Re: sxml-match bad pattern syntax

[Thien-Thi Nguyen]

() <tomas@tuxteam.de>
() Tue, 28 Dec 2021 08:19:04 +0100

   I /think/ the ellipsis is at a wrong place there. Note that
   I'm coming from "traditional" match, and I just have wrapped
   half of my brain around that (which, BTW, is somewhat painful
   :-)

Haha, i know exactly how you feel.

   So take this with a grain of salt.

   * Phenomenology (aka: I barely know what I'm doing):

   If you replace your ellipses above by ". ,rest", things seem
   to work:

    (define (unbogus x)
      (sxml-match x
        ((a (@ . ,attrs) . ,rest)
         `(a (@ ,@(delete '(shape "rect") attrs)) . ,rest))))

    (unbogus '(a (@ (shape "rect") (href "foo.html")) "kid"))

    => (a (@ (href "foo.html")) "kid")

Cool.  I guest the ". ,rest" is a shortcut for the ellipses.

   * Philosophy (aka blah, blah)

   If those ellipses resemble what match do, I think they are
   wrong there:

     (@ . ,attrs) ...

   would mean zero or more times the shape "(@ . ,attrs)". I
   think this isn't what you want.

Ah, right!  The ellipses are a tail that need to follow a head.
I guess i was confused by the documentation's use of ellipses in
the conventional sense rather than the literal sense:

 (define (album->html x)
   (sxml-match x
     [(album (@ (title ,t)) (catalog (num ,n) (fmt ,f)) ...)
      `(ul (li ,t)
           (li (b ,n) (i ,f)) ...)]))

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

#!/usr/bin/guile -s
!#
(use-modules
 (ice-9 pretty-print)
 (sxml match))

(define (pp x)
  (pretty-print x)
  (newline))

(define (unbogus x)
  (sxml-match x
    ((a (@ . ,attrs) . ,rest)
     `(a (@ ,@(delete '(shape "rect") attrs)) . ,rest))
    (,otherwise
     (if (string? x)
         x
         `(,(car x) ,(cadr x)
           ,@(map unbogus (cddr x)))))))

(define one '(a (@ (shape "rect") (href "foo.html")) "kid"))
(newline)
(pp one)
(pp (unbogus one))

(define bad `(p (@) "some text and " ,one " and " ,one))
(newline)
(pp bad)
(pp (unbogus bad))

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

-- 
Thien-Thi Nguyen -----------------------------------------------
 (defun responsep (query)               ; (2021) Software Libero
   (pcase (context query)               ;       = Dissenso Etico
     (`(technical ,ml) (correctp ml))
     ...))                              748E A0E8 1CB8 A748 9BFA
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