gen-merge

gen-merge is a pure-Nix, nixpkgs.lib-free module merge engine — the drop-in replacement for lib.evalModules plus lib.types-merge in the pure-gen module system. evalModuleTree collects a tree of modules, ties the self-referential config fixpoint, and resolves per-option definitions by priority.

The problem it owns: NixOS’s own module system is powerful but drags nixpkgs.lib and its whole option-type machinery along with it. gen-merge is that machinery, rebuilt without the dependency, so the pure substrate can have module merging without pulling in nixpkgs to get it.

It depends on gen-memo, gen-prelude, and gen-types — gen-memo for the incremental decision layer folded into evaluation, gen-types for the leaf checkers evalModuleTree accepts. Two roster members build on gen-merge in turn: gen-aspects and gen-schema — the latter being the roster’s one ruled exception to the strata ordering, reaching sideways from substrate into modules for exactly this dependency.

Take it alone

let
  genMerge = import (fetchGit "https://github.com/sini/gen-merge").outPath {
    prelude = genPrelude;
    types = genTypes;               # the leaf checkers
  };
  inherit (genMerge) evalModuleTree mkOption mkForce;
  t = genMerge.types;

  result = evalModuleTree {
    modules = [
      { options.name = mkOption { type = t.str; default = "anon"; }; }
      { name = mkForce "pinned"; }
    ];
  };
in
  result.config                     # ⇒ { name = "pinned"; }

evalModuleTree returns { config; options; type; provenance; undeclared; deprecations }. .config is the merged output; .options is the merged descriptor map; .provenance is a lazy per-location record of where each value came from.