I accidentally found a rarely encountered omission in Cabal (the lib, because via Setup.hs) by building Haskell Platform as shared libs from source. It is rare because you have to use both --enabled-shared and --package-db=blah together to run into it. --package-db=blah is already rare enough (probably only Haskell Platform's build script uses it); --enable-shared is my addition to make it even rarer. (Also I have only tried this on linux x86 32-bit.) Following the essence of scripts/build.sh plus my own --enable-shared: Create an empty lookaside package database with: echo '[]' > /tmp/lookaside Install a package X that depends only on base (say), ask for shared libs, register in /tmp/lookaside only: ./Setup configure --package-db=/tmp/lookaside \ --ghc-pkg-option=--package-conf=/tmp/lookaside \ --enable-shared ./Setup build ./Setup register --inplace Install another package Y that depends on base and X only (say), ask for shared libs. There is a problem now related to X known to /tmp/lookaside only, but not known to the global or the user database: ./Setup configure --package-db=/tmp/lookaside \ --ghc-pkg-option=--package-conf=/tmp/lookaside \ --enable-shared ./Setup build -v2 Now it bombs. As the verbose build messages show, there is no problem creating the static *.a version; it bombs when creating the shared *.so version, and it is because it can't find X, and it is because -package-conf=/tmp/lookaside is not given to ghc at the *.so stage. (Though, the *.dyn_o stage works correctly.) My current fix is: ./Setup configure --package-db=/tmp/lookaside \ --ghc-pkg-option=--package-conf=/tmp/lookaside \ --ghc-option=-package-conf=/tmp/lookaside \ --enable-shared I surely hope we no longer need --ghc-pkg-option and --ghc-option in a week! :)