Erlang Code Auto Reloader - Auto Run Unit Tests and Hot Load Code After a Build

I wrote this post a few months ago when I was developing a new Erlang application. I eventually ended up using the reloader I mentioned at the end, but never documented it. Now I’m about to start writing a new app, and I’m on a different machine.

First, get the reloader. It was developed as part of mochiweb but functions just fine on it’s own. Just put it somewhere accessible - I use the ebin directory in my home folder. (yes, I just copied & pasted the source) Then compile it like so:

erlc reloader.erl

Now you can load the reloader automatically on startup by adding this to your ~/.erlang config file:

code:load_abs("/path/to/your/ebin/reloader"). reloader:start().

Change your path to match your reloader. Do not add the .beam extension. See the code module documentation.

When you start your Eshell, you’ll automatically get the reloader running. If you didn’t do it right, you’ll get output that looks like this:

computer:~ jhaddad$ erl Erlang R14B (erts-5.8.1) [source] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]

{“init terminating in do_boot”,{undef,[{reloader,start,[]},{init,start_it,1},{init,start_em,1}]}}

After loading a module, if you rebuild it, it’ll automatically get reloaded. If you’ve added the eunit .hrl file, it’ll also run your unit tests for you.

There’s not a lot of documentation out there on this incredibly useful library - hopefully this helps a few of you out.

If you found this post helpful, please consider sharing to your network. I'm also available to help you be successful with your distributed systems! Please reach out if you're interested in working with me, and I'll be happy to schedule a free one-hour consultation.