condition.rb

# Raises a condition. If a handler is set up, it can restart the condition
# and return to this place.
#
# name - The Symbol name of the condition.
#
# Returns nothing if the condition was restarted, and doesn't return at all
#   otherwise.
def condition(name)
  callcc do |cont|
    throw name, cont
  end
end

# Handles a condition, possibly restarting it.
#
# name - The Symbol name of the condition.
# options - A Hash of options:
#           :restart - true if the condition should be restarted, false otherwise.
#
# Returns nothing.
def handle(name, options = {})
  cont = catch(name) do
    yield

    # If no conditions were raised, this will return from the method.
    return
  end

  # We'll only reach these lines if a condition was raised. If :restart => true
  # was passed, we resume the place where #condition was called. Otherwise, we
  # just ignore it. We could possibly:
  #
  # - Raise an exception.
  # - Do some logging.
  # - Perform a use-specific restart, e.g. passing a directive to the raise site,
  #   such as "use this value", "skip the item", etc.
  if options[:restart]
    cont.call
  end
end


handle :deprecation, :restart => false do
  puts "A"
  condition :deprecation
  puts "B"
end
# Outputs:
#
#  A

handle :deprecation, :restart => true do
  puts "A"
  condition :deprecation
  puts "B"
end
# Outputs:
#
#  A
#  B

goto.rb

$LABELS = {}

def label(name)
  callcc do |cont|
    $LABELS[name] = cont
  end
end

def goto(name)
  $LABELS[name].call
end


i = 0

label :loop

puts "LOOPIN'!"
i += 1

goto :loop if i < 100