[Haskell-cafe] Best way to find an "undefined" error?

[Don Stewart]

saynte:
> Hello all,
> 
> Does anyone favourite technique to track down an "undefined" call? I'm
> 99% sure that my code is not the offender (I've grepped for undefined
> occurrences and turned them all into "error" calls). Supposing that
> this is happening in some other package or library that I'm using,
> what is the best way to find out where it is? I have profiling enabled
> on all libraries that I use, so I'm ok there I think. Any magic
> combination of +RTS .. -RTS?
> 

You can use the profiler to get a stack trace, or use the new 
GHCi debugger to step backwards from the exception to the source.

I wrote a bit of a tutorial for this here:

http://cgi.cse.unsw.edu.au/~dons/blog/2007/11/14#no-exceptions

-- Don