What are Lua coroutines even for? Why doesn't this code work as I expect it?

Question

I\'m having trouble understanding this code... I was expecting something similar to threading where I would get an output with random \"nooo\" and \"yaaaay\"s interspersed w

Answer 1

Since there have been a number of comments asking how to implement the wait function that would make deft_code's example work, I've decided to write a possible implementation. The general idea is that we have a scheduler with a list of coroutines, and the scheduler decides when to return control to the coroutines after they give up control with their wait calls. This is desirable because it makes asynchronous code be readable and easy to reason about.

This is only one possible use of coroutines, they are a more general abstraction tool that can be used for many different purposes (such as writing iterators and generators, writing stateful stream processing objects (for example, multiple stages in a parser), implementing exceptions and continuations, etc.).

First: the scheduler definition:

local function make_scheduler()
    local script_container = {}
    return {
        continue_script = function(frame, script_thread)
            if script_container[frame] == nil then
                script_container[frame] = {}
            end
            table.insert(script_container[frame],script_thread)
        end,
        run = function(frame_number, game_control)
            if script_container[frame_number] ~= nil then
                local i = 1
                --recheck length every time, to allow coroutine to resume on
                --the same frame
                local scripts = script_container[frame_number]
                while i <= #scripts do
                    local success, msg =
                        coroutine.resume(scripts[i], game_control)
                    if not success then error(msg) end
                    i = i + 1
                end
            end
        end
    }
end

Now, initialising the world:

local fps = 60
local frame_number = 1
local scheduler = make_scheduler()

scheduler.continue_script(frame_number, coroutine.create(function(game_control)
    while true do
        --instead of passing game_control as a parameter, we could
        --have equivalently put these values in _ENV.
        game_control.wait(game_control.seconds(5))
        game_control.start_eruption_volcano()
        game_control.wait(game_control.frames(10))
        s = game_control.play("rumble_sound")
        game_control.wait( game_control.end_of(s) )
        game_control.start_camera_shake()

        -- more stuff

        game_control.wait(game_control.minutes(2))
    end
end))

The (dummy) interface to the game:

local game_control = {
    seconds = function(num)
        return math.floor(num*fps)
    end,
    minutes = function(num)
        return math.floor(num*fps*60)
    end,
    frames = function(num) return num end,
    end_of = function(sound)
        return sound.start+sound.duration-frame_number
    end,
    wait = function(frames_to_wait_for)
        scheduler.continue_script(
            frame_number+math.floor(frames_to_wait_for),
            coroutine.running())
        coroutine.yield()
    end,
    start_eruption_volcano = function()
        --obviously in a real game, this could 
        --affect some datastructure in a non-immediate way
        print(frame_number..": The volcano is erupting, BOOM!")
    end,
    start_camera_shake = function()
        print(frame_number..": SHAKY!")
    end,
    play = function(soundname)
        print(frame_number..": Playing: "..soundname)
        return {name = soundname, start = frame_number, duration = 30}
    end
}

And the game loop:

while true do
    scheduler.run(frame_number,game_control)
    frame_number = frame_number+1
end