A goroutine really is implemented as a coroutine though. It's just that without this runtime optimization, there hasn't been a way to interact with them without letting a multithreaded scheduler manage their execution.
I don't remember where I came across this, but many years ago I saw python-style generators termed "semi-coroutines" which I'm a fan of. Python (frustratingly) doesn't implement them this way, but the beauty of generators is that by limiting the scope where execution can be suspended to the highest-level stack frame, you can statically determine how much memory is needed for the (semi-)coroutine, so it doesn't require a single allocation.
Zig takes that a step farther by considering recursive functions second-class, which lets the compiler keep track of the maximum stack depth of any function, and thereby allow you to allocate a stack frame for any non-recursive function inside of another stack frame, enabling zero-allocation full coroutines, as long as the function isn't recursive.
That would... probably be overkill for Go, since marginal allocations are very cheap, and you're already paying the runtime cost for dynamic stack size, so the initial allocation can be tiny.
I would love to see full proper coroutine support make it to Go, freeing users of the overhead of the multithreaded scheduler on the occasions where coroutine patterns work best. I remember back in 2012 or so, looking at examples of Go code that showed off goroutine's utility as control flow primitives even when parallelism wasn't desired and being disappointed that those patterns would likely be rare on account of runtime overhead, and sure enough I hardly ever see them.
I don't remember where I came across this, but many years ago I saw python-style generators termed "semi-coroutines" which I'm a fan of. Python (frustratingly) doesn't implement them this way, but the beauty of generators is that by limiting the scope where execution can be suspended to the highest-level stack frame, you can statically determine how much memory is needed for the (semi-)coroutine, so it doesn't require a single allocation.
Zig takes that a step farther by considering recursive functions second-class, which lets the compiler keep track of the maximum stack depth of any function, and thereby allow you to allocate a stack frame for any non-recursive function inside of another stack frame, enabling zero-allocation full coroutines, as long as the function isn't recursive.
That would... probably be overkill for Go, since marginal allocations are very cheap, and you're already paying the runtime cost for dynamic stack size, so the initial allocation can be tiny.
I would love to see full proper coroutine support make it to Go, freeing users of the overhead of the multithreaded scheduler on the occasions where coroutine patterns work best. I remember back in 2012 or so, looking at examples of Go code that showed off goroutine's utility as control flow primitives even when parallelism wasn't desired and being disappointed that those patterns would likely be rare on account of runtime overhead, and sure enough I hardly ever see them.