[racket-dev] PLaneT(2): Single vs multi-collection packages

At Tue, 4 Jun 2013 07:41:53 -0400, Eli Barzilay wrote:
> Yesterday, Jay McCarthy wrote:
> > and you should deal with the non-proof of concept method of
> > specifying it in, for instance, the info file, which is now package
> > info AND collect info.
> 
> This shouldn't be a problem

At Tue, 4 Jun 2013 10:32:12 +0200, Laurent wrote:
> As Greg suggested, the single-collection package would have only one
> info.rkt file (which is simpler for the user[*]), and would contain both
> the info for the package and for the collection.

I agree with both of you.

My intent in echoing Jay's comment about "info.rkt" was to point out
areas that need attention to fill out the patch --- not to suggest that
it isn't possible to make things work.

More generally, I hope I haven't come across as being firmly opposed to
the idea of single-collection packages. I intended to come across as
being opposed to implementing the idea myself. :)

But it seems that I haven't convinced anyone to take on the
implementation task (understandably), and I've arrived back to a point
where I want single-collection packages (in the ongoing
collection-splitting experiment). So, the enclosed patch is a
suggestion on how to have single-collection packages.


With the patch, a single-collection package must have an "info.rkt"
file containing a definition of `single-collection' as a string, and
the string is used as the package's collection name. That is, you don't
have to know from the outside what kind of package you're installing;
it's up to the package author to choose single-collection mode or
multi-collection mode within the package's implementation. A package in
either mode can be upgraded to the other mode. If you install a package
with `--link' and you want to change modes, the only available
"upgrade" process (at the moment) is to remove and re-install the
package.

Some other the details:

 * A package's mode is recorded in the installed-package table.
   Otherwise, a linked package could switch modes just because the
   package directory's content changes, which would be difficult to
   keep in sync with the low-level table of links.

 * Both modes are handled in-place instead of trying to reshape one
   mode's directory structure to the other.

 * In a single-collection package, "info.rkt" doubles as the package's
   "info.rkt" and the collection's "info.rkt".

 * ".plt"-format files cannot encode single-collection packages. It's a
   deprecated format, anyway.

I experimented with various ways of distinguishing single-collection
packages from multi-collection packages. An "info.rkt" definition seems
best among the things I tried, at least. (A discarded experiment was to
treat a directory a single-collection package when it has any ".rkt"
file other than "info.rkt".)

As Jay pointed out to me, putting the collection name in the "info.rkt"
file --- instead of using the package name as the collection name ---
makes the package content work the same even when the package is
renamed. Otherwise, you have to know whether the package is
single-collection or multi-collection to know whether changing the
package's name changes its content.

It's debatable whether single-collection or multi-collection would be a
better default, but I favor multi-collection mode.

We can try different ways of distinguishing single-collection package
from multi-collection packages by changing the implementation of
`pkg-single-collection', as long as we stick to the distinction being
inside the package instead of outside.
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