>         (declare-file
>             (indent (define-html-template 2)))
>
>         at the top of every source file using `define-html-template`.
>
>     Yes, that would be fine.  My point was just that it would no longer
>     be a zero-configuration printer.
>
> I don't think putting stuff in files is general enough, because not all
> pretty-printing is of code.  Indeed, Emacs should be used for that (I do
> even though I don't edit with Emacs).  It's pretty-printing data,
> especially at the REPL, that's the most important use of pp IME.

Indeed, formatting code and pretty-printing an arbitrary S-expression
are two different tasks. The declare-file stuff should only affect the
formatting the source file it's in.

Emacs scheme-mode should follow the same rules as all other Scheme code
formatters. But it's useful to have formatters callable from outside
Emacs: for example, batch formatters (`make format` or similar), linters
for pre-commit hooks (is my code formatted right when I commit it?),
continuous integration jobs, etc. And something to call from Vim :) Who
knows, maybe it's even useful to format Scheme code from the REPL.

In the same vein, there should be a "one true Common Lisp coding style"
and auto-formatters for that, etc. I've dealt with auto-formatters for
lots of languages, and us Lispers have it very easy.