Sometimes we want to build things from source and use it in a project we already have. `asdf` is excellent, but building from source has its merits as well. It's good to verify release candidates, new features and potentially identify issues if needed. I've asked myself the question above, so here is the answer!

What has worked for me in this case is to have both projects under a folder. I usually call this folder `oss` (short for open source software).

# on Mac:
$ brew install autoconf wxmac wxwidgets libtool automake fop

# on ubuntu:
$ sudo apt-get install build-essential autoconf libncurses5-dev openssl libssl-dev fop xsltproc unixodbc-dev

# Erlang
# Per https://github.com/erlang/otp/blob/master/HOWTO/DEVELOPMENT.md:
# from /otp recently cloned
# inside your_path/oss
$ git clone git@github.com:erlang/otp.git
$ cd otp
$ ./otp_build configure && make

# Elixir
# Per https://github.com/elixir-lang/elixir?tab=readme-ov-file#compiling-from-source
# inside your_path/oss
$ git clone git@github.com:elixir-lang/elixir.git
$ cd elixir
$ make clean && make

Now, we configure paths so we choose which Erlang and Elixir to pick up. We will set that up for a specific shell, you may update your dotfiles accordingly so it's picked up in every new shell.

# from a folder that has both otp and elixir repos under them, in my case, `oss`
# set vars up
oss $ export ERL_TOP=`pwd`/otp
oss $ echo $ERL_TOP
/Users/pg/dev/oss/otp
oss $ export LOCAL_ELIXIR_PATH=`pwd`/elixir
oss $ echo $LOCAL_ELIXIR_PATH
/Users/pg/dev/oss/elixir

# update PATH
oss $ export PATH=$ERL_TOP/bin:$PATH
oss $ export PATH=$LOCAL_ELIXIR_PATH/bin:$PATH

# and verify!
$ iex
Erlang/OTP 28 [DEVELOPMENT] [erts-15.0] [source-cf8c426765] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1]

Interactive Elixir (1.18.0-dev) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)>

Now you can use whatever Erlang and Elixir versions you like!