domain.go

package domain

type Name struct {
	value string
}

func NewName(name string) (Name, error) {
	if len(name) < 3 || len(name) > 100 {
		return Name{}, fmt.Errorf("%w: name length must be less than 3 and gather than 100", ErrInvalidNameLength)
	}

	return Name{value: name}, nil
}

func (n Name) String() string {
	return n.value
}

type UserID struct {
	value int64
}

func NewUserID(value int64) (UserID, error) {
	if value <= 0 {
		return UserID{}, fmt.Errorf("%w: user id must be greater than 0, but got %d", ErrInvalidID, value)
	}

	return UserID{value: value}, nil
}

func (uid UserID) Int64() int64 {
	return cid.value
}

handlers.go

package service

type CreateUserCommand struct {
	Name      domain.Name
}

func NewCreateUserCommand(userName string) (*CreateUserCommand, error) {
	name, err := domain.NewName(userName)
	// handle err

	return &CreateUserCommand{
		Name:      name,
	}, nil
}

func (s *Service) CreateUser(ctx context.Context, cmd *CreateUserCommand) (domain.UserID, error) {
	id, err := s.repo.NextID(ctx)
    // handle err

    user := domain.NewUser(id, cmd.Name, nil)

    err = s.repo.Save(ctx, user)
    // handle err

    return id, nil
}

presentation.go

package presentation

func (h *Handlers) CreateUser(
	ctx context.Context,
	req *grpc.Request,
) (*grpc.NewUserResponse, error) {
	cmd, err := service.NewCreateUsertCommand(
		req.GetName(),
	)
    // handle err

	userID, err := h.service.CreateUser(ctx, cmd)
	// handle err

	return &grpc.CreateUserResponse{UserId: userID.Int64()}, nil
}