I use this in my shell

{ pkgs ? (import <nixpkgs> {}), ... }:
with pkgs;
let
  otp = beam.packages.erlangR25;
  basePackages = [
    otp.elixir_1_13
  ];
  # Hot reloading stuff
  inputs = basePackages ++ lib.optionals stdenv.isLinux [ inotify-tools ]
    ++ lib.optionals stdenv.isDarwin
    (with darwin.apple_sdk.frameworks; [ CoreFoundation CoreServices ]);
in pkgs.mkShell {
  buildInputs = inputs;

  # keep your shell history in iex
  ERL_AFLAGS = "-kernel shell_history enabled";

  shellHook = ''
    # this isolates mix to work only in local directory
    mkdir -p .nix-mix .nix-hex
    export MIX_HOME=$PWD/.nix-mix
    export HEX_HOME=$PWD/.nix-hex

    # make hex from Nixpkgs available
    # `mix local.hex` will install hex into MIX_HOME and should take precedence
    export MIX_PATH="${otp.hex}/lib/erlang/lib/hex/ebin"
    export PATH=$MIX_HOME/bin:$HEX_HOME/bin:$PATH
  '';
}

While I like Scheme, I personally find Guix derivations harder to read than Nix. Also almost hostility towards non-free packages (in FSF sense) and host systems make Guix no-go zone for me.

For me FSF approach towards the software is on the “absurd” side of the spectrum, just like Oracle or Nintendo is on the exact opposite of spectrum in the “absurd” region.

Idea is glorious, but execution - not much. I am happy user of Nix on Darwin and while I always wanted to try Guix, their community and approach to people with other requirements/goals is something that keeps me out of it. Fortunately Nix is learning their lesson and they are fixing CLI into more useable and manageable form.

Alright so the following seems to work in the nix config:

    beam.packages.erlangR25.elixir_1_14
    beam.packages.erlangR25.erlang
    beam.packages.erlangR25.hex

Now how did you know there was a beam package?

On the website, the beam seems to be a haskell thing : NixOS Search (2nd- result)