Hi Stanislaw,
What is going on here? Why am I getting different errors with the new cabal version?
No idea. Perhaps remove the sandbox for the hakyll build and start a new one.
What is the relationship between system-wide packages and those in the sandbox? I thought sandbox is meant to be an isolated environment?!?
Some packages are fixed with the GHC version, like the base one.
How do I get the list of all the packages currently installed in the sandbox? Is this:
$ ls ./lib/x86_64-linux-ghc-7.6.3/ the way?
Yes.
Why is 'base' not on the list, although the new cabal is not complaining about it anymore?
The last hakyll version supporting your ghc compiler 7.6.3 was 4.7.5.2. I don't know why cabal gets stuck here, it should take this hakyll version, but you might help cabal with its dependency resolution by directly installing 4.7.5.2: cabal install hakyll-4.7.5.2 Greetings, Daniel