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1 Symmetric Coroutine

Lua支持asymmetric coroutine。對于symmetric coroutine，名為Coroutines in Lua的一篇論文，提供了如下的解決方案：

coro = {}
coro.main = function() end
coro.current = coro.main

function coro.create(f) 
  return coroutine.wrap(function(val) return nil, f(val) end)
end

function coro.transfer(k, val)
  if coro.current ~= coro.main then
    return coroutine.yield(k, val)
  else
    while k do
      coro.current = k
      if k == coro.main then
        return val
      end
      k, val = k(val)
    end
    error("coroutine ended without transfering control...")
  end
end

代碼雖然不長，但是十分精致。通過yield/resume，以及一個dipatching循環，巧妙地實現了symmetric coroutine。此外在Revisiting Coroutines論文中，甚至用asymmetric coroutine實現了one-shot continuation。

2 Reference

Coroutines in Lua Ana L´ucia de Moura , Noemi Rodriguez , Roberto Ierusalimschy
Revisiting Coroutines Ana L´ucia de Moura and Roberto Ierusalimschy