Cabal: What does "only already installed instances can be used" mean?

Question

I'm new to Cabal, and this phrase "only already installed instances can be used" doesn't make any 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 communicate here?

Auriscope

Answer 1

Cabal won't let you install base. I ran into this issue when trying to install the profiling version of base. Instead, you need to do something like:

sudo apt-get install haskell-platform-prof

See this thread for a discussion by cabal developers.

Ben