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If you use Emacs to write Erlang, you might have heard of EDTS. It's a recent Erlang development suite for Emacs, and it has some impressive features, but it can be a beast to get setup and working properly with CouchDB and the BigCouch merge branch.

Current CouchDB master's folder hierarchy is non standard for Erlang applications, and as a result it does not work well with EDTS. There is work being done in the BigCouch merge branch and the RCouch merge branch to restructure things in a standard OTP compliant hierarchy. This guide is for Emacs + EDTS + the BigCouch merge branch.

If you just want to load a vanilla VM with no running applications, then just follow the standard EDTS config, but that's boring. This guide will get you setup connecting Emacs/EDTS to your local BigCouch dev cluster, and provides all the nice EDTS features like automatic code reloading in the running VM, Emacs repl with tab completion and documentation, show callers and everything else in EDTS.

WARNING this is quite a hack, and involves a number of changes and updates. I hope that by writing this up I'll get some feedback on how to do things cleaner, but it works, and that's the important part ;-)


We'll need to update the EDTS start script, make an .edts config file, and use a hacked together function for spinning up the repl.

Also worth noting is that this setup expects you to manually run ./rel/ in the BigCouch branch to spin up the VM. If you forget to do that, EDTS will spin up a VM for you and then when you try to boot the dev cluster you'll get errors from trying to run multiple Erlang VMs with the same name.

Update EDTS start script

We need to update the base EDTS VM start script to use -name instead of -sname so that it can talk to the CouchDB nodes. I also like to add -hidden so that th EDTS VM doesn't show up in the list of nodes in the three node dev cluster. Here's my diff to the start script:

{% highlight diff %} diff --git a/start b/start index de68605..c744e72 100755 --- a/start +++ b/start @@ -27,8 +27,9 @@ PLUGINDIR=$EDTSHOME/plugins

cd $EDTSHOME exec $ERL - -sname edts + -name edts@ -edts projectdatadir ""$PROJDIR"" -edts plugindir ""$PLUGINDIR"" -pa $EDTSHOME/{lib,plugins}/*/ebin - -s edtsapp + -s edtsapp + -hidden {% endhighlight %}

Add a .edts config

EDTS works by having a .edts config in the top level of your project. We need to let the config know where to find all the projects, and we also need to specify the name of the VM. It's important that the name matches the name of the local dev cluster node you want to connect to, as EDTS will connect to the existing VM we start with the dev script, rather than starting a new VM. This way any changes you make will be immediately reflected in the running VM, allowing you to curl again or work in remsh without having to manually reload the modules or dev server. Here's my config, it's pretty simple:

{% highlight common-lisp %} :name "dev1" :lib-dirs '("src") {% endhighlight %}

Now when you open a .erl file within the CouchDB source folder, EDTS will start and then you will connect to the running dev server. At this point you'll be able to save files and have them loaded in the VM, but you won't have a repl available to you. For some reason EDTS does not start a repl for nodes in remsh's into.

Opening an inferior Erlang remsh

I cobbled together a little function to spin up an inferior Erlang shell and remsh into the dev cluster. This works, and let's you interact with the running VM, but you lose out on all the niceties of the EDTS shell, like tab completion and inline documentation boxes.

{% highlight common-lisp %} ;; adapted from: (defun erlang-shell-connect-to-node (name) (interactive "MNode name to connect to: ") (let* ((inferior-erlang-machine-options (list "-hidden" "-name" (format "emacs-remsh-%s" name) "-remsh" (format "%s@" name)))) (erlang-shell-display))) {% endhighlight %}

Hacking together an EDTS remsh

I decided I really wanted to have the proper EDTS repl, so I dove in and hacked one out today. I updated edts-shell.el with a new hardcoded function to connect to dev1 through a remsh. It's ugly, hardcoded, and it even throws an error at you when you run it, but it works!

{% highlight common-lisp %} (defun edts-shell-dev1 (&optional pwd switch-to) "Start an interactive erlang shell." (interactive '(nil t)) (edts-ensure-server-started) (let((buffer-name "dev1-shell") (node-name "dev1@") ;; (command (list "erl" "-name dev1-remsh -hidden -remsh" node-name)) (command (list "/Users/russell/src/dotfiles/emacs.d/plugins/edts/start_dev1")) (root (expand-file-name (or pwd default-directory)))) (let ((buffer (edts-shell-make-comint-buffer buffer-name node-name root command))) (edts-init-node-when-ready node-name node-name root nil) (when switch-to (switch-to-buffer buffer)) buffer))) {% endhighlight %}

I was having trouble properly passing the Erlang VM command line options, so I switched to using a little shell script to start the VM. Ideally the commented out command declaration should do the trick, but I was having issues with it. Here's my start_dev1 script:

{% highlight bash %}

/usr/bin/env bash

exec /usr/local/bin/erl -name dev1-remsh -hidden -remsh dev1@ {% endhighlight %}

Hopefully someone has a cleaner approach for this, but it works for now.


With this in place I've got a solid Emacs + EDTS dev environment for hacking on (big)CouchDB. If you have any feedback or suggestions, please don't be shy as this is definitely a work in progress and I see it as an minimal working version, not a finished configuration. I would also like to better understand interacting with running VMs with EDTS, it seems to be designed with the assumption you're working locally with a VM started by EDTS, not a running environment. I'm curious if the author of EDTS has any suggestions on better approach here.


Some random things to keep in mind.

Single node interaction only

Right now you connect to an individual node, in this case dev1. The local dev cluster spins up dev2 and dev3 as well, but this does not connect to those nodes, and does not load code with nl to upgrade all nodes with your modifications. I would like to fix this to at least do an nl. You'll occasionally run into whacky behavior on the dev cluster, just restart it and run M-x edts-project-node-init to reconnect.

Disconnected from dev1 VM

If you restart the dev server, you'll lose the connection from EDTS and will get an error when it tries to push your changes. I just do M-x edts-project-node-init to get back up and running.

Show source under cursor doesn't go into Erlang/OTP source

I was having issues following code into the full OTP source, and I tracked this down to the compile info stored in the beams pointing to a directory that did not exist on my system. I'm not sure what happened there, I was using erlbrew but ended up going back to erln8 and now I can follow code down into the Erlang/OTP source.

Built in manual

Don't forget to generate the manual with M-x edts-man-setup RET so you can go directly to the man pages inside emacs and also get inline function docs, very handy!