/.emacs.d/init.el

I added those two lines, restarted Emacs and after calling the function again got a much more detailed stack trace:

From reading this stack trace we learn that the problem happens when the inc function is called with a parameter of 'nil'. We’d see the same thing if we called it directly:

Although Clojure error messages do baffle me at times, I hope things will be better now that I’ll be able to see on which line the error occurred.