Re: How to create a music function properly?

[David Kastrup]

Marc Hohl <address@hidden> writes:

> Hello list,
>
> after having written mails way too early to this list recently
> while overlooking the obvious, I think I stumbled upon a *real*
> problem now :-/
>
> Please have a look at the attached file. This is not the real world problem,
> but I boiled it down to a nearly minimal example.
>
> Only version 3 is giving the desired output in standard notation,
> but ignoring the \override.
>
> What am I doing wrong here?
>
> Thanks in advance!
>
> Marc
>
> \version "2.13.32"
>
> % thanks to Neil for this function
> #(define (make-harmonic mus)
>   (let ((elts (ly:music-property mus 'elements))
>         (elt (ly:music-property mus 'element)))
>        (cond
>       ((pair? elts)
>        (map make-harmonic elts))
>       ((ly:music? elt)
>        (make-harmonic elt))
>       ((music-is-of-type? mus 'note-event)
>        (set! (ly:music-property mus 'articulations)
>              (append
>                (ly:music-property mus 'articulations)
>                (list (make-music 'HarmonicEvent))))))
>        mus))
>
>
> harmonicTestOne = #(define-music-function (parser location music) (ly:music?)
>   (let* ((test 2)
>          (result (/ test 2)))
>
>   #{
>     \override TabNoteHead #'transparent = ##t
>   #}
>   (make-harmonic music)
>   (display "\nDummy output to check the let-block: ")(display result)
>   #{
>     \revert TabNoteHead #'transparent
>   #}))

Uh, #{ #} creates and returns a value.  Only the last value in a Scheme
function is the return value of the function.

> harmonicTestTwo = #(define-music-function (parser location music) (ly:music?)
>   (let* ((test 4)
>          (result (/ test 2)))
>   #{
>     \override TabNoteHead #'transparent = ##t
>
>   #(begin
>      (display "\nDummy output to check the let-block: ")
>      (display result)

Uh, you need $result here, or the global result will be used.  Also, a #
expression is evaluated when it is encountered.  That may be before the
\override is considered complete.

>      (make-harmonic $music))
>     \revert TabNoteHead #'transparent
>   #}))
>
> harmonicTestThree = #(define-music-function (parser location music) 
> (ly:music?)
>   (let* ((test 6)
>          (result (/ test 2)))
>
>   #{
>     \override TabNoteHead #'transparent = ##t
>   #}
>   (begin
>      (display "\nDummy output to check the let-block: ")
>      (display result)
>      (make-harmonic music))))

You don't return your override as the resulting music function
expression.  Expressions in #{ ... #} don't "happen" when encountered.
They "happen" by returning them to the music function caller.  So the #{
#} is a do-nothing here.

> %{
> harmonicTestFour = #(define-music-function (parser location music) (ly:music?)
>   #{
>   #(let* ((test 8)
>           (result (/ test 2)))
>
>     \override TabNoteHead #'transparent = ##t
>   #(begin
>      (display "\nDummy output to check the let-block: ")
>      (display result)
>      (make-harmonic music)))
>   #})
> %}

Outcommented.

>
> test = {
>  c2^"Test 0" < c g >2
>  \harmonicTestOne { c4^"Test 1" < c g >4 }
>  \harmonicTestTwo { c4^"Test 2" < c g >4 }
>  \harmonicTestThree { c4^"Test 3" < c g >4 }
>  %\harmonicTestFour { c4^"Test 4" < c g >4 }
> }
>
> \score {
>   <<
>     \new Staff {
>       \new Voice {
>         \clef "treble_8"
>         \test
>       }
>     }
>     \new TabStaff {
>       \new TabVoice {
>         \test
>       }
>     }
>   >>
> }

-- 
David Kastrup