Problems with types.oneOf and submodules

GrafBlutwurst · February 8, 2022, 4:16pm

I just had the same issue and found this post. After a couple hours of digging through the sources of types.nix I think I understand what’s happening. There’s two pertinent points where it goes wrong.

First the merge function of either / oneOf at

github.com

NixOS/nixpkgs/blob/ba3e91ed43c05a4a0984a6faa948949612fd113c/lib/types.nix#L644


      
              check = flip elem values;
              merge = mergeEqualOption;
              functor = (defaultFunctor name) // { payload = values; binOp = a: b: unique (a ++ b); };
            };
          
          
# Either value of type `t1` or `t2`.
          either = t1: t2: mkOptionType rec {
            name = "either";
            description = "${t1.description} or ${t2.description}";
            check = x: t1.check x || t2.check x;
            merge = loc: defs:
              let
                defList = map (d: d.value) defs;
              in
                if   all (x: t1.check x) defList
                     then t1.merge loc defs
                else if all (x: t2.check x) defList
                     then t2.merge loc defs
                else mergeOneOption loc defs;
            typeMerge = f':
              let mt1 = t1.typeMerge (elemAt f'.wrapped 0).functor;

merge = loc: defs:
  let
    defList = map (d: d.value) defs;
  in
  if all (x: t1.check x) defList
  then t1.merge loc defs
  else if all (x: t2.check x) defList
  then t2.merge loc defs
  else mergeOneOption loc defs;

It checks whether all defs that are to be merged are either of the left type (t1) or the right type (t2). This makes sense since if we’re merging we only want to have one side of the either, presumably else the merging is probably not well defined.

Now if submodules come into play inside the either we’ll have to look at how those do check at:

github.com

NixOS/nixpkgs/blob/ba3e91ed43c05a4a0984a6faa948949612fd113c/lib/types.nix#L568


      
                _module.args.name = lib.mkOptionDefault "‹name›";
              }] ++ modules;
            };
          
          
  freeformType = base._module.freeformType;
          
          
in
          mkOptionType rec {
            name = "submodule";
            description = freeformType.description or name;
            check = x: isAttrs x || isFunction x || path.check x;
            merge = loc: defs:
              (base.extendModules {
                modules = [ { _module.args.name = last loc; } ] ++ allModules defs;
                prefix = loc;
              }).config;
            emptyValue = { value = {}; };
            getSubOptions = prefix: (base.extendModules
              { inherit prefix; }).options // optionalAttrs (freeformType != null) {
                # Expose the sub options of the freeform type. Note that the option
                # discovery doesn't care about the attribute name used here, so this

check = x: isAttrs x || isFunction x || path.check x;

This is where it breaks all appart if both your type parameters to either are submodules because isAttrs is going to be true thus the merge in either will always think it’s the left type parameter (t1).

I hacked together a solution that seems to work so far but I only just got done so I don’t know yet if it’s just gonna break other things but here it is: A hacky fix for the either submodule problem · GitHub