On Thu, 01 Sep 2005, Frederik Eaton
But getting a stack backtrace when there is an error should be a pretty basic feature. It's very hard to debug a large program when you can randomly get messages like "*** Exception: Prelude.head: empty list" and have no idea where they came from.
From the GHC FAQ:
My program is failing with head [], or an array bounds error, or some other random error, and I have no idea how to find the bug. Can you help? Compile your program with -prof -auto-all (make sure you have the profiling libraries installed), and run it with +RTS -xc -RTS to get a “stack trace” at the point at which the exception was raised. See Section 4.14.4, “RTS options for hackers, debuggers, and over-interested souls” for more details. I tried this out under GHC 6.4/Linux and got a segmentation fault instead of a stack trace. Under GHC 6.2.2 it seemed to work, though. -- /NAD