On Tue, Dec 28, 2021 at 02:58:56AM -0500, Thien-Thi Nguyen wrote:
> 
> () <tomas@tuxteam.de>
> () Tue, 28 Dec 2021 08:19:04 +0100
> 
>    I /think/ the ellipsis is at a wrong place there [...]

> Haha, i know exactly how you feel.

%-)

> Ah, right!  The ellipses are a tail that need to follow a head.

No, the ellipses tell the matcher that the symbol to its left acutally
stands for "zero or more of this". Consequently, you have to somehow [1]
use those ellipses on the right hand side (aka template) whenever you use
them on the left hand side (aka pattern) and vice versa.

> I guess i was confused by the documentation's use of ellipses in
> the conventional sense rather than the literal sense:

I think of the ellipses as a kind of funny Kleene star. Not really at
home in any world: in the regular world, because it isn't a star, in
the Scheme world, because it is postfix.

So, to come back on your original example,

  (@ . ,attrs) ...

would match either empty (remember: zero or more) or

  (@ (foo 1) (bar 2))

or

  (@ (meep 1)) (@ "this ain't what we expected))

... so zero or more of (@ . stuff) where stuff stands for any
S-expression (note that it's sloppy: it will match things which aren't
lists of well-formed attribute lists [2]).

It won't match

  (@ (foo 1)) (@ (bar 2)) ("rumpelstilzchen")

for example.

> Anyway, thanks to your help, i've managed to cobble together the
> following code, attached here for the benefit of future self:

[...]

Thanks. And thanks for the code :)

> Maybe it's idiomatic.  Feedback on how to improve it welcome!

For the idiomatic part, I'll have to defer to those with more chevrons
;-)

Cheers

[1] Yes, some handwaving here.
[2] which isn't what you want to match, anyway. You want to match things
   with one attribute list in them.

Cheers
-- 
t