RE: cabal design

On 16 September 2005 18:03, Isaac Jones wrote:

"Simon Marlow" writes:

...

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>

to Cabal? It would be another option in the --user/--global family. The idea isn't new - I'm sure we've discussed this before. It seems to suffer from none of the problems of the other suggestions, but perhaps I'm missing something. For Hugs, the <file> would actually specify a directory. Additionally, --package-db=<file> is a step on the way to other things that we want to do (--in-place).

The problem I have with --package-db= as a directory for Hugs (and nhc, and jhc) is that it conflicts with the meaning of the --prefix flag. That is, if you specify a package database, it changes the location of the installation, which is not what you might expect. Also, we'd have to make sure people don't try to use --prefix and --package-db together in the case of Hugs (unless they're equal).

Ah, I has read the section "Hugs Package Database" on the wiki and for some reason my brain had accepted it as The Truth rather than just a proposal. Nevertheless, such a setup is already possible using symbolic links, at least on Unix. I think that assuming a level of indirection between the package database and the actual installed location of the package is not a bad thing - Hugs will catch up. This isn't so bad. To be precise: --package-db works fully for GHC --package-db works fully for Hugs on Unix-like systems, using symbolic links. --package-db will not work on Windows with Hugs, currently.

Another option, since this will only really work for GHC, is to add --ghc-options and --ghc-pkg-options and people can specifcy the database they want, and it'll be clear that it's not a portable option.

That's ok, but not as attractive (IMO). Other systems will want to have the same facilities, and we'll end up with users of Cabal having to know the right --<foo>-options flags to pass to Cabal for their compiler. If we standardise on the idea of a package database as a filesystem path, which should be enough for everyone, then we can provide a portable interface.

So for the sake of those who haven't read the wiki page, here's my --root= proposal. This basically directly implements "virtualization" in Cabal:

* For GHC o --prefix now becomes $(ROOT)/$(PREFIX) o the ghc-pkg file is stored in $(ROOT)/.whatever o Cabal adds package-conf related flags to ghc and ghc-pkg to look at $(ROOT)/.whatever.

*this* is the bit I have a problem with. You haven't specified how we figure out the value of ".whatever", and it looks like it should vary depending on the value of $(ROOT), at least for GHC. Also, having both --root and --prefix is quite confusing - I don't think I fully grasp the significance of having both options, and I'm sure it'll confuse users. This just doesn't look as simple as the --package-db flag. We want the ability to specify the package db anyway, and this gives us virtualisation too. Two birds with one stone! Cheers, Simon