SXML and optional attributes

SXML and optional attributes

[Luis Felipe]

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Hi,

I'd like to add attributes to SXML elements conditionally, but I don't 
see how.

I'm looking for a hypothetical "if*" that allows me to do the following:

(define* (special-input #:key name (required? #true))
   "Return an SXHTML INPUT element for typing text."
   `(input
     (@ (type "text")
        (name ,name)
        ,(if* required? '(required "")))))

The built-in "if" doesn't work here because it will return 
#<unspecified> when the condition is false, which is not an acceptable 
attribute. I'm not aware of any value that can be returned in an "else" 
expression that would mean "nothing" to SXML; the empty list is not it:

   ,(if required? '(required "") '())

I'm hoping to find something better than the following workaround:

(define* (special-input #:key name (required? #true))
   "Return an SXHTML INPUT element for typing text."
   (if required?
       `(input (@ (type "text") (name ,name) (required "")))
       `(input (@ (type "text") (name ,name)))))

Any ideas?

-- 
Luis Felipe López Acevedo
https://luis-felipe.gitlab.io/


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RE: SXML and optional attributes

[Maxime Devos via General Guile related discussions]

Unquote-splicing (,@) is your friend:

`(foo ,@(if p? '(a (b) c) '()) d)

When p? -> (foo a (b) c d).
When (not p?) -> (foo d).

Now do this for SXML …

Re: SXML and optional attributes

[Luis Felipe]

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On 22/12/24 21:38, Maxime Devos wrote:
>
> Unquote-splicing (,@) is your friend:
>
> `(foo ,@(if p? '(a (b) c) '()) d)
>
> When p? -> (foo a (b) c d).
>
> When (not p?) -> (foo d).
>
> Now do this for SXML …
>
Ah, splicing... yes Maxime, that works :)

So I applied it as follows:

(define* (special-input #:key name (required? #true))
   "Return an SXHTML INPUT element for typing text."
   `(input
     (@ (type "text")
        (name ,name)
        ,@(if required? '((required "")) '()))))

And now converting to XML I get the results I expected:

(sxml->xml (special-input #:name "author"))
→ <input type="text" name="author" required="" />

(sxml->xml (special-input #:name "author" #:required? #false))
→ <input type="text" name="author" />

Thank you very much,

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