How to create and compile a custom module in MongooseIM - github

Question

System Info:
MongooseIM version: 3.0.0
Installed from: pkg
Erlang/OTP version: 18
Ubuntu 16.04
I am having trouble creating a standard base for a custom module. I want to create a simple hello world program as outlined in the documentation for ejabberd.
However, I cannot get it to work for MongooseIM. Are there any instructions for how to do this? As a beginner I am just looking for building blocks to creating my own modules, and everything I look at is a little too complex for what I am trying to achieve at the moment.
Here is the code for my module: (taken from ejabberd) https://docs.ejabberd.im/developer/extending-ejabberd/modules/#mod-hello-world
And here is my log error:
I have added the following line in my config file with all other running modules:
{mod_hello_world, []}
I am assuming it has something to do with the compilation and there being no .beam file created for the modules as well as some syntax errors specific to MongooseIM. I am also unfamiliar with documentation for compiling modules when using a pre-built pkg as opposed to installing from source.

Answer 1

DISCLAIMER: I'm a MongooseIM developer working for Erlang Solutions.
The link you posted hints at the answer to the immediate question:
If you compiled ejabberd from source code, you can copy that source code file with all the other ejabberd source code files, so it will be compiled and installed with them. If you installed some compiled ejabberd package, you can create your own module dir, see Managing Your Own Modules.
MongooseIM (a.k.a. MIM) does not support the latter method of managing modules, i.e. it's not possible to drop source code into some predefined location when MIM is installed from a package and let it just compile and run the module. If we want to write a custom module, we have to build MongooseIM from source.
To be precise, we don't have to build the whole server from source and package it ourselves. We have to, however, clone the repository, place the new module source there (due to build time requirements like header files) and build it there. Once we get a .beam file of the new module we can just drop it into an installed MongooseIM's code path.
To be even more precise, let's say we have installed MIM from mongooseim_3.0.0-1~ubuntu~artful_amd64.deb available from the Downloads page at erlang-solutions.com, therefore we want to build a module compatible with 3.0.0:
Clone MIM: git clone https://github.com/esl/mongooseim
cd mongooseim
git checkout 3.0.0
Place mod_hello_world.erl under ./src/
rebar3 compile
Once rebar3 finishes get ./_build/default/lib/mongooseim/mod_hello_world.beam and copy to the target host where we installed MIM from a package.
Please note, though, that an example taken straight from ejabberd documentation might not work "as is" in MongooseIM. In this simple module, for example, we'll not be able to include logger.hrl as MongooseIM doesn't have such a header file - we would have to -include("mongoose_logger.hrl"). instead.