The interactive_editor gem lets you launch an instance of Vim within an IRB session.

It brings up a scratch buffer, and when you :wq (save+quit) Vim, you’re back in IRB, with any methods/classes you defined now available. Even better, if you relaunch Vim it will load the code you were editing previously, make it easy to work incrementally and try ideas out in the repl.

To use this on NixOS, I needed to write a nix derivation for it. So I created a simple Gemfile:

mkdir interactive-editor &&
    cd interactive-editor &&
    cat <<EOF > Gemfile
source ''
gem 'interactive_editor'

Then I used bundix to generate a gemset.nix describing interactive_editor and its dependencies.

nix-shell --packages bundix --run 'bundix --magic'
chmod 644 gemset.nix

The chmod is a workaround for a small bug in bundix that creates gemset.nix with 0600 permissions, preventing read access for users other than the file owner. Since I want a root-owned copy of this file incorporated into my nixos configuration, setting 0644 permissions ensures non-root users can still use it.

I submitted a fix to bundix for the permissions issue, but until the fixed version makes it into the nixpkgs tree, chmod does the trick.

Next, I copied Gemfile, Gemfile.lock, and gemset.nix into an overlay in my nixos confg. I keep most of my custom packages in an overlay under /etc/nixos/overlays/core/. I copied the relevant files to an interactive-editor/ subdirectory, and wrote a derivation with bundlerEnv in default.nix:

sudo bash -c '
    mkdir /etc/nixos/overlays/core/interactive-editor &&
        cp /home/ivan/path/to/interactive-editor/{Gemfile{,.lock},gemset.nix} \
        cat <<"EOF" > /etc/nixos/overlays/interactive-editor/default.nix
{ lib, bundlerEnv, ruby }:

bundlerEnv rec {
  name = "interactive-editor-${version}";

  version = (import gemset).interactive-editor.version;
  inherit ruby;
  # expects Gemfile, Gemfile.lock and gemset.nix in the same directory
  gemdir = ./.;

  meta = with lib; {
    description = "Interactive editing in irb";
    homepage    =;
    license     = with licenses; mit;
    platforms   = platforms.unix;

I should call out a small gotcha in the above bash snippet. The opening heredoc delimiter needs to be quoted ("EOF"), otherwise Bash will try to expand ${version} and substitute the expansion (an empty string, since we haven’t defined any such variable in our shell) in the text we’re writing to default.nix.

We want the ${version} to be inserted literally (it’s a Nix variable, not a Bash variable). This is made all the more confusing by the fact that the heredoc is already inside a single-quoted block. We’re using the single-quotes to wrap a block of code we want Bash to interpret (bash -c '...'). This way, sudo runs mkdir, cp, and cat all with elevated privileges.

With the derivation in place, I added it to my overlay:

self: super: {
  # ...

  interactive-editor = super.callPackage ./interactive-editor { };

  # ...

I aliased nirb to launch IRB with interactive_editor available:

nix-shell --packages ruby interactive-editor --command irb

Finally, in order for nix-shell to find packages in my overlay, I added a nixpkgs-overlays entry to my NIX_PATH.