Tagged coroutines (nested coroutines with a lower-level interface)
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README.md

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Tagged Coroutines 1.0.0

This module is is a replacement to the standard coroutine module that adds tagged coroutines. Functions create and wrap now receive a tag and a function, instead of just a function. Function yield now also needs a tag as the first argument. The tag can be any Lua value.

A yield with a specific tag yields to the dynamically closest resume of a coroutine with that tag (making an analogy with exceptions: the coroutine is like a try/catch block, yield is like a throw, and the tag is analogous with the type of the exception). If there is no coroutine with the tag an error is thrown at the point of the yield.

On a successful yield, any coroutine that has been passed through in the search for the coroutine that handled that yield becomes stacked, a new status string returned by the status function. Attempting to directly resume a stacked coroutine is an error. Resuming the coroutine that handled the yield rewinds the whole stack, resuming the stacked coroutines along the way until reaching and finally continuing from the point of the original yield.

A failed yield can be an expensive operation, so if you are unsure if you can yield you can use the extended isyieldable function, which now expects a tag and will return true only if yielding with this tag will succeed.

The function coroutine.yield is an untagged yield. A tagged coroutine passes an untagged yield along, unless its parent is the main coroutine: in this case, the yield is supressed, and the source resumes from the untagged yield with the call to yield returning nil and "untagged coroutine not found". Unfortunately there is no way to make the untagged yield fail as if it had tried to yield outside a coroutine.

When an untagged yield reaches an untagged parent, the parent will suspend as if the yield was intended for it; when the parent resumes the whole stack will be resumed, ultimately resuming from the point of the untagged yield. This way you can have a stack of tagged coroutines on top of an untagged coroutine (allowing the use of existing coroutine schedulers, for example).

A tagged yield that reaches an untagged coroutine fails at the point of the call to yield as if it had reached the main coroutine.

A new function call resumes a coroutine as if it had been wrapped by wrap: any uncaught errors while running the coroutine will be propagated. But the stack is not unwound: you can still get a traceback of the full stack of the dead coroutine (including all of the coroutines that were stacked above it) using the new traceback function. It is similar to debug.traceback, except that it includes a full traceback, following source to reach the source of the error and tracing parent back to the main thread.

A new tag function returns the tag of a coroutine. A parent function returns the coroutine that last resumed a coroutine. A source function returns, for a given coroutine, either the coroutine where the last yield came from, in case of a suspended coroutine, or where an error originated, in case of a dead coroutine. You can use these two functions to walk a dead stack of coroutines with the debug functions in case traceback is not enough.

Finally, the function fortag receives a tag and returns a set of tagged coroutine functions specialized for that tag. For compatibility with lua-coronest there is also a make function that is like fortag except it generates a fresh tag if none is given.

There is both a C and a pure Lua implementation. The C implementation is more efficient, and produces better stacktraces, but requires stock Lua 5.2 or higher (it will not work with LuaJIT 2). The pure Lua implementation should work on LuaJIT 2, Lua 5.2, or Lua 5.3, but the isyieldable might give a false positive if there are pending unyieldable C calls in the stack on any Lua version except Lua 5.3.

Install it by running luarocks make on one of the provided rockspec files (taggedcoro for the C version, taggedcoro-purelua for the Lua version). The contrib folder has sample libraries that implement some abstractions on top of coroutines that can be freely composed with tagged coroutines. The samples folder has sample scripts that exercise these higher-level libraries. Some of them depend on the thread library and on a branch of Cosmo that requires tagged coroutines.