After you have ran brew install proper you should check where it has been installed and then do as the guide says to first add PropEr's base directory to Erlang's path, either by modifying your .bashrc file or your .erlang file to point to the installation path.

In the case that under no circumstances you are able to find the installation directory, you can always add PropEr as a dependency of the project you want to use it in (e.g., in your rebar.config add {proper, "1.3.0"})

Thank you, with rebar3 I'm actually able to use it, setting manually the path is still not working but now is a lower priority.

Okay, I got a colleague who has a Mac to help me out fixing this issue. At the end of the output you get from running brew install proper you can find where it has been installed.

So, you will find PropEr installed at /usr/local/Cellar/proper/<version> (version is the latest release, e.g., 1.3). If you use that path you will be able to add it to Erlang's path either with ERL_LIBS or code:load_abs/1. If by any chance PropEr wasn't installed there, I think you can run brew info proper to get its installation path.

Having the same issue as @mezzomondo:

> brew install proper && export ERL_LIBS=/usr/local/Cellar/proper/1.4 && erl
Warning: proper 1.4 is already installed and up-to-date.
To reinstall 1.4, run:
  brew reinstall proper
Erlang/OTP 24 [erts-12.1.5] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit] [dtrace]

Eshell V12.1.5  (abort with ^G)
1> c(my_sort).
my_sort.erl:3:14: can't find include lib "proper/include/proper.hrl"
%    3| -include_lib("proper/include/proper.hrl").
%     |              ^

README seems to be wrong.
I also use rebar3 now.

We clearly need help here from some user who has access to a MacOS environment and knows / can check what needs to be done there.

Happy to jump in in a call or something to sort it out.

We need some PR to the README that describes the actions that homebrew users need to make in order to be able to use PropEr (from the Erlang shell). I have no idea about how homebrew stores packages, but surely there is some (a single?) place in the file system where PropEr's ebin and include dirs are stored and use them somehow.

Perhaps it's just a matter of manually adding symbolic links to ebin and include under /usr/local/Cellar/proper and bypassing the 1.4 dir, with commands such as:

cd /usr/local/Cellar/proper
ln -s 1.4/ebin .
ln -s 1.4/include .

and things will magically work from then on.

If we need a call to sort out this, my skype ID is of the form: name_surname

Update:

It turns out it's a Homebrew issue, the folder created by brew doesn't follow Erlang naming conventions.

See this discussion: Homebrew/discussions#2537
And the relative PR: Homebrew/homebrew-core#89963

Since the PR was merged to Homebrew's repo, I am closing the issue.

Thanks to all and to @mezzomondo in particular for opening the issue and for his help.