lib.modules.mergeDefinitions

The following is the current implementation of this function.

mergeDefinitions = loc: type: defs: rec {
    defsFinal' =
      let
        # Process mkMerge and mkIf properties.
        defs' = concatMap (m:
          map (value: { inherit (m) file; inherit value; }) (addErrorContext "while evaluating definitions from `${m.file}':" (dischargeProperties m.value))
        ) defs;

        # Process mkOverride properties.
        defs'' = filterOverrides' defs';

        # Sort mkOrder properties.
        defs''' =
          # Avoid sorting if we don't have to.
          if any (def: def.value._type or "" == "order") defs''.values
          then sortProperties defs''.values
          else defs''.values;
      in {
        values = defs''';
        inherit (defs'') highestPrio;
      };
    defsFinal = defsFinal'.values;

    # Type-check the remaining definitions, and merge them. Or throw if no definitions.
    mergedValue =
      if isDefined then
        if all (def: type.check def.value) defsFinal then type.merge loc defsFinal
        else let allInvalid = filter (def: ! type.check def.value) defsFinal;
        in throw "A definition for option `${showOption loc}' is not of type `${type.description}'. Definition values:${showDefs allInvalid}"
      else
        # (nixos-option detects this specific error message and gives it special
        # handling.  If changed here, please change it there too.)
        throw "The option `${showOption loc}' was accessed but has no value defined. Try setting the option.";

    isDefined = defsFinal != [];

    optionalValue =
      if isDefined then { value = mergedValue; }
      else {};
  };