difference of spawn() and coroutine?

Coroutines are a very powerful feature that allow two executions to "work together".

Two coroutines use coroutine.resume and coroutine.yield to communicate with each other and "hand off" execution.

The simplest way for two coroutines to "work together" is to completely ignore each other and just take turns doing work. This is how ROBLOX gets the appearance of "multitasking": wait() causes the current coroutine to stop working and lets another coroutine start working.

If you're only using coroutine to make two threads work "at the same time", don't use coroutine.

Instead, use spawn to create a "thread" in the background.

See this thread for examples of (not) using spawn to great effect.

Consider the following function that produces prime numbers:

-- a prime number is any number larger than 1 not divisible
-- any smaller (prime) number
function computePrimes()
	local primes = {}
	
	while #primes < 1000 do
		local isPrime = true
		for _, smallerPrime in ipairs(primes) do
			-- if `i` is divisible by `smallerPrime`
			if i % smallerPrime == 0 then
				isPrime = false
			end
		end

		if isPrime then
			table.insert(primes, i)
		end
	end

	return primes
end

The above function produces a list; you can use a simple loop to print out the first 1000 primes:

for _, prime in ipairs(computePrimes()) do
	print(prime)
end

Or you could do something like, compute the sum of all of the primes under 10000:

local sum = 0
for _, prime in ipairs(computePrimes()) do
	if prime >= 10000 then
		break
	end
	sum = sum + prime
end
print(sum)

There are two main problems with this approach.

1. computePrimes() might not have made enough primes; it's possible it stopped too early
2. computePrimes() might have wasted time making way more primes than we need; it's possible the loop doesn't even cover most of the list.

We could plumb this information into computePrimes, but that would make computePrimes much more complicated.

The solution is to use coroutines; then computePrimes will be a generator that produces 1 prime at a time:

primeComputer = coroutine.wrap(function()
	local primes = {}
	
	for i = 2, math.huge do
		local isPrime = true
		for _, smallerPrime in ipairs(primes) do
			-- if `i` is divisible by `smallerPrime`
			if i % smallerPrime == 0 then
				isPrime = false
			end
		end

		if isPrime then
			table.insert(primes, i)
			-- `prime` is the value that this generator
			-- will return each time it is called
			coroutine.yield(i)
		end
	end
end)

I use coroutine.wrap as a convenience, I won't give an example here that uses coroutine.resume / coroutine.create manually.

Now, we can write our loop and compute exactly the set of primes we want:

local sum = 0
while true do
	-- ask for one more prime
	local prime = primeComputer()

	if prime >= 10000 then
		break
	end
	sum = sum + prime
end

print(sum)