Re: Accessing context properties (e.g. the current key) from a music function?

[Thomas Morley]

2015-05-01 1:23 GMT+02:00 David Nalesnik <address@hidden>:
> Hi Leah,
>
> On Thu, Apr 30, 2015 at 3:16 PM, Leah Velleman <address@hidden>
> wrote:
>>
>> I'm looking to write a \transpose-like music function that will need to be
>> able to "find out" what the current key is.
>>
>> (In case it matters: The reason I'm doing this is to make it possible to
>> enter music using movable-do solfege. The idea is to have a function
>> \movableDo that will transpose from C (i.e. "do" in a fixed-do system) to
>> the tonic of the current key, so that
>>
>> \key a \major
>> \movableDo { do re mi fa so }
>>
>>
>> will produce the same output as
>>
>> \key a \major
>>
>> a b cs d e
>>
>>
>> Of course, it would be possible to require the user to repeat the key
>> information, by writing this:
>>
>> \key a \major
>>
>> \movableDo a { do re mi fa so }
>>
>> But this is inconveniently redundant, and makes errors more likely in
>> large source files, since the key must now be written in many different
>> places. So I'm trying to avoid doing it this way if possible.)
>>
>> In any case, I'm having a very difficult time figuring out how a music
>> function can get access to the key. It seems that the issue is that the key
>> is a context property; music functions do not take a context as an argument,
>> and thus can't get access to context properties the way a function written
>> with make-apply-context could. But I can't see how to get around that
>> limitation, or indeed whether it would be possible to get around it at all.
>
>
> I think the only way you'll be able to get access to the key from a music
> function is actually to parse the music expression, looking for the relevant
> music events.
>
> Here's something that's pretty limited, but it show that such a thing is
> possible.
>
> Note that in this sketch, you have to apply the function to each key area
> individually.  It would be nice if you could apply the function to the whole
> music expression, but first things first :)
>
> Hope this gets you started--
>
> David
>
> %%%%%%%%%%%%%%%%%%%%%%%%%%
>
> \version "2.18"
>
> \language "italiano" % the closest preset :)
>
> test =
> #(define-music-function (parser location music)
>    (ly:music?)
>    (let ((t (ly:make-pitch 0 0))) ; C is the default tonic
>      (music-map
>       (lambda (mus)
>         (if (music-is-of-type? mus 'key-change-event)
>             (set! t (ly:music-property mus 'tonic)))
>         (if (music-is-of-type? mus 'note-event)
>             #{
>               \transpose #(ly:make-pitch 0 0) #t #mus
>             #})
>         mus)
>       music)
>      music))
>
>
> \new Staff {
>   \new Voice {
>     \test {
>       \key re \major
>       do'4 mi' sol' do'' <do' mi' sol'>1
>     }
>     \test {
>       \key fa \major
>        do'4 mi' sol' do'' <do' mi' sol'>1
>     }
>     \test {
>       \key sol \minor
>        do'4 mib' sol' do'' <do' mib' sol'>1
>     }
>   }
> }
>

This seems to work as well, or am I missing something?

\test
\new Staff {
  \new Voice {
     {
      \key re \major
      do'4 mi' sol' do'' <do' mi' sol'>1
    }
     {
      \key fa \major
       do'4 mi' sol' do'' <do' mi' sol'>1
    }
     {
      \key sol \minor
       do'4 mib' sol' do'' <do' mib' sol'>1
    }
  }
}


Cheers,
  Harm