> It does look very nice, and the last point release was in June:
> https://bitbucket.org/ktakashi/sagittarius-scheme/wiki/Home

Yeah. It's one of the most actively maintained Schemes. People just
don't know about it.

> Chibi Scheme is simpler, follows the truth in advertising principle.
> It's small and for me more comprehensible that the other currently
> maintained, up to date with R7RS-small Schemes I've looked at like
> Sagittarius or Chicken.  Also uses Gnu Make vs. e.g. CMake, which
> I'm not familiar with.

A normal build is "cmake . && make && make install".

It is quite a bit bigger than Chibi though, so they target different
niches. That's what I like about Scheme. Any environment or deployment
scenario you could possibly need to target, there's already an
implementation for the job. Now it's just a matter of polishing the
implementations and increasing portability to make it easier to share
code and jump around as requirements change.

> I will support R7RS in the fullness of time, and will be happy to
> cooperate with R6RS porting.  FFI porting will likely be a bigger
> task, depending on how easy it is for an implementation.

Chibi is perhaps the purest R7RS Scheme so you already have that :)

It'd be nice to have more portable FFI. Every language eventually has
that problem. I have some ideas but haven't had time to look at
r6rs-pffi or implement anything yet.

> Yes, the cond-expands for the PostgreSQL library are impressive, not
> just for Scheme implementations but also for the presence or absence
> of some other SRFIs it depends on.  (chibi net) appears to be a
> sufficient subset of SRFI 106 that little or nothing was needed there.

Yes :) Portable Scheme is already easier than most people think. Many
individuals are actively doing it but it's not much discussed, so
people's general impression is that Scheme is not portable.

Even R6RS<->R7RS converters are easy to write for a lot of real code.
Here's one in 65 lines:
<https://github.com/lassik/srfi-175/blob/master/srfi/r7rs-to-r6rs.scm>.
Akku comes with a generic converter that can handle most/all of R7RS by now.

The wildest cond-expands I've seen are in the SRFI 64 unit test
framework implementation:
<https://github.com/retroj/srfi-64-chicken/blob/master/srfi-64/srfi-64-port.scm>.
It's nice to be able to do that but it also shows why standards and code
portability are important :)