nexREPL

An experimental (proof of concept) Elixir nREPL server.

Demo

See this tweet.

Rationale and Motivation

Why?

Elixir ships with an excellent REPL (IEx), which even supports remote shells out-of-the-box. However, there's no standard way to evaluate the code you're writing in your editor directly in the context of an IEx session.

The Clojure community uses nREPL to integrate their editor with their REPL. nREPL doesn't seem to have taken off outside of the Clojure ecosystem, but the protocol is language-agnostic, and there are already ready-made clients for most editors.

If IEx could understand the nREPL protocol, that would theoretically turn all the red cells to green in the IEx column of Thomas Millar's Elixir scripting spreadsheet (see this tweet).

Why Not?

This project validates that an Elixir nREPL server works with existing (non-Elixir) nREPL clients.

However, to actually make this approach viable for development, nREPL features like evaluating the expression at the current point need to work. This requires some understanding of the structure of Elixir code; the core logic for teasing out the structure could live in the nREPL client exclusively, or alternatively primarily on the server.

In either case, unfortunately, client modifications will be necessary. Thus, one of the major benefits of using nREPL in the first place (existing clients are already written for most editors) is no longer that helpful.

A Potential Path Forward

What if instead we implemented similar functionality using the LSP, perhaps piggybacking on a new "eval code" Code Action request? With this approach, the client problem is solved, and the LSP server (which understands code structure) could send the code directly to a networked IEx shell for evaluation, providing the same benefits as nREPL.

Additional Discussion

See this thread on Elixir Forum.

Installation

To install from GitHub, run:

mix archive.install github mjrusso/nexrepl

To build and install locally, first ensure any previous archive versions are removed:

$ mix archive.uninstall nexrepl

And then run:

$ MIX_ENV=prod mix do archive.build, archive.install

Usage

First, ensure that your editor has an nREPL client installed and configured (see this list).

For example, here's what configuration in Emacs could look like (at least for those of use that use straight.el with use-package):

(use-package rail
  :straight (rail :type git :host github :repo "Sasanidas/Rail"))

(Rail is a generic nREPL client for Emacs that does not assume a Clojure server.)

Next, start the nREPL server on the default port (7888):

iex -S mix nexrepl

Then start editing a new or existing Elixir file.

Finally, using the nREPL client you've already configured, start a new nREPL session, and start evaluating code. (For example, in Emacs with Rail: start a new session with M-x rail, select a region, and then evaluate the region with M-x rail-eval-region.)

License

nexREPL is released under the terms of the Apache License 2.0. Includes vendored code from Bento; see NOTICE for details.

Copyright (c) 2024, Michael Russo.