On Fri, Sep 16, 2005 at 01:16:26PM +0100, Simon Marlow wrote:
Ok, I've read through this discussion. Could somebody explain to me why this whole "virtualization" thing can't be solved by simply adding the flag
--package-db=<file>
Nope, can't explain it! Seems like a good solution to me too. Although I think it would be nice if Cabal were able to automatically pick a "standard" package database which is a function of the 'prefix' (hence the --default-package-db option I mention on the wiki); that is not crucial, if we think that it would add too much complexity. The --user and --global options would let users specify the standard databases in the most common cases.
... It seems to me that the requirement is basically this:
- To be able to install packages in a group, such that all packages in the group are registered in a single, separate, package database.
the fact that in the proposed use cases the packages are also installed under a pseudo-root is a red herring as far as Cabal is concerned. We don't care where you install your packages or what files they contain.
That seems correct. I might say that, although it is not in itself a requirement, it certainly satisfies all the other requirements, and there is no reason not to be simple and general.
So the question becomes how to specify which "package group" a package should be registered in. I propose the simplest thing: tell Cabal the location of a package database. The other proposals (--prefix and --root) require Cabal to magically derive the location of the package database from the "root" directory, but that's magic that we don't need/want IMO.
I'm happy to add support for GHC_PACKAGE_PATH, as requested in this discussion.
Excellent. (Jacques Frechet, the author of Toast, has spent some time modifying Toast so that versions of GHC it installs have their binaries wrapped to support this environment variable, but I thought his solution was always a bit hacky. It would be good to have actual support in GHC.) Frederik -- http://ofb.net/~frederik/