amb.rb

require "continuation"

$breadcrumbs = []

def current_continuation
  callcc { |cc| cc }
end

def fail!
  raise "No more breadcrumbs to follow" if $breadcrumbs.empty?
  next_stop = $breadcrumbs.pop
  next_stop.call next_stop
end

def amb(*choices)
  current_stop = current_continuation
  if choices.empty?
    fail!
  else
    $breadcrumbs.push current_stop if current_stop
    choices.shift
  end
end

def assert!(cond)
  unless cond
    fail!
  else
    true
  end
end