Hi, The Yhc.Core Binary format is about to change, the Haskell interface will remain the same - but you'll need to recreate all core files you have floating around - anything .ycr or .yca. I am sorry to say that opening an old Core file with the new instances seems to non-terminate, rather than giving an error message. This is unfortunate, but hopefully future Core changes should not have this side effect. The reason for this change is the move from our hand-made Binary instance to the standard Data.Binary one. This should give us a speed up, and means we are keeping less of our own code in our tree. It probably means that we require either GHC 6.6 or the fps package to be installed on GHC 6.4. This change also means that to generate the Binary instance we are no longer relying on DrIFT but instead on Derive [1]. This means that anyone should be able to regenerate the Binary instance, rather than just me with a weird batch file. Thanks Neil [1] http://www-users.cs.york.ac.uk/~ndm/derive/