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“Argh!  When working on GHC, or in any Haskell in general, it is occasionally useful to know something about what was going on when a program crashed.  Currently we have several ways of doing this; for example the ghci-debugger or using the cost-center-stacks that are part of the profiling tools in GHC.  For my internship here at MSR, I’ve been looking at putting another tool in the GHC-users toolbelt, that of selective explicit-call-stack (ecs) traces (in a theme of things described on this wiki page); and today I’ve had a breakthrough in terms of using the tool I’ve been working on to actually help debug a real issue I was having in GHC! So, our panic above was coming from this bit of source:” source...