nix-flake-organization

Nix flake organization

Keep flake/ as the public output layer and src/ as the implementation layer. The flake tree names, routes, and re-exports outputs; feature behavior, module bodies, package derivations, app scripts, and helper logic live under src/<name>/....

Apply codebase-design within those boundaries: keep each feature's implementation together and extract only to hide complexity or enable useful reuse. Nix output adapters are meaningful public entrypoints; additional internal forwarding files need their own purpose. Preserve Nix's roles rather than imposing TypeScript models/ and services/ directories.

When to use

• A user asks to reorganize or refactor a Nix flake layout.
• Flake outputs are crowded into flake.nix or mixed with implementation code.
• NixOS, nix-darwin, Home Manager, or flake-parts modules contain business logic directly in output wiring files.
• A repo needs a clear split between public flake API and implementation internals.

When NOT to use

• The task is only to add one package, app, or module without changing repo layout.
• The repo is not a Nix flake.
• The user explicitly wants a different established project layout preserved.

Canonical prompt

Use this improved prompt when turning the request into an implementation plan:

Reorganize this repo so its public flake surface lives in a top-level flake/ directory: flake/apps, flake/packages, flake/lib, and flake/modules/{flake-parts,home-manager,darwin,nixos} where applicable. Keep flake.nix as a minimal entrypoint and keep every file under flake/ thin: it may declare output names, import implementation files, compose flake-parts modules, and adapt public calling conventions, but it must not contain derivation logic, app scripts, option/config bodies, service behavior, or helper implementations. Keep those implementation details together in feature-oriented src/<name>/... paths. Preserve existing output attribute names during structural moves; follow the repository's compatibility policy when changing the public interface. Verify with nix flake show, nix flake check, and representative package/module evals or builds.

Target shape

Prefer this structure, adapting names to the existing repo:

flake.nix
flake/default.nix
flake/apps/default.nix
flake/packages/default.nix
flake/lib/default.nix
flake/checks/default.nix       # if checks exist
flake/devShells/default.nix    # if devShells exist
flake/overlays/default.nix     # if overlays exist
flake/modules/flake-parts/default.nix
flake/modules/home-manager/default.nix
flake/modules/darwin/default.nix
flake/modules/nixos/default.nix
src/<name>/app.nix
src/<name>/package.nix
src/<name>/lib.nix
src/<name>/modules/flake-parts.nix
src/<name>/modules/home-manager.nix
src/<name>/modules/darwin.nix
src/<name>/modules/nixos.nix

flake/modules/flake-parts belongs beside the platform module families when flake-parts modules exist, even if the user's shorthand example omits it.

Feature-oriented means grouping by domain concept, not by output type. If a concept vpn has a package, app, and NixOS module, keep them under src/vpn/. If a package has no associated modules, src/<pkg-name>/package.nix is fine. When a repo is already organized by platform, preserving that structure under src/ is acceptable, but do not mechanically recreate the whole flake/ tree under src/.

Thin layer rule

Thin does not mean empty. A thin file can adapt calling conventions, pass inputs, self, or pkgs, and preserve public attribute names. It should be understandable without reading implementation details.

Keep configured file limits. If a cohesive module exceeds one, first look for an independent responsibility; otherwise document a narrow increase or exception. Keep options, defaults, assertions, and configuration local when they explain one feature, even when that makes a module longer.

For module outputs, thin means the flake/modules/<platform>/default.nix file is a re-export point. It imports the full module from src/ and wires it into the public output attribute. It does not mean splitting options from config inside a single module; options and config stay together in src/<name>/modules/<platform>.nix.

# flake/modules/nixos/default.nix
{ ... }:
{
  flake.nixosModules.vpn = import ../../../src/vpn/modules/nixos.nix;
}

# src/vpn/modules/nixos.nix
{ config, lib, pkgs, ... }:
{
  options.services.vpn = { ... };
  config = lib.mkIf config.services.vpn.enable { ... };
}

In flake-parts repos, perSystem often owns packages, apps, checks, and devShells. Keep perSystem composition thin in flake/modules/flake-parts; package derivations still belong in src/<name>/package.nix, app behavior in src/<name>/app.nix, and dev shell/check implementation in src/<name>/.... Do not define the same output through both direct flake/packages wiring and flake-parts perSystem wiring.

