Extensions

    public static class ModuleHandleExtensions
    {
        private static readonly Logger logger = LogManager.GetCurrentClassLogger();

        public static object Handle<T>(
            this NancyModule m,
            Action<T> handle,
            Func<T, object> successResult,
            Func<T, object> failureResult = null)
        {
            if (successResult == null) throw new Exception("Result not set");
            if (failureResult == null) failureResult = successResult;

            var model = m.Bind<T>();
            if (logger.IsTraceEnabled)
            {
                logger.Trace("Executing {0}: {1}".Fmt(m.Request.Url, JsonConvert.SerializeObject(model)));
            }
            try
            {
                handle(model);
                return successResult(model);
            }
            catch (ValidationException ex)
            {
                m.ModelValidationResult = CreateValidationResult(ex);
                return failureResult(model);
            }
        }

        public static object Handle<T, TR>(
            this NancyModule m,
            Func<T, TR> handle,
            Func<T, TR, object> successResult,
            Func<T, TR, object> failureResult = null)
        {
            if (successResult == null) throw new Exception("Result not set");
            if (failureResult == null) failureResult = successResult;

            var model = m.Bind<T>();
            if (logger.IsTraceEnabled)
            {
                logger.Trace("Executing {0}: {1}".Fmt(m.Request.Url, JsonConvert.SerializeObject(model)));
            }
            try
            {
                var result = handle(model);
                return successResult(model, result);
            }
            catch (ValidationException ex)
            {
                m.ModelValidationResult = CreateValidationResult(ex);
                return failureResult(model, default(TR));
            }
        }

        private static ModelValidationResult CreateValidationResult(ValidationException ex)
        {
            var errors = GetErrors(ex.Errors);
            return new ModelValidationResult(errors);
        }

        private static IEnumerable<ModelValidationError> GetErrors(IEnumerable<ValidationFailure> errors)
        {
            return errors.Select(error => new ModelValidationError(new[] { error.PropertyName }, error.ErrorMessage));
        }
    }

Nancy + MediatR

    public class LoginModule : NancyModule
    {
        public LoginModule(IMediator mediator)
        {
            Get["/login"] = _ => View["login"];
            Post["/login"] = _ => this.Handle<Login.Model, Guid>(
                mediator.Send,
                (m, r) => this.LoginAndRedirect(r),
                (m, r) => View["login", m]);

            Get["/logout"] = _ => this.LogoutAndRedirect("~/");
        }
    }