This is a shell script 'solution' I've put together so we can have this functionality even before it is implemented in cabal.

It looks for the latest version of the specified package, adds it to the *.cabal file then does a cabal install --only-dependencies.

Installation:

$ git clone git@github.com:Wizek/cabal-install-save.git
$ # Export to path:
$ echo 'export $PATH:'`pwd`'/cabal-install-save/bin' >> ~/.*hrc

Usage:

$ cd /to/a/cabal/project
$ cabal-install-save foobar

Questions, comments, issues and pull requests are welcome.