I hope these lies and oversimplifications will help others understand coroutines. See below for an increasingly accurate explanation. A special thanks to miguel on the #tcl freenode channel for taking the time to explain this to me.
proc allNumbers {} { yield set i 0 while 1 { yield $i incr i 2 } } coroutine nextNumber allNumbers for {set i 0} {$i < 10} {incr i} { puts "received nextNumber" } rename nextNumber {}
The first thing to understand is the yield command (which is in the coroutine's code). [yield $foo] acts like [return $foo], but it suspends execution; when execution is resumed, the proc restarts on the line after the previous call to yield.
The procedure "allNumbers" is a proc which first calls yield, and so suspends immediately (line 2); when it resumes it begins execution at line 3, after which it reaches another yield within a while loop on line 5. After producing each number, it yields its value ... and suspends until it is called again.
Now this line:
coroutine nextNumber allNumbers
This line will (a) run allNumbers until the first yield (b) create a command called 'nextNumber' that resumes execution of allNumbers when nextNumber is called.
So after this we have allNumbers suspended right before 'set i 0', and a new command 'nextNumber' that continues it where it left off.
Get it?
MS notes that he kind of lied to jblz: yield is a bit more complicated, but behaves exactly as described for this example.
ZB Wrong. It's the thing, that once confused me: the proc restarts not "on the line after previous call to yield", but resumes - one can say - directly in that lately left yield field, because there it can get ev. arguments "from outside". Consider the yield field as a "gate". You're leaving the proc - and getting back using this gate. Changed an example a bit - in my opinion, now it better explains the role of yield; changing theValue (and restarting the loop) one can see, how the increment step is different.
proc allNumbers {} { yield set i 0 while 1 { set howMuch yield $i incr i $howMuch } } coroutine nextNumber allNumbers set theValue 2 for {set i 0} {$i < 10} {incr i} { puts "received nextNumber $theValue" }