Hi folks, I'm sorry to say that in GHC 6.12.2, as a result of fixing a couple of bugs in 6.12.1, we (well, I) managed to introduce some more serious ones: http://hackage.haskell.org/trac/ghc/ticket/4038 http://hackage.haskell.org/trac/ghc/ticket/4041 The first causes gtk2hs programs to crash, and we have a report of it affecting hmatrix too. There's no workaround unfortunately: the problem is that we changed some data structures related to FFI calls, with the result that a nested callback (calling out to C, back to Haskell, out to C again and back to Haskell in the same thread) can result in a crash or a bogus result. The second causes lazy bytestring's hGetContents to block waiting for a full buffer, when it should just return the data already read. You can work around this by using bytestring-0.9.1.5 instead of the supplied 0.9.1.6. We'll put out 6.12.3 with fixes for these as soon as possible. In the meantime, if you are still on 6.12.1 and not encountering any problems, I don't recommend upgrading. Sorry for the inconvenience. Cheers, Simon