lib.types.either

The following is the current implementation of this function.

either =
    t1: t2:
    mkOptionType rec {
      name = "either";
      description =
        if t1.descriptionClass or null == "nonRestrictiveClause" then
          # Plain, but add comma
          "${t1.description}, or ${
            optionDescriptionPhrase (class: class == "noun" || class == "conjunction") t2
          }"
        else
          "${optionDescriptionPhrase (class: class == "noun" || class == "conjunction") t1} or ${
            optionDescriptionPhrase (
              class: class == "noun" || class == "conjunction" || class == "composite"
            ) t2
          }";
      descriptionClass = "conjunction";
      check = {
        __functor = _self: x: t1.check x || t2.check x;
        isV2MergeCoherent = true;
      };
      merge = {
        __functor =
          self: loc: defs:
          (self.v2 { inherit loc defs; }).value;
        v2 =
          { loc, defs }:
          let
            t1CheckedAndMerged =
              if t1.merge ? v2 then
                checkV2MergeCoherence loc t1 (t1.merge.v2 { inherit loc defs; })
              else
                {
                  value = t1.merge loc defs;
                  headError = checkDefsForError t1.check loc defs;
                  valueMeta = { };
                };
            t2CheckedAndMerged =
              if t2.merge ? v2 then
                checkV2MergeCoherence loc t2 (t2.merge.v2 { inherit loc defs; })
              else
                {
                  value = t2.merge loc defs;
                  headError = checkDefsForError t2.check loc defs;
                  valueMeta = { };
                };

            checkedAndMerged =
              if t1CheckedAndMerged.headError == null then
                t1CheckedAndMerged
              else if t2CheckedAndMerged.headError == null then
                t2CheckedAndMerged
              else
                rec {
                  valueMeta = {
                    inherit headError;
                  };
                  headError = {
                    message = "The option `${showOption loc}` is neither a value of type `${t1.description}` nor `${t2.description}`, Definition values: ${showDefs defs}";
                  };
                  value = lib.warn ''
                    One or more definitions did not pass the type-check of the 'either' type.
                    ${headError.message}
                    If `either`, `oneOf` or similar is used in freeformType, ensure that it is preceded by an 'attrsOf' such as: `freeformType = types.attrsOf (types.either t1 t2)`.
                    Otherwise consider using the correct type for the option `${showOption loc}`.  This will be an error in Nixpkgs 26.05.
                  '' (mergeOneOption loc defs);
                };
          in
          checkedAndMerged;
      };
      typeMerge =
        f':
        let
          mt1 = t1.typeMerge (elemAt f'.payload.elemType 0).functor;
          mt2 = t2.typeMerge (elemAt f'.payload.elemType 1).functor;
        in
        if (name == f'.name) && (mt1 != null) && (mt2 != null) then functor.type mt1 mt2 else null;
      functor = elemTypeFunctor name {
        elemType = [
          t1
          t2
        ];
      };
      nestedTypes.left = t1;
      nestedTypes.right = t2;
    };