Tcl Source Code

In the following script control eventually returns to p1, which sets $done to success. The fact that $done1 has a final value of failure shows that the attempt to yield back to p1 failed, so how did control end up in p1 again?

variable done
proc p0 {} {
	yield
	variable done0
	after 0 [list [namespace which c1]]
	vwait [namespace current]::done0
}


proc p1 {} {
	variable done0
	yield
	yieldto try "yieldto [list [info coroutine]]" on error {} "
		::set [list [namespace current]]::done1 failure
		::set [list [namespace current]]::done0 failure
	"
	set done0 success
}

coroutine c0 p0
coroutine c1 p1
after 1 [list [namespace which c0]]
vwait [namespace current]::done0
puts [list status $done0 $done1]

output:

status success failure

If the script doesn't attempt the inner yieldto, and just sets the variables instead, the result is different:

variable done
proc p0 {} {
	yield
	variable done0
	after 0 [list [namespace which c1]]
	vwait [namespace current]::done0
}


proc p1 {} {
	variable done0
	yield
	yieldto try "
		::set [list [namespace current]]::done1 failure
		::set [list [namespace current]]::done0 failure
	"
	set done0 success
}

coroutine c0 p0
coroutine c1 p1
after 1 [list [namespace which c0]]
vwait [namespace current]::done0
puts [list status $done0 $done1]

output:

status failure failure

So apparently in the first script the failure to yield back to c1 affects some internal state and changes the outcome.