class Rack::ShowExceptions

def pretty(env, exception)
  req = Rack::Request.new(env)
  # This double assignment is to prevent an "unused variable" warning.
  # Yes, it is dumb, but I don't like Ruby yelling at me.
  path = path = (req.script_name + req.path_info).squeeze("/")
  # This double assignment is to prevent an "unused variable" warning.
  # Yes, it is dumb, but I don't like Ruby yelling at me.
  frames = frames = exception.backtrace.map { |line|
    frame = OpenStruct.new
    if line =~ /(.*?):(\d+)(:in `(.*)')?/
      frame.filename = $1
      frame.lineno = $2.to_i
      frame.function = $4
      begin
        lineno = frame.lineno - 1
        lines = ::File.readlines(frame.filename)
        frame.pre_context_lineno = [lineno - CONTEXT, 0].max
        frame.pre_context = lines[frame.pre_context_lineno...lineno]
        frame.context_line = lines[lineno].chomp
        frame.post_context_lineno = [lineno + CONTEXT, lines.size].min
        frame.post_context = lines[lineno + 1..frame.post_context_lineno]
      rescue
      end
      frame
    else
      nil
    end
  }.compact
  template.result(binding)
end