Redd Webapp Example (from https://redd.it/61p1hj)

app.rb

require 'bundler/setup'
require 'sinatra'
require 'redd/middleware'

use Rack::Session::Cookie
use Redd::Middleware,
    user_agent:   'Redd:Your App:v1.0.0 (by /u/<your username>)',
    client_id:    '<your client id>',
    secret:       '<your app secret>',
    redirect_uri: '<your redirect uri>',
    scope:        %w(identity),
    via:          '/login'

get '/' do
  '<a href="/login">login with reddit</a>'
end

get '/redirect' do
  redirect to('/profile') if request.env['redd.error'].nil?

  if request.env['redd.error'].message == 'access_denied'
    'Sorry, you clicked decline. <a href="/login">Login again?</a>'
  elsif request.env['redd.error'].message == 'invalid_state'
    'Did you login through our website? <a href="/login">(No)</a>'
  else
    puts "Error while logging in!"
    raise request.env['redd.error'] # Raise a 500 and make a mental note to look at the logs later
  end
end

get '/profile' do
  "Hi, #{request.env['redd.session'].me.name}! <a href='/logout'>Log out</a>"
end

get '/logout' do
  request.env['redd.session'] = nil
  redirect to('/')
end

Gemfile

source 'https://rubygems.org'

gem 'sinatra', '2.0.0.rc2' # Currently the latest Sinatra.
gem 'redd', '~> 0.8'