Hello, GHC can spot (some) non-terminating computations and terminate with blackhole: <<loop>> instead of running indefinitely. With exception handling one can handle such situations. The following program 'launches missiles': import Control.Exception main = do handle go_ahead don't_launch_first launch_missiles don't_launch_first = don't_launch_first go_ahead :: SomeException -> IO () go_ahead _ = putStr "go ahead, " launch_missiles = putStr "fire!" The output of this program is go ahead, fire! although, don't_launch_first is a non-terminating computation. Without black-hole detection this code would never 'launch missiles'. Is the above output intended? The idea behind black-hole detection is that one bottom is as good as another [1]. Consequently, exception handling may not distinguish between non-termination and other errors. Is the equation _|_ `catch` const a = a supposed to hold in general (for every kind of bottom)? Or is it a bug in GHC that programmers can handle black-hole errors? Sebastian [1] A semantics for imprecise exceptions http://portal.acm.org/citation.cfm?id=301637 -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)