Scot:
 > Jens Axel:

>> My intution says that it's relatively seldom one needs to use
>>
>>more than one implementation at a time (but it can happen).
>>
>>
>
>I see this more frequently than you'd think.  Its not so much that *you*
>would write code that uses two collections of the same type, but it
>happens frequently when joining in modular code from more than one
>subsystem, especially where that system publishes an API which you write
>for where you need to be deliberately unaware of certain implementation
>details.
>
You have a point.

>This is especially important for maintenance of the resulting
>system.  Especially as Scheme systems start (and continue) to offer
>compiled modules, programmers or sysadmins may want to upgrade package
>Foo.  If Foo changes its selection of the underlying set collection, it
>would be onerous to require all packages that depend on it to be
>modified at the source code level to remain compatible.
>
Yes - but isn't it the role of the module system to take care of that?

[I'm for the sake of argument ignoring the fact that we don't have a
common module system.]

As an afterthought: It wouldn't be too difficult to support both
solutions in systems
with a module system.

--
Jens Axel Søgaard