atomi/nix

AtomiCloud's nix merger

Purpose

Deep-merges Nix configuration files when multiple templates or layers contribute files to the same path. The resolver parses Nix files into structured data, merges entries across layers, and pretty-prints the result. Files not matching a known pattern fall back to last-write-wins (highest layer).

Configuration Schema

This resolver requires no configuration.

Resolution Strategy

The resolver dispatches to a file-type-specific merger based on the file's basename or relative path:

File Pattern	Merger	Strategy
flake.nix	mergeFlake	Deep merge: inputs (URL LWW), outputs (union), registries (expr LWW), packages inherit (union)
nix/env.nix	mergeEnv	Category union: deduplicate and sort packages within each category
nix/fmt.nix	mergeFmt	Deep merge: programs (enable true-wins, extra_args LWW), projectRootFile LWW
nix/packages.nix	mergePackages	Sub-block merge: function args (union), inherit IDs (LWW per identifier), assignments (LWW)
nix/shells.nix	mergeShells	Shell merge: buildInputs (union, dedupe, sort) per shell name
nix/pre-commit.nix	mergePrecommit	Deep merge: hooks (enable true-wins, string fields LWW, array fields concat+dedupe)
any other .nix	fallback	Last-write-wins: highest layer wins

Field-level merge rules

Rule	Applies to
Union + dedupe	Input URLs, output params, function args, buildInputs, inherit IDs, excludes, stages
Enable true-wins	fmt programs, pre-commit hooks
String LWW	src, projectRootFile, hook name/description/entry/files/language/package
Boolean LWW	pass_filenames, fmt extra_args
Array concat	excludes, stages

Commutativity and Associativity

The resolver ensures deterministic output regardless of input ordering:

• Files are sorted by layer number (ascending), then template name (alphabetical) before merging
• Unioned collections (inputs, outputs, packages, inherit IDs) are deduplicated via Set and sorted alphabetically on output
• LWW fields always use the value from the highest layer, which is stable regardless of input order
• True-wins booleans produce the same result whether processed in any order (a || b is commutative)
• Comment groups are preserved from the first layer that defines them (not reordered by re-processing)
• Conflicting registries in packages.nix sub-blocks throw an error rather than silently choosing

Merge Examples

Example 1: flake.nix input merge

Input files (both flake.nix):

Origin Template	Origin Layer	Content
base	0	inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
infra	1	inputs.flake-utils.url = "github:numtide/flake-utils";

Resolved output:

inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
    flake-utils.url = "github:numtide/flake-utils";
  };

Example 2: nix/env.nix category union

Input files (both nix/env.nix):

Origin Template	Origin Layer	Content
base	0	{ pkgs }: { dev = [ nodejs_22 typescript ]; }
frontend	1	{ pkgs }: { dev = [ nodejs_22 eslint ]; }

Resolved output:

{ pkgs }:
{
  dev = [
    eslint
    nodejs_22
    typescript
  ];
}

Example 3: nix/shells.nix buildInputs union

Input files (both nix/shells.nix):

Origin Template	Origin Layer	Content
base	0	Shell default with buildInputs = [ pkgs.go ];
tools	1	Shell default with buildInputs = [ pkgs.go pkgs.gopls ];

Resolved output (deduped, sorted):

  default = pkgs.mkShell {
    buildInputs = pkgs.go ++ pkgs.gopls;
  };

Example 4: nix/pre-commit.nix hook deep merge

Input files (both nix/pre-commit.nix):

Origin Template	Origin Layer	Content
base	0	prettier.enable = true; prettier.excludes = ["*.snap"];
frontend	1	prettier.enable = false; prettier.excludes = ["dist/*"];

Resolved output (enable true-wins, excludes concat+dedupe):

    prettier = {
      enable = true;
      excludes = [
        "*.snap"
        "dist/*"
      ];
    };

Example 5: Fallback LWW for unknown .nix files

Input files (both nix/overlay.nix):

Origin Template	Origin Layer	Content
base	0	self: super: { }
custom	1	self: super: { go = super.go_1_22; }

Resolved output (highest layer wins):

self: super: { go = super.go_1_22; }

Integration

Reference this resolver in a template's cyan.yaml:

resolvers:
  - resolver: atomi/nix:version
    config: {}
    files: ['**/*.nix']