callcc.lua

-- An implementation of call-with-current-continuation in Lua.
-- (c) 2009 Zachary Voase

function callcc (fun)
  
  function coro ()
    -- The coroutine which will hold the value with which it is called, then
    -- yield it back again later.
    local val = coroutine.yield()
    coroutine.yield()
    coroutine.yield(val)
  end
  
  cont = coroutine.create(coro)
  coroutine.resume(cont) -- Gets us to the first coroutine.yield() call.
  
  -- Callback sends the args it is called with into the coroutine.
  function callback (...)
    return coroutine.resume(cont, arg)
  end
  
  fun(callback)
  
  -- Status and value pulled out of the coroutine.
  local status, value = coroutine.resume(cont)
  return unpack(value)
end