Re: [Haskell-cafe] How to debug GHC

Quoting Nils Anders Danielsson :

On Thu, 01 Sep 2005, Frederik Eaton wrote:

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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.

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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 ¡Èstack trace¡É at the point at which the exception was raised. See Section 4.14.4, ¡ÈRTS options for hackers, debuggers, and over-interested souls¡É 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.

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. Maybe you'll find it useful. Jason