It's somewhat annoying when using cabal-nix that whenever I call cabal update, a subsequent cabal-nix might start installing a bunch of new packages. (Somewhat related to @hvr 's way-back feature.)
Btw I actually love that cabal new-build rebuilds stuff against the latest versions by default, some stuff I'm working on relies on this very property. Freezing the package db should be something the user explicitly requests (e.g. by writing something like hackage-snapshot: 2015-12-24T14:43 into the cabal.project file).
Ah, yes, I'm not suggesting freezing should happen by default.
I hope I'll get to forward-porting the index-wayback branch very soon so we can glue it all together and see how it interoperates with nix-local-build, cabal.project & hackage-security
PS: See #3604
Note that always defaulting the latest and greatest of all packages also means we end up with tons of different permutations of packages with their dependencies in the store. Still not convinced that this is the most sensible default.
There is now cabal new-freeze which should have the desired effect.