For sandboxing, users want to be able to use only local package dbs. To handle rts, base etc, they copy those specific packages from the global package db into a local package db.

Currently however there is no sensible way to tell ghc to look only at the listed package dbs, and not the global db. There is a flag -no-user-package-conf to avoid looking at the per-user one, but no equivalent to ignore or to specify the global one.

(Note: as a quirky non-sensible way, you can use GHC_PACKAGE_PATH="" to clear the package db stack, but this should be possible using command line flags.)

The user interface we're now using in cabal is like this:

• --global sets the initial db stack to [GlobalPackageDB]
• --user sets the initial db stack to [GlobalPackageDB, UserPackageDB]
• --package-db=clear clears the db stack to []
• --package-db=global pushes GlobalPackageDB on the top of the stack
• --package-db=user pushes UserPackageDB on the top of the stack
• --package-db=${otherfile} pushes SpecificPackageDB ${otherfile} on the top of the stack

It seems to me we could do something similar with ghc. Currently in ghc we have a -hide-all-packages flag that resets the package set to empty. We could do with an equivalent for the package database stack. I suggest something like:

• -clear-package-conf
• -global-package-conf
• -user-package-conf (and the existing -no-user-package-conf)
• -package-conf file

The default would remain that the global and user dbs are used, but this can be reset using -clear-package-conf.

Personally I prefer the nomenclarture "package db" rather than "package conf" here, but it's not that important.