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Re: [Haskell-cafe] install-dirs on Mac OS X

Subject: Re: [Haskell-cafe] install-dirs on Mac OS X
From: wren ng thornton
Date: Wed, 23 Dec 2009 01:34:26 -0500
Mark Lentczner wrote:
Taking a cue from the various preinstalled language systems on Mac OS X, up 
over in /Library might be a better place:

        Python puts installed packages in:
        /Library/Python/<version>/site-packages

        Ruby puts installed packages in:
        /Library/Ruby/Gems/<version>
        /Library/Ruby/Site/<version>

        Java appears to use
        /Library/Java/Extensions
        and has a link to the packages that come the framework as:
        /Library/Java/Home
Which is just part of the symlink chain:

/Library/Java/Home
/System/Library/Frameworks/JavaVM.framework/Home
/System/Library/Frameworks/JavaVM.framework/Versions/Current/Home
/System/Library/Frameworks/JavaVM.framework/Versions/<shortVersion>/Home
/System/Library/Frameworks/JavaVM.framework/Versions/<longVersion>/Home

Which is rather convoluted, but is designed to allow switching over the default Java used internally by OSX. (Java6 is only available on 10.5+intel. And it's not recommended to switch the version registered for OSX, though it's fine to munge your $PATH to pick the new one since OSX internals doesn't use $PATH to resolve things.)


Overall, I'd like taking the cue to break things out by version number and then have some symlinking (or tool) in order to select one of many installed versions. This would be helpful for people who want to have multiple versions installed for debugging purposes. Even though the compiler version is already interposed on the lib dir path, giving a high-level separation by version makes it easier to deal with the different sets of libraries compiled by/for each version*compiler.
However, I would be opposed to storing anything in /Library or /System.
Those are for system use and sysadmin experience tells me never to mess
with the system installs. For things used by the system, it's liable to
break the system; and for everything, it's liable to be broken on system
upgrade. OSX does have their framework packaging system, which has been
used successfully in previous versions of GHC; if anything happens in
/Library or /System then it has to go through the framework packaging
system IMO, which means having someone as the OSX maintainer. But I
think the framework system isn't really a great option for the kinds of
development we end up doing. I.e., many Haskell hackers are devoted
developers and many of the packages require other *nix tools to be
installed as well. Because of that, I think it'd be a lot nicer to try
to integrate with the way other *nix installs are done on OSX: that is,
via Fink or MacPorts, or in /usr/local.
Besides, if we used something like a $HASKELL_PATH, or a tool for
querying and recording installation paths, then it doesn't really matter
where the default is since people can choose their own. All that matters
is the structure of the tree rooted there.


2) Structure of package pieces

I notice that cabal/cabal-install's default layout of where a package's pieces 
go, and GHC's layout of its base packages don't agree. Further, 
cabal/cabal-install's are not set up so that one can easily delete an installed 
package without hunting down its parts.
Again, it's good not to mess with system internals (where GHC is the
"system" here). In general we *want* Cabal to install things in a
different location than the GHC base libraries. Cabal packages can be
upgraded and removed and maybe even have multiple versions installed. If
someone borks their set of Cabal packages, they shouldn't have to
reinstall GHC as well in order to fix things. Similarly, the libraries
used by Cabal itself should be separate from the packages for users to
use. Both GHC and Cabal libraries could be accessible if the user
packages can't satisfy some dependency, but their maintenance should be
kept distinct.
I'm not saying the current Cabal defaults are the best or shouldn't be
changed, but those questions are orthogonal to the issue of interacting
with GHC's base libraries.

I think it best if everything for a package is in one place - making removal 
very easy:
   executables: --prefix--/packages/--pkgid--/bin
   libraries:   --prefix--/packages/--pkgid--/lib/--compiler--
   data:        --prefix--/packages/--pkgid--/share
   doc:         --prefix--/packages/--pkgid--/doc
   html:        --prefix--/packages/--pkgid--/doc
I put the "packages" level at the top, so that other things, like a master 
Haddock index dir could be put easily directly under the prefix.
I would be supportive of a setup with ./packages/<packageName>/ as the
root of <packageName>'s files. This is the strategy used by stow[1], as
well as texmf trees for LaTeX[2]. It works really well for avoiding and
resolving conflicts. There's some development overhead for setting up a
tool for merging the trees, but I think it's worth it (i.e., I'd be
willing to help write it).
The stow approach to tree merging is to symlink everything together into
one canonical directory, but that's not the only option. The texmf
approach is to direct queries through a program which maintains database
information, rather than directing queries through the filesystem. Both
approaches are used by other projects as well. The latter has a higher
development cost, but is ultimately more powerful and configurable.

[1] http://www.gnu.org/software/stow/
[2] http://www.tug.org/tds/tds.html (namely ./tex/latex/<pkg>/* stuff)

--
Live well,
~wren
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