Hi, everybody! I think there's either an error in cabal-install, or I don't have an idea on haskell packages. I tried to install one particular library that depends on containers-0.2.0.1 and got the following error: Data/IntMap.hs:182:7: Could not find module `Data.Data': It is a member of the hidden package `base'. Perhaps you need to add `base' to the build-depends in your .cabal file. Use -v to see a list of the files searched for. Then I tried to install containers-0.2.0.1 separately, andin the process of the struggle I found that issuing the following command in containers-0.2.0.1 directory: ~/.cabal/bin/cabal install --constraint="base >= 4" -v ... prints this: Resolving dependencies... selecting containers-0.2.0.1 (hackage) selecting array-0.3.0.1 (installed or hackage) and discarding array-0.1.0.0, 0.2.0.0 and 0.3.0.0 selecting base-3.0.3.2 (installed) and 4.2.0.1 (installed) and discarding syb-0.1.0.0, 0.1.0.1, 0.1.0.2, 0.1.0.3, 0.2 and 0.2.1 selecting ffi-1.0 (installed) selecting ghc-prim-0.2.0.0 (installed) selecting integer-gmp-0.2.0.1 (installed) selecting rts-1.0 (installed) selecting syb-0.1.0.2 (installed) In order, the following would be installed: containers-0.2.0.1 (new version) Configuring containers-0.2.0.1... Dependency array ==0.3.0.1: using array-0.3.0.1 Dependency base ==3.0.3.2: using base-3.0.3.2 And proceeds to building the library: /usr/local/bin/ghc --make -package-name containers-0.2.0.1 -hide-all-packages -fbuilding-cabal-package -i -idist/build -i. -idist/build/autogen -Idist/build/autogen -Idist/build -Iinclude -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package-id array-0.3.0.1-fb28b07fd38a6011f4f73844e0d8281f -package-id base-3.0.3.2-260693a92016991a03206b5a55f6a411 -O -XDeriveDataTypeable -XMagicHash -XRank2Types -XCPP Data.Graph Data.IntMap Data.IntSet Data.Map Data.Sequence Data.Set Data.Tree where, as you see, it really uses base-3.0.3.2, and thus fails. So, my qusetion is, whether I'm doing something wrong, or there are really two errors: 1) cabal-install makes a wrong choice for dependency version 2) cabal-install does not respect the constraint, and proceeds to building the library without any warning