:l QSort.hs
:set -fbreak-on-exception
:trace qsort ("abc" ++ undefined)

-- Back up to the (filter (<=a) as) call
:back
:back

putStrLn "Printing 1"
:print as

putStrLn "Forcing"
:force as

-- this should print the exception
putStrLn "Printing 2"
:print as