This post originated from an RSS feed registered with Ruby Buzz by Eigen Class. Original Post: Continuations are so much easier to grasp than 2ch and Japanese Feed Title: Eigenclass Feed URL: http://feeds.feedburner.com/eigenclass Feed Description: Ruby stuff --- trying to stay away from triviality. Latest Ruby Buzz Posts Latest Ruby Buzz Posts by Eigen Class Latest Posts From Eigenclass

ko1 (of YARV fame) pointed me to an interesting question on 2ch (a large Japanese BBS).

Given some #open() and #close() methods, you can define trivially

def open_block(fn)
  fh = open(fn)
  yield fh
  close(fh)
end 

which accepts a block, passes the file handler corresponding to a given filename and closes it when done (you'd normally use an ensure clause, but the original message carried the above definition). This is a standard idiom. Now, given a working #open_block, can you define #open and #close?

As I read that, I knew it would involve callcc. #open was trivial, #close seemed harder. Actually hard enough to deserve being done. I was game.

It was pretty late (~1AM), but callcc is always fun and anyway it would only take a few minutes to solve, so I quickly came up with

def _open(name); puts "open: #{name}"; "IO<#{name}>" end
def _close(name); puts "close: #{name}" end

def open_block(fn); fh = _open(fn); yield fh; _close(fh); end

open_block("hoge"){|x| puts "Got #{x}" }

def open(x)
  callcc do |cc|
    r = nil
    open_block(x) do |fh|
      r = callcc do |cc2|
        fh.instance_variable_set("@_closecc", cc2)
        cc.call(fh)
      end
    end
    r.call
  end
end

def close(x)
  callcc do |cc|
    x.instance_variable_get("@_closecc").call(cc)
  end
end

a = open("foo")
puts "----"
close(a)
puts "end"