lib.types.enum

The following is the current implementation of this function.

enum =
        values:
        let
          inherit (lib.lists) unique;
          show =
            v:
            if builtins.isString v then
              ''"${v}"''
            else if builtins.isInt v then
              builtins.toString v
            else if builtins.isBool v then
              boolToString v
            else
              ''<${builtins.typeOf v}>'';
        in
        mkOptionType rec {
          name = "enum";
          description =
            # Length 0 or 1 enums may occur in a design pattern with type merging
            # where an "interface" module declares an empty enum and other modules
            # provide implementations, each extending the enum with their own
            # identifier.
            if values == [ ] then
              "impossible (empty enum)"
            else if builtins.length values == 1 then
              "value ${show (builtins.head values)} (singular enum)"
            else
              "one of ${concatMapStringsSep ", " show values}";
          descriptionClass = if builtins.length values < 2 then "noun" else "conjunction";
          check = flip elem values;
          merge = mergeEqualOption;
          functor = (defaultFunctor name) // {
            payload = { inherit values; };
            type = payload: types.enum payload.values;
            binOp = a: b: { values = unique (a.values ++ b.values); };
          };
        };