>     Making Emacs format a file from the command line is basically a one-line shell script.

Unfortunately Emacs is heavy and fickle to have as a build dependency.
I've had cheap VPS servers run out of disk space due to installing Emacs
from the OS package manager. It also adds bloat to Docker images.
Ephemeral containers are very convenient for CI builds (spin up
container, run build script, save build products, destroy container)
among other purposes.

>     Got it. Line breaking seems to be the hard part, based on the
>     above-linked article and the descriptions in the TeXBook, because of
>     the combinatorial explosion.

It may actually be that the line breaking algorithms in auto-formatters
are descendants of the TeX paragraph filling algorithm :)

> I once implemented a pretty printer for generated C++ code based on this
> paper: Strictly Pretty (Citeseer
> <http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.34.2200>).  The
> algorithm is clearly described, and I'm sure that it would be easy to
> apply to Scheme.

PDF: <https://lindig.github.io/papers/strictly-pretty-2000.pdf>

Very nice. Would you be willing to re-implement one for Scheme?

I suspect that for a really good job we'll end up needing a
penalty-points system like clang-format. It's probably the best
auto-formatter I've used, does a terrific job of C code and very rarely
produces anything ugly. Then again, C's block structure is way simpler
than Lisp since there's a clear separation between statements and
expressions, and a cultural attitude that it's the programmer's own
fault if complex expressions get ugly.

> (Alas, the references give no credit to earlier pretty
> printers in Lisp.  It's written as if Wadler invented pretty printing.)

CS literature can be weird. Perhaps because the field is so young that
we have few authorities.