The OS X version of the Glasgow Haskell Compiler compiles Haskell into 32-bit code. Unfortunately, this means that if you are on a system where it is the default for libraries to be built in 64-bit mode, you tend to get errors when linking Haskell code telling you that you are trying to link 32-bit code against 64-bit code.

A problem that has plagued my GHC installation for a while is that whenever I tried to install any non-trivial package I would get a horrible link error like this:

I recently tracked down the cause of the persistent “thread blocked indefinitely in an MVar operation” exceptions I was getting from the GHC runtime when I used my parallel-io library. There doesn’t seem to be much information about this exception on the web, so I’m documenting my experiences here to help those unfortunate souls that come across it.

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:

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