Is there a way to find why cabal installed a certain package?

Question

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?

Answer 1

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)

Update

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.

Answer 2

I found cabal-db to be helpful. For example, you can run

cabal-db revdeps semigroupoids

and it will tell you

zippers: semigroupoids (>=4 && <5)
wl-pprint-extras: semigroupoids (>=3 && <5)
vector-instances: semigroupoids (>=3)
validation: semigroupoids (>=4.0)
transformers-abort: semigroupoids (>=1.2)
these: semigroupoids (>=1.0 && <4.1)

etc...