When installing a package with cabal-install, it will also indirectly install all the dependencies. Given a certain package in my .cabal/packages folder that I didn't directly install, is there a way to find what other package(s) it was a dependency of?
I found this command somewhere (can't remember where now) and use it regularly to produce a dependency graph of my installed packages:
ghc-pkg dot | tred | dot -Tpng > pkgs.png
Note that it's actually ~/.ghc which contains the installed package information, rather than ~/.cabal.
You can also use:
ghc-pkg unregister <pkgname>
which will print a list of packages which would break if you uninstalled this package, which is effectively what you are looking for:
$ ghc-pkg unregister aeson
ghc-pkg: unregistering aeson would break the following packages: criterion-0.8.0.0 yesod-1.2.4 .... (use --force to override)
Using dot -Tsvg > pkgs.svg in the above command also allows you to use text searches (if you open the file in a browser, for example).
Also, the cab utility is very useful for showing dependencies and reverse dependencies, amongst other things.
For stack users stack dot --external can be used from your project directory in place of the above ghc-pkg dot.