The proposal for coroutines, which seems to be one of the "accepted" ones, bothers me.

When a suspended block is called again, parameters are ignored and it resumes where it left off. Great for iterator, right? Well, what happens if you want to use the same iterator within the body being controlled by the iterator already?

Basically, a line like

while (my $node = $root->next_inorder()) { print $node->{data}; }
found in a module will test fine, but will suddenly and mysteriously fail if that code is called (even indirectly, by many levels) from another use of the iterator, even if it's on a different collection.

Or, are they implying that a specific occurance of the function call in the code keeps its own state, like the flip-flop operator does today? I wonder, because their example is calling itself recursivly and passing it an argument (which presumably is not ignored!).

We also need a "quit" feature. Say I'm iterating over a collection and break out of the loop when I find the element I want. Just last it, and don't look at the rest. Well, next time that code is called, it will still be incomplete...