With GHC 1ece7b27a11c6947f0ae3a11703e22b7065a6b6c zlib fails to build, apparently due to Safe Haskell (bug 5610 [1]). The error is specifically, $ cabal install zlib Resolving dependencies... Configuring zlib-0.5.3.1... Preprocessing library zlib-0.5.3.1... Building zlib-0.5.3.1... [1 of 5] Compiling Codec.Compression.Zlib.Stream ( dist/build/Codec/Compression/Zlib/Stream.hs, dist/build/Codec/Compression/Zlib/Stream.o ) Codec/Compression/Zlib/Stream.hsc:857:1: Unacceptable argument type in foreign declaration: CInt When checking declaration: foreign import ccall unsafe "static zlib.h inflateInit2_" c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt Codec/Compression/Zlib/Stream.hsc:857:1: Unacceptable argument type in foreign declaration: CInt When checking declaration: foreign import ccall unsafe "static zlib.h inflateInit2_" c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt Codec/Compression/Zlib/Stream.hsc:857:1: Unacceptable result type in foreign declaration: IO CInt Safe Haskell is on, all FFI imports must be in the IO monad When checking declaration: foreign import ccall unsafe "static zlib.h inflateInit2_" c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt ... This is a little strange since, a) It's not clear why Safe Haskell is enabled b) The declarations in question seem to be valid Does this seem like a compiler issue to you? Cheers, - Ben [1] http://hackage.haskell.org/trac/ghc/ticket/5610