app.rb

require 'sinatra'
require 'haml'
require 'better_errors'

configure :development do
  use BetterErrors::Middleware
  BetterErrors.application_root = __dir__
end

get '/' do
  @title = 'HAML Test'
  haml :index
end

config.ru

require './app'
run Sinatra::Application

Gemfile

source "https://rubygems.org"

group :development do
  gem "better_errors"
  gem "binding_of_caller"
  gem "rerun"
end

gem "sinatra"
gem "haml"

index.haml

%h1 Title: #{@title}
%p Hello HAML!

layout.haml

!!! 5
%html
  %head
    %meta{ charset: 'utf-8' }
    %title= @title
    %link{ rel: 'stylesheet', href: '//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css' }
    %link{ rel: 'stylesheet', href: '//maxcdn.bootstrapcdn.com/font-awesome/4.2.0/css/font-awesome.min.css' }
    %script{ src: '//code.jquery.com/jquery-2.1.1.min.js' }
    :css
      header { border-bottom: 1px solid #eee; }
      main { margin-top: 25px; }
  %body
    .container
      %header
        %h1.text-muted= @title
      %main
        = yield