How to transpile a complex Erlang project with Elixir Flavoured Erlang: erldns

A user (yes, there's another one!) of Elixir Flavoured Erlang asked why a project wasn't transpiling correctly, I went to look and here are the notes:

Assuming you have the efe escript in your PATH, cd to a folder of your choice and do:

git clone https://github.com/dnsimple/erldns.git
git clone https://github.com/dnsimple/dns_erlang.git

Create a file called erldns.conf with the following content:

#{
   includes => ["../include/", ".", "../priv/", "../../"],
   macros => #{},
   encoding => utf8,
   output_dir => "./out/"
}.

And then run:

efe pp erldns.conf erldns/src/*.erl

Now the context:

We clone dns_erlang because erldns includes headers files from it, like here include_lib("dns_erlang/include/dns_records.hrl")

Since we want to find files using include_lib that refer to dns_erlang, we also include ../../ (which will find any project cloned at the same level as erldns)

erldns also includes headers from the include and priv folders, paths in include are relative to the source file, that's why both start with ../

Currently efe will silently remove parts of the code that can't find or parse properly, that's why you may notice that code that references external records or constants in header files not found in the includes list will silently be missing, in the future I may warn about that.