Comp.lang.tcl questioners frequently demand some sort of gadget to monitor-an-ongoing-process-and-display-the-result-in-a-text. Tailing widget is the simplest answer CL knows that meets the common intent of these requests.
There are lots of variations on this theme, though. tailf [L1 ] carefully implements the functionality of the Unix tail(1), and adds a bit of filtering capability. I've written a few examples [L2 ] that show how different combinations of IPC, fileevent, after, ... can co-operate to monitor continuing processes.