I spent a little time the other day building all of Hackage with GHC HEAD. Here's a quick writeup of what I found. Total packages: 5564 Succeeded with 7.6.3: 3234 Succeeded with HEAD: 2061 Succeeded with 7.6.3 *but failed with HEAD: 1186* (A few dozen packages built with HEAD, but not with 7.6.3, which is why the numbers don't add up.) I think the condensed version of the story is that we're actually in good shape, the above numbers notwithstanding. There are only a few categories of build regression, which I detail below. Except for one genuine bug, the rest (at least those I've seen so far) are upgrade details that could be written about in release notes or a blog post. There currently exists just one panic-related bug, which prevents all 23 packages in the Snap web framework from building: ghc: panic! (the 'impossible' happened) (GHC version 7.7.20130923 for x86_64-unknown-linux): dmdTransformDictSelSig: split failedhttp://ghc.haskell.org/trac/ghc/ticket/8329 Of the 1186 packages that build with 7.6.3 but not HEAD: 311 fail due to cabal being unable to solve dependency constraints. As usual, most (195) have excessively tight constraints on base. 37 packages fail due to GeneralizedNewtypeDeriving and the new role infrastructure, due to the following failures in packages upon which they depend. 12 language-c-0.4.2 the last parameter of ‛Error’ is at role Nominal 11 logfloat-0.12.1 the last parameter of ‛IArray’ is at role Nominal 5 diagrams-lib-0.7.1.1 the last parameter of ‛Transformable’ is at role Nominal 4 smallcheck-1.0.4 the last parameter of ‛MonadLogic’ is at role Nominal 2 monadiccp-0.7.6 the last parameter of ‛ReaderM’ is at role Nominal 2 histogram-fill-0.8.1.0 the last parameter of ‛BinEq’ is at role Nominal 1 acme-schoenfinkel-0.1.1 the last parameter of ‛ArrowApply’ is at role Nominal There's quite a bit of transitive primop-related breakage. 364 template-haskell-2.8.0.0 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 48 primitive-0.4.1 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 6 primitive-0.5.0.1 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ Most of the breakage could be fixed by loosening the dependencies on the template-haskell and primitive packages, and here are the rest. 63 uulib-0.9.15 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 20 utf8-light-0.4.0.1 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 19 safeint-0.5.3 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 16 permutation-0.4.1 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 9 floatshow-0.2.3 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 9 bytestring-show-0.3.5.5 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 8 STMonadTrans-0.3.1 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 7 hashtables-1.1.0.2 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 2 word24-1.0.3 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 2 arithmoi-0.4.0.3 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 1 word8-0.0.3 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ 1 IORefCAS-0.2.0.1 Couldn't match expected type ‛Bool’ with actual type ‛Int#’ Roughly 45 packages are broken by changes to Typeable.