the usage of resume and yield

test_routine.lua

local function testRoutine()
   co = coroutine.create(function (x)
          for i = 1, 10 do
--            print("co", i)
            coroutine.yield(i)
          end
        end)
   print(coroutine.resume(co))
   print(coroutine.resume(co))
   print(coroutine.resume(co))
end

--[[
A call to resume returns, after the true that signals no errors, 
any arguments passed to the corresponding yield.
Note, though from the point of the coroutine, the yield has not 
yet return back when suspended, but arguments passed to yield will 
be passed to the resume, and resume will return those values.
output:
true 30 10
--]]

local function testRoutine1()
    co = coroutine.create(function (a, b)
        coroutine.yield (a+b, a-b)
       end
    )
  print(coroutine.resume(co, 20, 10))
end
--[[
Symmetrically, yield returns any extra arguments passed to the corresponding resume:
output:
co1 hi
true  mark
co2 1 2
--]]
local function testRoutine2()
    co = coroutine.create(function (x)
    print("co1", x)
    print("co2", coroutine.yield("mark"))   
        end)
  print(coroutine.resume(co, "hi"))
  coroutine.resume(co, 1, 2)
end

local function main()
--  testRoutine()
--  testYield()
  testRoutine2()
end

main()