Flash

Private mechanism to send data across requests.

### Use case

```ruby
require 'lotus/controller'

module Comments
  class Index
    include Lotus::Action
    include Lotus::Action::Session

    expose :comments

    def call(params)
      @comments = CommentRepository.all
    end
  end

  class Create
    include Lotus::Action
    include Lotus::Action::Session

    params do
      param :text, type: String, presence: true
    end

    def call(params)
      comment = Comment.new(params)
      CommentRepository.create(comment) if params.valid?

      redirect_to '/comments'
    end
  end
end
```

The validation errors caused by `Comments::Create` are available **after the redirect** in the context of `Comments::Index`.

Again, this is marked as private API.

Closes #10