On Fri, Jan 24, 2020 at 7:30 AM Lassi Kortela <xxxxxx@lassi.io> wrote:

Arthur and John know best about this topic, but I got the impression
that SRFIs are not intended to have optional parts. If optional features
are desired, the custom is to put them in a separate SRFI which has a
dependency on the other SRFI containing the core procedures. Cross-SRFI
dependencies are fine when they serve a purpose, and there are many
existing examples of them.

The SRFI process document doesn't say anything about optional features one way or the other. There are some SRFIs that specify features that are required only conditionally, e.g. if the core implementation has the full numeric tower. I'd generally prefer that SRFIs only have that kind of optional feature, but that's just a preference.