In many discussions we make guesses about how much code proposals like Functor => Monad would break. You can use https://github.com/dterei/Hackager to build all of Hackage (preferably in a VM). Of course many packages have external dependencies, so I'd like to share the following list of packages to save you some time. (These are the names of packages suited for a Ubuntu 13.04 VM, and with these installed, 2800 packages are successfully built and 1700 failing, so it's quite good coverage. If you find more packages that improve on this, please add them.) acl-dev attr-dev binutils-dev cfitsio-dev expect-dev freeglut3-dev libadns1-dev libalure-dev libasound-dev libaspell-dev libatlas-dev libaugeas-dev libavcodec-dev libavfilter-dev libavformat-dev libavutil-dev libbibutils-dev libbluetooth-dev libbz2-dev libcal3d12-dev libcmph-dev libcrack2-dev libcrypto++-dev libcsound64-dev libctemplate-dev libcurl-dev libcv-dev libcwiid-dev libdb-dev libdevil-dev libdpkg-dev libev-dev libevent-dev libexif-dev libfam-dev libfcgi-dev libfftw3-dev libfltk1.3-dev libfreenect-dev libftgl-dev libfuse-dev libgd2-xpm-dev libgeoip-dev libglfw-dev libglpk-dev libgmime-2.6-dev libgnome-keyring-dev libgnutls-dev libgnutls-dev libgpcl-dev libhighgui-dev libimlib2-dev libinsighttoolkit4-dev libjasper-dev libjudy-dev libkyotocabinet-dev liblapack-dev libldap2-dev libldap2-dev libleveldb-dev libmagic-dev libmarkdown2-dev libmecab-dev libmpfr-dev libmtp-dev libmx-dev libmysqlclient-dev libncurses-dev libncurses-dev libnotmuch-dev libobjc-4.7-dev libodbc1 libode-dev libogg-dev libois-dev libopenal-dev libpam0g-dev libpcap-dev libpoker-eval-dev libportaudio-dev libpulse-dev libqd-dev libqrencode-dev libraw1394-dev libreadline-dev libscsynth1 libselinux-dev libsnappy-dev libsndfile1-dev libsqlite3-dev libssl-dev libssl-dev libst-dev libsvm-dev libtalloc-dev libtbb-dev libtheora-dev libtiff-dev libtokyocabinet-dev libtokyotyrant-dev libtre-dev libuuid-dev libv4l-dev libvxl1-dev libxapian-dev libxen-dev libxerces-c-dev libxine-dev libxmmsclient-dev libxosd-dev libxqilla-dev libxss-dev libxtst-dev libyaml-dev libz-dev libzephyr-dev libzmq-dev linux-libc-dev tcl-dev wx2.8-headers xmms2-dev