Kabbalah: What does "only already installed instances" mean?

Question

I am new to Cabal, and this phrase “only already installed instances can be used” makes no sense to me.

$ cabal install base==4.7.0.0 Resolving dependencies... cabal: Could not resolve dependencies: next goal: base (user goal) rejecting: base-4.7.0.1 (global constraint requires ==4.7.0.0) rejecting: base-4.7.0.0 (only already installed instances can be used) rejecting: base-4.6.0.1/installed-8aa..., 4.6.0.1, 4.6.0.0, 4.5.1.0, 4.5.0.0, 4.4.1.0, 4.4.0.0, 4.3.1.0, 4.3.0.0, 4.2.0.2, 4.2.0.1, 4.2.0.0, 4.1.0.0, 4.0.0.0, 3.0.3.2, 3.0.3.1 (global constraint requires ==4.7.0.0) Dependency tree exhaustively searched. 

What is Cabal trying to contact here?

Answer 1

Cabal will not allow you to set up a base. I encountered this problem when trying to install the profiling version of the database. Instead, you need to do something like:

 sudo apt-get install haskell-platform-prof 

See this thread for discussion by cabal developers.