Migration workflow

1. Inventory current public outputs: packages, apps, lib, checks, devShells, overlays, nixosModules, darwinModules, homeManagerModules, and flake-parts imports.
2. Preserve the public output schema first. Move files without renaming output attributes unless the user requested a breaking change.
3. If current outputs are interleaved in one file, first separate each output family enough that it can move independently; avoid a big-bang rewrite.
4. Create the flake/ directories as output shims and move one output family at a time.
5. Move implementation into feature-oriented src/<name>/...; avoid recreating the flake/ tree under src/ unless the repo is already platform-oriented.
6. Keep shared implementation in src/<name>/lib.nix or src/shared/<name>.nix, not in flake/lib.
7. Follow the repository's compatibility policy for renamed outputs. Keep aliases only when that policy requires them; otherwise update consumers and remove obsolete paths.
8. Give outputs without an example directory, such as formatter, their own thin flake/<output>/ shim or leave them in the nearest existing composition file; do not fold them into unrelated directories.
9. Verify after each family move with nix flake show, nix flake check, and targeted builds/evals.

Common mistakes

• Moving flake.nix into flake/; Nix expects flake.nix at the repo root.
• Treating flake/ as the new implementation home instead of a thin public layer.
• Hiding implementation in flake/lib because it feels like a shared bucket.
• Putting option declarations, config, assertions, or services in flake/modules/*.
• Forgetting that packages.${system} and apps.${system} are system-specific while module outputs usually are not.
• Putting perSystem build logic in flake/modules/flake-parts instead of importing implementations from src/<name>/....
• Creating both direct output wiring and flake-parts perSystem wiring for the same derivation without a deliberate compatibility reason.
• Creating circular imports between flake/lib, flake/modules, and src.
• Changing public output names during a move-only refactor.
• Skipping verification because the change looks like only path shuffling.

Templated scripts

When packaging shell scripts, keep reusable script logic in its own source file instead of embedding long scripts inline in Nix strings. Use Nix only to substitute package-specific defaults.

Prefer nixpkgs-style @name@ placeholders with replaceVars:

writeTextFile {
  name = "tool";
  destination = "/bin/tool";
  executable = true;
  text = builtins.readFile (replaceVars ./tool.sh {
    default_config = builtins.toJSON config;
  });
}

In the script, group all Nix substitutions at the top and make runtime values overrideable by arguments or environment variables:

default_config() {
  cat <<'JSON'
@default_config@
JSON
}

: "${TOOL_JQ:=jq}"

tool_config() {
  if [ -n "${TOOL_CONFIG_JSON:-}" ]; then
    printf '%s\n' "$TOOL_CONFIG_JSON"
  else
    default_config
  fi
}

This keeps scripts usable outside Nix, makes the Nix coupling easy to audit, avoids quote-heavy inline shell strings, and matches current nixpkgs conventions now that substituteAll has been replaced by replaceVars.

Review checklist

• flake/ files are mostly imports, attr names, re-exports, and public calling-convention adapters.
• src/<name>/... contains derivations, scripts, options, config, services, assertions, and helpers.
• Existing public output names still work or have deliberate migration notes.
• Module families are separated: home-manager, darwin, nixos, and flake-parts do not leak platform-specific logic into each other.
• Verification commands cover both system-specific outputs and module evaluation paths.
• Templated scripts live in standalone files, use @name@ placeholders via replaceVars, and keep Nix-specific defaults grouped at the top with environment/argument overrides for reusable behavior.

Tools

None. This is a pure prompt and review skill. Use the repo's existing Nix commands for verification.

Reference

No separate reference files. Use the canonical prompt, thin layer rule, migration workflow, and review checklist above.