[plt-scheme] scribblings on PLT Scheme 4.0

In general, the more quickly PLT Scheme versions break backwards
compatibility the less useful old code is, on PLaneT and elsewhere.
But in the particular case of making pairs immutable by default, it
doesn't look like this will be a major source of incompatibility for
PLaneT packages: only 30 modules (not packages, module files) in all
most-current-versions of PLaneT packages (1359 total) contain any uses
of set-car! or set-cdr!.

This is one reason why default-mutable pairs are so frustrating: very
little code actually takes advantage of the ability to mutate pairs,
and code that does could almost always be rewritten easily; but the
fact that pairs _could_ be mutated breaks the soundness of a whole lot
of reasoning.

-jacob

On 5/24/07, Grant Rettke <grettke at acm.org> wrote:
> I'm all for breaking changes, but what will happen to Planet?
>
> Here is the worst case scenario:
>
> New users try out DrScheme and look to Planet as a resource of
> interesting libraries only to find out that the ones that they
> download are broken.
>
> On 5/23/07, Matthew Flatt <mflatt at cs.utah.edu> wrote:
> > At the language level, we are considering a few changes for version
> > 4.0. The most significant are:
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