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

[Leah Velleman]

Ah! I hadn't even gotten as far as thinking about key-change events *within* the scope of the music function, though you're right that those would be an issue too (and your suggestion looks like a sensible way of dealing with them). 

What I was trying to get at was the key that was current before the music function was called. That is, the test case I was thinking about would look like this:

\new Staff {

  \new Voice {

    \key re \major   

    \test {

      do'4 mi' sol' do'' <do' mi' sol'>1

    }

  }

}

On Thu, Apr 30, 2015 at 7:23 PM, David Nalesnik <address@hidden> wrote:

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

    }

  }

}