Re: unusual let binding (of lp) - in function element->xml from sxml simple

[Damien Mattei]

Hi Andreas,
a good explanation is here:
https://stackoverflow.com/questions/31909121/how-does-the-named-let-in-the-form-of-a-loop-work
personally i avoid to use it, as i can do the same with internal
definitions .
regards,
Damien

On Sat, Feb 3, 2024 at 10:49 PM Andreas Reuleaux <rx@a-rx.info> wrote:

> Hi,
>
> I have a hard time understanding the let binding lp ...
> in the function element->xml - from module sxml simple -
> repeated here for convenience.
>
>
> --8<---------------cut here---------------start------------->8---
> (define (element->xml tag attrs body port)
>   (check-name tag)
>   (display #\< port)
>   (display tag port)
>   (if attrs
>       (let lp ((attrs attrs))
>         (if (pair? attrs)
>             (let ((attr (car attrs)))
>               (display #\space port)
>               (if (pair? attr)
>                   (attribute->xml (car attr) (cdr attr) port)
>                   (error "bad attribute" tag attr))
>               (lp (cdr attrs)))
>             (if (not (null? attrs))
>                 (error "bad attributes" tag attrs)))))
>   (if (pair? body)
>       (begin
>         (display #\> port)
>         (let lp ((body body))
>           (cond
>            ((pair? body)
>             (sxml->xml (car body) port)
>             (lp (cdr body)))
>            ((null? body)
>             (display "</" port)
>             (display tag port)
>             (display ">" port))
>            (else
>             (error "bad element body" tag body)))))
>       (display " />" port)))
> --8<---------------cut here---------------end--------------->8---
>
> Some background: sxml simple is - as the name implies -
> simple (or should be so). - It's implements
>
> * functions for parsing xml to s-expression
>   xml->sxml, ...
>
> * and functions for serializing sxml to xml
>   sxml->xml, ... (and among them: element->xml, as above)
>
> This second task (group of functions) is even simpler
> (than the parsing stuff) - I would say.
>
> Now these function (sxml->xml, element->xml) work for
> me: I can test them with some simple s-expression foo for example
>
> --8<---------------cut here---------------start------------->8---
> (define foo
>   '(d:para (d:emphasis (@ (role "strong")) "system analyst") " for "
> (d:link (@ (xl:href "http://www.blah.com";)) (d:emphasis (@ (role
> "strong")) "Blah")) " in Berlin, and so on"))
> --8<---------------cut here---------------end--------------->8---
>
>
> --8<---------------cut here---------------start------------->8---
> scheme@(guile-user) [13]> (sxml->xml foo)
> <d:para><d:emphasis role="strong">system analyst</d:emphasis> for
> <d:link xl:href="http://www.blah.com";><d:emphasis
> role="strong">Blah</d:emphasis></d:link> in Berlin, and so on</d:para>
> --8<---------------cut here---------------end--------------->8---
>
>
> sxml->xml making use of element->xml above.
> Now with regards to the binding / usage of lp above: it is
> bound / used several times:
>
>
>       (let lp ((attrs attrs)) ...
>       let lp be a function that ... does what?
>
>
>       (lp (cdr attrs)))
>       now call this function with (cdr attrs).
>       What does it do?
>       set attrs to (cdr attrs)?
>
>
>       then similar below
>       (let lp ((body body)) ...
>       ???
>
>
>       (lp (cdr body)))
>       ???
>
>
> For one thing I am used to
>
> (let bindings body)
>
> with bindings being of the form
>  ((x 'foo)
>   (y 'bar))
>
> (i.e. bind x with 'foo, y with 'bar when executing body)
>
> for example, lp is not of this style. Maybe if there is just one variable
> bound
> one can shortcut this to
>
>   let x 'foo body
>
> Is this what's going on above? And if so:
> then lp is bound to a list of one pair? :
>
> ((attrs attrs))
>
> how come can we call this as a function then later.
>
> And if attrs is a function (which I doubt) that what
> is the application of attrs to attrs?
>
> Anyway: confused. - Thanks in advanc,
>   A'
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