interface SqmFunctionTemplateRegistry {
  void register(String name, SqmFunctionTemplate functionTemplate);

  /** I think `#builder` is better than `#build`... more accurately describes what we are doing */
  SqmFunctionTemplateBuilder patternTemplateBuilder(String name, String pattern);
  ...
}

interface SqmFunctionTemplateBuilder {
  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // convenient in that only one "factory"
  //       - but meh
  default SqmFunctionTemplateBuilder minArgumentCount(int minCount) {
    return argumentValidator( ArgumentsValidator.min( minCount ) );
  }

  default SqmFunctionTemplateBuilder maxArgumentCount(int maxCount) {
    return argumentValidator( ArgumentsValidator.max( minCount ) );
  }

  default SqmFunctionTemplateBuilder ofTypes(Class<?>... types) {
    return argumentValidator( ArgumentsValidator.types( types ) );
  }

  default SqmFunctionTemplateBuilder ofType(Class<?> type) {
    return argumentValidator( ArgumentsValidator.type( type ) );
  }
  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  SqmFunctionTemplateBuilder argumentValidator(ArgumentValidator validator);

  SqmFunctionTemplateBuilder functionReturn(Class<?> javaType);

  ...

  void register();
}