A little-known feature of GHC is the ability to run a program (that has been compiled with profiling) with the +RTS -xc option. Now, every time an exception is raised a stack trace will be dumped to stderr. This is the only way get stack traces for your Haskell code currently. Unfortunately:

• This stack trace is constructed from the cost-centre stack, so you’ll only see names from modules you compiled in profiling mode with -auto-all or similar

• The stack trace is shown regardless of whether the exception in question is actually handled or not - so you are likely to see lots of spurious stack traces where exceptions have been caught and handled

It’s still better than the alternative (i.e. wailing about the lack of proper Haskell stack traces) though.

Now, the question is how to interpret the mess you get out. Here is an example stack trace from my current debugging session (I’ve added linebreaks to aid comprehension):

<Control.Concurrent.ParallelIO.Local.parallelE,
Control.Concurrent.ParallelIO.Local.parallel,
Control.Concurrent.ParallelIO.Local.parallel_,
Control.Concurrent.ParallelIO.Local.withPool,
Development.Shake.shake,Development.Shake.CAF>


(I’m trying to figure out why OpenShake reliably dies with the dreaded “blocked indefinitely on MVar” exception.)

The answer lies in the GHC source code (see fprintCSS in Profiling.c). The items at the front of the stack trace are at the top of the stack and hence correspond to the approximate to location from which the exception was thrown.

Looking forward, it would be awesome if e.g. the Google Summer of Code could sponsor a student to take my work on GHC ticket 3693 further. Personally I would find stack trace support for Haskell totally invaluable, and I’m sure I’m not alone.