lib.types.nullOr

The following is the current implementation of this function.

nullOr = elemType: mkOptionType rec {
      name = "nullOr";
      description = "null or ${optionDescriptionPhrase (class: class == "noun" || class == "conjunction") elemType}";
      descriptionClass = "conjunction";
      check = x: x == null || elemType.check x;
      merge = loc: defs:
        let nrNulls = count (def: def.value == null) defs; in
        if nrNulls == length defs then null
        else if nrNulls != 0 then
          throw "The option `${showOption loc}` is defined both null and not null, in ${showFiles (getFiles defs)}."
        else elemType.merge loc defs;
      emptyValue = { value = null; };
      getSubOptions = elemType.getSubOptions;
      getSubModules = elemType.getSubModules;
      substSubModules = m: nullOr (elemType.substSubModules m);
      functor = (defaultFunctor name) // { wrapped = elemType; };
      nestedTypes.elemType = elemType;
    };