A Ruby Version of Steve Losh's Lightning Talk at BACON 2012: Pulling Lists out of Thin Air

magic-lists.rb

# Pulling List's out of Thin Air
# Ruby Implementation of Steve Losh's Lightning Talk at BACON 2012

def prepend item, list
  ->(h) {
    return h ? item : list
  }
end

def head list
  list.call(true)
end

def tail list
  list.call(false)
end

def get list, index
  if index == 0
    head(list)
  else
    get(tail(list), index - 1)
  end
end

def length list
  if list.nil?
    0
  else
    1 + length(tail(list))
  end
end

def append item, list
  if list.nil?
    prepend(item, list)
  else
    prepend(head(list), append(item, tail(list)))
  end
end

def reverse list
  if list.nil?
    nil
  else
    append(head(list), reverse(tail(list)))
  end
end

greeting = nil
greeting = append("Hello", greeting)
greeting = append("BACON", greeting)
greeting = append("I'm Chris", greeting)

puts "greeting.size: #{length(greeting)}"
puts "greeting[0]: #{get(greeting, 0)}"
puts "greeting[1]: #{get(greeting, 1)}"
puts "greeting.reverse[0]: #{get(reverse(greeting), 0)}"