Re: how to repeat a scheme function that creates a Score

[Jean Abou Samra]

Le 07/04/2022 à 07:55, Jeff Olson a écrit :
> On 4/6/2022 12:35 AM, Jean Abou Samra wrote:
> > > The attached MWE file has lots of my failed attempts commented out 
> > > (I'm at a teachable moment if someone has a moment to teach).  Or 
> > > just point me to the right manual page(s).
> >
> >
> > Here is a piece of code that works:
> >
> >
> > $@(map (lambda (i) (scr)) (iota 10))
> >
> >
> > Now let's go through your attempt to see why they are failing. 
>
> This is an amazing user group! Thank you times three to Aaron, 
> Valentin and Jean for your immediate and expert responses!
>
> And special thanks to you, Jean, for giving me a full course 
> explaining why all my previous attempts had failed.  That was a very 
> valuable learning experience!
>
> The code I'm using now is:
>
> %%%%
> % scors generates a specified number of scores
> % e.g. invoke $@(scors 10) from lilypond to generate ten scores
> #(define (scors num) (map (lambda (i) (scr)) (iota num)))
> %%%%
>
> This works nicely as I can make a series of invocations in different 
> bookparts (simulating separate chapters) while preserving the overall 
> sequence of piece numbers.
>
> I'd like you to check my learning on "$@" ...
>
> Per the ER section 1.2.1, "There are also ‘list splicing’ operators 
> |$@| and |#@| that insert all elements of a list in the surrounding 
> context."
>
> So it's not part of scheme but part of lilypond, as a bridge between 
> scheme and lilypond.


Yes.


> And that it tells lilypond to preserve all the elements of scheme list 
> value as a sequence of expressions/values to be parsed/interpreted by 
> lilypond, as opposed to getting just the value of the last element of 
> the list.


Not the last element of the list, but the list itself. If you do
myVariable = #(list ...), it assigns a list value to myVariable.
Assuming the list we're talking about is #(list a b), referencing
it with #x gives you #(list a b), whereas #@x is (modulo subtle
parsing details I think) equivalent to #a #b.


> And so it is analogous (at least mnemonically) to the meaning of "$%" 
> in bash (if I remember correctly), which preserves a sequence of 
> command line arguments as separate values rather than combining them 
> as one argument.


I think you mean '$@', but it's kind of the same thing, yes.


> But may I ask, why are "$@" and "#@" called 'list splicing' 
> operators?  The same terminology is used in Perl for what appears to 
> me to be very different behaviors (list editing, insertion and 
> removal), so I'd appreciate learning why 'splicing' is considered 
> descriptive in lilypond.


Well, it's about the same, isn't it? As far as I can read from
Perl tutorials, list splicing allows you to insert all elements
from a list to replace some sequence in another list. For LilyPond,
the inspiration is not Perl but rather Scheme (of course), which
has an unquote-splicing operator, shortened as ",@" (hence the
spellings "#@" and "$@").

scheme@(guile-user)> (define lst '(1 2 3))
scheme@(guile-user)> `(a b ,lst c)
$1 = (a b (1 2 3) c)
scheme@(guile-user)> `(a b ,@lst c)
$2 = (a b 1 2 3 c)

The role of ",@" is to insert all elements from its argument
into the surrounding list. That's quite similar to what "#@"
and "$@" do.

Best,
Jean