awkward_decorator.rb

def memoize(&blk)
  cache = {}
  lambda do |*args|
    return cache[args]  if cache.has_key?(args)
    cache[args] = blk.call(*args)
  end
end

define_singleton_method(:fib, &memoize{ |n|
  if n < 2
    n
  else
    fib(n-2) + fib(n-1)
  end
})

puts fib(45)