Assuming that Debug.Trace offers similar functionality as "observe" does, I have to briefly mention that this tracing might not be overly helpful, precisely because of the lacking control over evaluation order, as you write later on. I had to write a parser using both, combinatoric and monadic style, and debugging was a major turn off because I could not easily see what went wrong (though I seem to remember that using observe the ouptut was reverse to actual program flow, at least the one I have in my head.)