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A flash. For your Rack apps. env["rack.session"][:flash] looks like a hash. Put something in it (like an error message), and you can get it later. Once you've retrieved what you stored, it'll be cleared out at the end of the session. So if in one request you

env["rack.session"][:flash][:error] = "Email address is blank."

and in the next request (maybe caused by redirecting after the last one) you render

- if env["rack.session"][:flash][:error]
  #error= env["rack.session"][:flash][:error]

(that's Haml) you'll see your error message in the second request. When the user's next request comes, the message will be gone.

Using Rack::Flash

Rack::Flash is a Rack middleware. How you use it depends on how you run your Rack app. If you're running a Rackup file, just add the following somewhere before your run command:

require 'rack/flash'
use Rack::Flash

However, Rack::Flash needs a session to work its magic, so make sure you use one before Rack::Flash. Something like:

require 'rack/flash'
use Rack::Session::Cookie
use Rack::Flash

(Rack comes with a few session middlewares to choose from; any of them will work.) Your session key will have to be the default, "rack.session". This might become configurable one day.

If you're running Sinatra, you've already got a session for free. Add the code above (the one without the session line) to your Sinatra file near the top. Also, Sinatra gives you a helper to access the session, so for you, the flash is as easy as

session[:flash][:error] = "Email address is blank."
# ...
haml "#error= session[:flash][:error]"

Full Example

Here's a full Rackup file demonstrating Rack::Flash. It's also in the tree as examples/ To run it, run rackup examples/

When you go to http://localhost:9292/ you'll see links to /view_error and /cause_error. The former displays the current error stored in the flash. The latter puts an error in the flash and redirects to /view_error. If you just go to /view_error, you won't see an error. If you go via /cause_error you'll see one, but the next time you go to /view_error the error message will be gone. That's the magic of the Flash!

require 'rack'
require 'rack/flash'

use Rack::Session::Cookie
use Rack::Flash

app = lambda do |env|
  case env["PATH_INFO"]
  when "/cause_error"
    env["rack.session"][:flash][:error] = "Something went wrong."
    [302, {"Content-type" => "text/html", "Location" => "/view_error"}, ""]

  when "/view_error"
    [200, {"Content-type" => "text/html"}, "Current Error: #{env["rack.session"][:flash][:error]}"]

    message  = "Try going to <a href='/view_error'>/view_error</a> "
    message += "or <a href='/cause_error'>/cause_error</a>"
    [200, {"Content-type" => "text/html"}, message]

run app

(This project is not affiliated in any way with

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