What is a coroutine?

What is a coroutine? How are they related to concurrency?

Coroutines and concurrency are largely orthogonal. Coroutines are a general control structure whereby flow control is cooperatively passed between two different routines without returning.

The 'yield' statement in Python is a good example. It creates a coroutine. When the 'yield ' is encountered the current state of the function is saved and control is returned to the calling function. The calling function can then transfer execution back to the yielding function and its state will be restored to the point where the 'yield' was encountered and execution will continue.

From Programming in Lua, "Coroutines" section:

A coroutine is similar to a thread (in the sense of multithreading): it is a line of execution, with its own stack, its own local variables, and its own instruction pointer; but it shares global variables and mostly anything else with other coroutines. The main difference between threads and coroutines is that, conceptually (or literally, in a multiprocessor machine), a program with threads runs several threads in parallel. Coroutines, on the other hand, are collaborative: at any given time, a program with coroutines is running only one of its coroutines, and this running coroutine suspends its execution only when it explicitly requests to be suspended.

So the point is: Coroutines are "collaborative". Even in multi-core system, there is only one coroutine running at any given time (but multiple threads can run in parallel). There is non-preemptive between coroutines, the running coroutine must relinquish the execution explicitly.

For "concurrency", you can refer Rob Pike's slide:

Concurrency is the composition of independently executing computations.

So during coroutine A's execution, it passes control to coroutine B. Then after some time, the coroutine B passes control back to coroutine A. Since there is dependency between coroutines, and they must run in tandem, so the two coroutines are not concurrency.

I find most of the answers too technical even though it is a technical question. I had a hard time trying to understand the coroutine process. I kind of get it but then I don't get it at the same time.

I found this answer here very helpful:

https://dev.to/thibmaek/explain-coroutines-like-im-five-2d9

To quote from Idan Arye:

To build on your story, I'd put it something like this:

You start watching the cartoon, but it's the intro. Instead of watching the intro you switch to the game and enter the online lobby - but it needs 3 players and only you and your sister are in it. Instead of waiting for another player to join you switch to your homework, and answer the first question. The second question has a link to a YouTube video you need to watch. You open it - and it starts loading. Instead of waiting for it to load, you switch back to the cartoon. The intro is over, so you can watch. Now there are commercials - but meanwhile a third player has joined so you switch to the game And so on...

The idea is that you don't just switch the tasks really fast to make it look like you are doing everything at once. You utilize the time you are waiting for something to happen(IO) to do other things that do require your direct attention.

Definitely check the link, there are much more that I cannot quote everything.