How can I use GHC with a cabal sandbox that's not in the current working directory?

Question

If I create a cabal sandbox with cabal sandbox init, I can use cabal repl or cabal exec ghc(i) to work with those packages without creating a project:

$ mkdir /tmp/example && cd /tmp/example
$ cabal sandbox init
$ cabal install QuickCheck
$ cabal exec ghci
Prelude> :m Test.QuickCheck
Prelude Test.QuickCheck>

However, if I change the path to something else, even to a subdirectory, I cannot access the packages anymore:

$ mkdir -p /tmp/example/sub && cd /tmp/example/sub
$ cabal exec ghci
Prelude> :m Test.QuickCheck
<no location info>:
    Could not find module ‘Test.QuickCheck’
    It is not a module in the current program, or in any known package.

Is there any way to use the contents from the sandbox, without copying its content?

Answer 1

The problem is that cabal will only respect sandboxes in the current working directory. However, there are several options where you can specify a sandbox location for cabal or the package databse for GHC.

Using cabal features

You can use cabal's --sandbox-config-file option to specify a sandbox configuration, e.g.

$ cabal --sandbox-config-file=/tmp/example/cabal.sandbox.config exec ghci
Prelude> :m Test.QuickCheck
Prelude Test.QuickCheck>   

This also enables you to change the sandbox from other places, which comes in handy if you just want to install random stuff into a temporary place:

$ cabal --sandbox-config-file=/tmp/example/cabal.sandbox.config install lens
$ cabal --sandbox-config-file=/tmp/example/cabal.sandbox.config repl
Prelude> :m Control.Lens
Prelude Control.Lens> :m Test.QuickCheck
Prelude Control.Lens Test.QuickCheck>

Since this gets cumbersome after a while, you should probably add an alias

$ alias sandboxed-cabal="cabal --sandbox-config-file=/tmp/example/cabal.sandbox.config"
$ sandboxed-cabal repl
Prelude>

Using ghc -package-db

Alternatively, you can directly specify the package database when you use GHC with -package-db:

$ ghci -package-db /tmp/example/.cabal-sandbox/<ARCH>-packages.conf.d
Prelude> :m Test.QuickCheck
Prelude Test.QuickCheck>

The <ARCH> depends on your system and the used GHC, e.g. on a 64bit Linux and GHC 7.10.3 it's x86_64-linux-ghc-7.10.3-packages.conf.d. You can then use all packages in that database:

$ ghci -package-db /tmp/example/.cabal-sandbox/<ARCH>-packages.conf.d
Prelude> :m Control.Lens
Prelude Control.Lens> 

Again, an alias should come in handy.

Using GHC_PACKAGE_PATH

Last, but not least, you can adjust an environment variable. However, if the environment variable GHC_PACKAGE_PATH exists, it will overwrite GHC's usual package databases, so you either need to check ghc-pkg list and add them too

$ GHC_PACKAGE_PATH=/opt/ghc/7.10.3/lib/ghc-7.10.3/package.conf.d/:/tmp/example/.cabal-sandbox/x86_64-linux-ghc-7.10.3-packages.conf.d ghci

or use -global-package-db and -user-package-db to reenable them:

$ GHC_PACKAGE_PATH=/tmp/example/.cabal-sandbox/x86_64-linux-ghc-7.10.3-packages.conf.d ghci -global-package-db -user-package-db