Lua Coroutines Create and use a coroutine

Example

All functions to interact with coroutines are avaliable in the coroutine table. A new coroutine is created by using the coroutine.create function with a single argument: a function with the code to be executed:

thread1 = coroutine.create(function()
            print("honk")
        end)

print(thread1)
-->> thread: 6b028b8c

A coroutine object returns value of type thread, representing a new coroutine. When a new coroutine is created, its initial state is suspended:

print(coroutine.status(thread1))
-->> suspended

To resume or start a coroutine, the function coroutine.resume is used, the first argument given is the thread object:

coroutine.resume(thread1)
-->> honk

Now the coroutine executes the code and terminates, changing its state to dead, wich cannot be resumed.

print(coroutine.status(thread1))
-->> dead

Coroutines can suspend its execution and resume it later thanks to the coroutine.yield function:

thread2 = coroutine.create(function()
        for n = 1, 5 do
            print("honk "..n)
            coroutine.yield()
        end
    end)

As you can see, coroutine.yield() is present inside the for loop, now when we resume the coroutine, it will execute the code until it reachs a coroutine.yield:

coroutine.resume(thread2)
-->> honk 1
coroutine.resume(thread2)
-->> honk 2

After finishing the loop, the thread status becomes dead and cannot be resumed. Coroutines also allows the exchange between data:

thread3 = coroutine.create(function(complement)
    print("honk "..complement)
    coroutine.yield()
    print("honk again "..complement)
end)
coroutine.resume(thread3, "stackoverflow")
-->> honk stackoverflow

If the coroutine is executed again with no extra arguments, the complement will still the argument from the first resume, in this case "stackoverflow":

coroutine.resume(thread3)
-->> honk again stackoverflow

Finally, when a coroutine ends, any values returned by its function go to the corresponding resume:

thread4 = coroutine.create(function(a, b)
    local c = a+b
    coroutine.yield()
    return c
end)
coroutine.resume(thread4, 1, 2)
print(coroutine.resume(thread4))
-->> true, 3

Coroutines are used in this function to pass values back to a calling thread from deep within a recursive call.

local function Combinations(l, r)
    local ll = #l
    r = r or ll
    local sel = {}
    local function rhelper(depth, last)
        depth = depth or 1
        last = last or 1
        if depth > r then
            coroutine.yield(sel)
        else
            for i = last, ll - (r - depth) do
                sel[depth] = l[i]
                rhelper(depth+1, i+1)
            end
        end
    end
    return coroutine.wrap(rhelper)
end

for v in Combinations({1, 2, 3}, 2) do
    print("{"..table.concat(v, ", ").."}")
end
--> {1, 2}
--> {1, 3}
--> {2, 3}

Coroutines can also be used for lazy evaluation.

-- slices a generator 'c' taking every 'step'th output from the generator
-- starting at the 'start'th output to the 'stop'th output
function slice(c, start, step, stop)
    local _
    return coroutine.wrap(function()
        for i = 1, start-1 do
            _ = c()
        end
        for i = start, stop do
            if (i - start) % step == 0 then
                coroutine.yield(c())
            else
                _ = c()
            end
        end
    end)
end


local alphabet = {}
for c = string.byte('a'), string.byte('z') do
    alphabet[#alphabet+1] = string.char(c)
end
-- only yields combinations 100 through 102
-- requires evaluating the first 100 combinations, but not the next 5311633
local s = slice(Combinations(alphabet, 10), 100, 1, 102)
for i in s do
    print(table.concat(i))
end
--> abcdefghpr
--> abcdefghps
--> abcdefghpt

Coroutines can be used for piping constructs as described in Programming In Lua. The author of PiL, Roberto Ierusalimschy, has also published a paper on using coroutines to implement more advanced and general flow control mechanics like continuations.