This good long thread prompted Don and I to take a look at the darcs source (this is current darcs-unstable). You can get the darcs source at: darcs get http://abridgegame.org/repos/darcs-unstable Here are some data points (without interpretation): In impossible.h we have the definitions: import Bug ( bug ) #define impossible (bug $ "Impossible case at "++__FILE__++":"++show (__LINE__ :: Int)++" compiled "++__TIME__++" "++__DATE__) #define fromJust (\m_fromJust_funny_name -> case m_fromJust_funny_name of {Nothing -> bug ("fromJust error at "++__FILE__++":"++show (__LINE__ :: Int)++" compiled "++__TIME__++" "++__DATE__); Just x -> x}) The second definition gives bug reports like the following: fromJust error at Push.lhs:136 compiled 11:51:58 Jun 16 2006 Please report this to bugs@darcs.net, If possible include the output of 'darcs --exact-version'. To see at a glance the various bug reports about fromJust you can search the bug database: http://bugs.darcs.net/issue?@columns=id%2Cactivity%2Ctitle%2Ccreator%2Cassignedto%2Cstatus&@sort=activity&@group=priority&@search_text=fromJust I count 7 bugs. Some other datapoints: $ grep fromJust *.lhs *.hs | wc -l 101 $ grep fromMaybe *.lhs *.hs | wc -l 7 $ grep maybe *.lhs *.hs | wc -l 120 I would be interested to see the results of static analysis tools (Catch?) or applying Oleg's strategy. Any volunteers? thanks! Jason