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Standard Libraries


Symmetric coroutines

local coro = require'coro'

Symmetric coroutines are coroutines that can transfer control freely between themselves, unlike Lua’s standard coroutines which can only yield back to the coroutine that resumed them (and are called asymmetric coroutines or generators because of that reason).


Using coroutine-based async I/O methods (like the read() and write() methods of async socket libraries) inside user-created standard coroutines is by default not possible because the I/O methods would yield to the parent coroutine instead of yielding to their scheduler. This can be solved using a coroutine scheduler that allows transferring control not only to the parent coroutine but to any specified coroutine.

This implementation is loosely based on the one from the paper Coroutines in Lua with some important modifications:

  • coro.transfer() can transfer multiple values between coroutines (without pressuring the gc).
  • the coro module reimplements all the methods of the built-in coroutine module such that it can replace it entirely, which is what enables arbitrary transfering of control from inside standard-behaving coroutines.
  • coro.safewrap() is added which allows cross-yielding.


coro.create(f) -> thread

Create a coroutine which can be started with either coro.resume() or with coro.transfer().

coro.transfer(thread[, ...]) -> ...

Transfer control (and optionally any values) to a coroutine, suspending execution. The target coroutine either hasn’t started yet, in which case it is started and it receives the values as the arguments of its main function, or it’s suspended in a call to coro.transfer(), in which case it is resumed and receives the values as the return values of that call. Likewise, the coroutine which transfers execution will stay suspended until coro.transfer() is called again with it as target.

Errors raised inside a coroutine which was transferred into are re-raised into the main thread.

A coroutine which was transferred into (as opposed to one which was resumed into) must finish by transferring control to another coroutine (or to the main thread) otherwise an error is raised.

coro.ptransfer(thread[, ...]) -> ok, ... | nil, err

Protected transfer: a variant of coro.transfer() that doesn’t raise.

coro.install() -> old_coroutine_module

Replace _G.coroutine with coro and return the old coroutine module. This enables coroutine-based-generators-over-abstract-I/O-callbacks from external modules to work with scheduled I/O functions which call coro.transfer() inside.

coro.yield(...) -> ...

Behaves like standard coroutine.yield(). A coroutine that was transferred into via coro.transfer() cannot yield (an error is raised if attempted).

coro.resume(...) -> true, ... | false, err, traceback

Behaves like standard coroutine.resume(). Adds a traceback as the third return value in case of error.

coro.current() -> thread, is_main

Behaves like standard coroutine.current() (from Lua 5.2 / LuaJIT 2).

coro.status(thread) -> status

Behaves like standard coroutine.status().

NOTE: In this implementation type(thread) == 'thread'.

coro.wrap(f) -> wrapper

Behaves like standard coroutine.wrap().

coro.safewrap(f) -> wrapper

Behaves like coroutine.wrap() except that the wrapped function receives a custom yield() function as its first argument which always yields back to the calling thread even when called from a different thread. This allows cross-yielding i.e. yielding past multiple levels of nested coroutines which enables unrestricted inversion-of-control.

With this you can turn any callback-based library into a sequential library, even if said library uses coroutines itself and wouldn’t normally allow the callbacks to yield.

Why it works

This works because calling resume() from a thread is a lie: instead of resuming the thread it actually suspends the calling thread giving back control to the main thread which does the resuming. Since the calling thread is now suspended, it can later be resumed from any other thread.

Last updated: 2 years ago | Edit on GitHub

Pkg type:Lua
Version: 9362362
Last commit:
License: Public Domain

Requires: none

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