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Re: select a note in a chord

From: David Kastrup
Subject: Re: select a note in a chord
Date: Fri, 18 Jan 2019 00:05:06 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

Valentin Villenave <address@hidden> writes:

> On 1/17/19, Davide Bonetti <address@hidden> wrote:
>> I worked on your example, and here is the result.
> Nice!
>> I'm sure there is a better way to write the inversion function, but I
>> haven't find a way to program the repetition of a function in scheme.
> This is certainly not the most elegant way, but it seems to work:
> inversion =
> #(define-music-function (num music) (integer? ly:music?)
>    (let ((str "")
>          (up? (> num 0)))
>      (map (lambda (x)
>             (set! str
>                   (string-append str
>                     (if up? "\\rise 1 " "\\drop 1 ")))
>             str)
>        (iota (abs num)))

There is append-map but seriously?

  (string-concatenate (make-list (abs num)
                                 (if (negative? num) "\\drop 1 " "\\rise 1 ")))

>      #{ $(ly:parser-include-string str) $music #}))

This is not really an issue for string-manipulation.

> Can you verify that it works as you intended? (If you’re running 2.18,
> you’ll need to add "parser" after ly:parser-include-string.)

And parser location before num in (num music).

Let's rather do this in a sane manner:

inversion =
#(define-music-function (num music) (integer? ly:music?)
  (let loop ((num num) (music music))
     (cond ((zero? num) music)
           ((negative? num) (loop (1+ num) (drop 1 music))
           (else (loop (1- num) (rise 1 music)))))))

Assuming 2.18, music functions are not directly callable from Scheme
which would render this as

inversion =
#(define-music-function (parser location num music) (integer? ly:music?)
  (let loop ((num num) (music music))
     (cond ((zero? num) music)
           ((negative? num) (loop (1+ num) #{ \drop 1 #music #}))
           (else (loop (1- num) #{ \rise 1 #music #})))))

This does not mess with internal parsers.  And the 2.19 version does not
even engage the parser for the looping.

David Kastrup

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