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4 Architecture Overview

Jotain is organised into distinct layers, each with a clear responsibility.

4.1 File Structure

jotain/
├── early-init.el       # Pre-package/GUI initialisation
├── init.el             # Entry point — loads modules in order
├── lisp/               # Modular Elisp config (init-*.el, one per concern)
├── emacs.nix           # GNU Emacs build from source (cache-parity invariant)
├── default.nix         # Non-flake entry point (flake-compat wrapper)
├── overlay.nix         # Standalone nixpkgs overlay (wraps nix/mk-overlay.nix)
├── module.nix          # Home Manager module for the daemon
├── module-system.nix   # Shared NixOS / nix-darwin module
├── module-nix-on-droid.nix  # nix-on-droid module (terminal-only build)
├── nix/
│   ├── mk-overlay.nix         # Overlay implementation — distribution assembly
│   ├── checks.nix             # Flake checks (lint, compile, tests, builds)
│   ├── use-package.nix        # use-package scanner (Elisp → epkgs mapping)
│   └── ...                    # docs, site, extra packages, treefmt helpers
├── devenv.nix          # Development shell (tooling only — no Emacs)
├── devenv.yaml         # devenv inputs (pinned to flake.lock revs)
├── flake.nix           # Flake entry point — source of truth for pins
├── Justfile            # Task runner recipes
├── docs/               # Documentation sources (rendered into the site,
│                       #   the Info manual, and the man page; docs.json
│                       #   defines page order for all three)
├── website/            # page.jylhis.com/jotain site shell (assembled by nix build .#site)
├── test/               # ERT tests (every *.el here is loaded by elisp-test)
├── bench/              # Startup-benchmark wrapper init files
├── templates/          # Tempel snippet templates (jotain.eld)
├── config/             # Auxiliary tool config (e.g. eca)
├── scripts/            # Bootstrap helpers (e.g. bootstrap-agent-env.sh)
└── journal/            # Development journal entries

4.2 Layers

4.2.1 Emacs Lisp Layer

The Elisp configuration is split into three parts:

  1. early-init.el — loaded before package.el, before the first frame, before init.el. Handles anything that must happen that early: the startup GC threshold, use-package-always-ensure, frame chrome defaults, native-comp and eln-cache redirection, terminal aliases.
  2. init.el — tiny entry point. Registers MELPA/NonGNU ELPA as fallback archives, puts lisp/ on the load-path, points custom-file at var/custom.el (write-only — never loaded back), and requires each module in order. Archive refresh is off the startup path entirely — there is no background warm-up; archives are fetched only when package-install finds an empty cache, or on an explicit M-x package-refresh-contents.
  3. lisp/init-*.el — one file per concern. See Modules for the full list and their responsibilities.

4.2.2 Nix Layer

The Nix layer provides reproducible Emacs builds:

  1. emacs.nix — wraps the git-based attrs from nix-community/emacs-overlay (git/unstable/igc; unstable is the default) or nixpkgs’ default emacs attribute (mainline, the cache-parity canary), with the supported build flags exposed as arguments. The build matrix is deliberately narrow: pgtk/Wayland GUI or terminal-only on Linux, patched NS GUI or terminal-only on Darwin — everything else is asserted away. A cache-parity invariant guarantees the Linux and terminal builds produce the same store paths as the matching prebuilt attrs, so their default-rev builds are cache hits against Hydra and nix-community.cachix.org. Custom rev pins and the Darwin GUI (whose nix-giant patches are on by default) run through overrideAttrs and intentionally rebuild from source. See Nix build layer for the invariant and its verification command.
  2. default.nix — flake-compat wrapper for the default package set.
  3. module.nix — Home Manager module that runs Jotain as a user-session Emacs daemon (services.jotain), generates a desktop entry for emacsclient, and can install itself as EDITOR/VISUAL. Supports systemd on Linux and launchd on macOS; services.jotain.package swaps in any other Jotain-shaped distribution.

4.2.3 Development Layer

  • devenv.nix — development shell, tooling only: there is no emacs binary in it (see its top-of-file note). It ships Nix tooling and linters (nil, nixfmt-rfc-style, treefmt, statix, deadnix), the language servers and CLIs the Elisp config shells out to, the docs toolchain, and the fonts the UI config probes. Build and launch the editor with just run-built (see the current-state note in Launching Emacs).
  • Justfile — every recipe you run day-to-day: run-built / run-built-debug (build via Nix, then launch), check, test, the build-* matrix, fmt, update, verify, clean, clean-all. The former direct-launch and in-shell compile recipes (run, debug, tty, check-elisp, compile, bench, …) were removed when Emacs left the dev shell; their coverage lives in the elisp-lint / elisp-compile / elisp-test flake checks.
  • Pinning — flake.lock is the single source of truth; default.nix and emacs.nix read it directly via fetchTarball so non-flake nix-build consumers get the same pin. devenv.yaml mirrors the flake’s shared input revs (nixpkgs, treefmt-nix, emacs-overlay), and just update keeps devenv.lock in lockstep.

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