about coroutines

Posted by Kevnuke on Mon 05 Aug 2019 08:27 PM — 6 posts, 25,201 views.

Kevnuke USA Mon 05 Aug 2019 08:27 PM #0

I've been reading about coroutines in the Lua 5.1 docs and had a few questions about it.

1) Is a coroutine related to multithreading or spawning a separate process? In other words, does it enable the use of multiple cores to allow the main process to happen on one core, and another process to happen in parallel, on a different core?

2) Do you recommend using them routinely instead of state variables? Pause a script with yield and resume it later when a condition is met, rather than repeatedly using an if statement to check the value of a variable (and wasting processor time as a result)?

3) What else are coroutines commonly used for, or are they a niche feature of the language?

Nick Gammon Australia Forum Administrator Mon 05 Aug 2019 10:17 PM #1

No. They are just co-operative multitasking. It is a way of pausing a script and keeping its state.
You have to have some reason to resume the script (and therefore the coroutine). Whatever that reason is, you will have to test for it, so you don't really get much further ahead by pausing a script rather than starting a new one.
They are useful in situations where there might be pauses, as illustrated here: http://www.gammon.com.au/forum/?id=4957 and here: http://www.gammon.com.au/forum/?id=4956

That lets you have a simple loop in a script, for example gathering lines of text from an inventory and building them into a table. The coroutine used there in the "wait" module just simplifies the code, it isn't really there to save time.

Kevnuke USA Mon 05 Aug 2019 11:33 PM #2

Alright, I know there's an example in PIL 9.4 using luasocket and coroutines to download multiple files simultaneously, but it's using HTTP instead of HTTPS. There's an API for characters in Achaea at https://api.achaea.com/characters.json that lists every online character's name and a url, such as https://api.achaea.com/characters/aegoth.json. You can follow each link to see basic info about that character.

I wanted to have a Lua table which stored that information for each character but I don't know how to go about it. Do I have to download and save the files before working with the data in them or can I do it in memory? I'd rather not have a local file for every logged-in character just to populate a Lua table with that data. I guess I'm looking for the simplest way to access the object at each url. So in the end I'd have this for each character:


player_data = {}
player_data.aegoth = {
    name          = "Aegoth",
    fullname      = "Holocaust King Aegoth Aristata, the Commandant of Baelgrim",
    city          = "mhaldor",
    house         = "legates",
    level         = "116",
    class         = "magi",
    mob_kills     = "116k",
    player_kills  = "7996",
    xp_rank       = "24",
    explorer_rank = "8"
}

Maybe something like what I use to get at GMCP json objects?


temp = json.decode(content)

Fiendish USA Global Moderator Tue 06 Aug 2019 04:42 PM #3

LuaSocket can't do HTTPS and instead just silently shunts you to HTTP ( https://www.gammon.com.au/forum/?id=12520&reply=4 )

I find coroutines hard to use, so in the aardwolf mushclient package I use lua-llthreads2 and lua-openssl (because luasec isn't thread-safe) for non-blocking https requests with actual OS threads.

Nick Gammon Australia Forum Administrator Tue 06 Aug 2019 08:21 PM #4

You could use a coroutine to manage a non-blocking HTTP request, but if it isn't thread-safe then you probably should not do more than one at a time.

Fiendish USA Global Moderator Tue 06 Aug 2019 08:35 PM #5

Quote:
You could use a coroutine to manage a non-blocking HTTP request

Plain HTTP is no longer sufficient, because many services (like this one) block non-TLS connections. Luasocket on its own won't work for the given URL because the server forces TLS.

Quote:
but if it isn't thread-safe then you probably should not do more than one at a time.

Luasec isn't safe, but Lua-openssl is and has an identical interface.