On Fri, 2 Sep 2005 dagit@eecs.oregonstate.edu wrote: ...
I was trying to debug a smallish program where I was getting this exact error and the trick with profiling did "work" on my system, but I remember it being almost useless for me. What what did end up working for me was: myhead :: [a] -> String -> a myhead [] s = error s myhead xs _ = head xs
And then I did a M-x occur head <RET>, replaced all the calls to head with |myhead xs "myhead callsite n"| and incremented n appropriately.
This technique is pedestrian, but it generalizes quite well and it will work in any situation given enough time to do all the search/replace. There are certainly better methods and it's not a replacement for a real debugger, but this one is easy for a beginner to come up with and it does work.
Just more or less as an aside, at its origin in April (!) this thread didn't mention any debugger - the question was just how to build ghc so that a stack trace would come out. A real debugger is no replacement for that (because you have to be on hand and know how to repeat the problem to get anywhere with a debugger), but that's just my opinion. Donn Cave, donn@drizzle.com