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reading properties

From: Han-Wen Nienhuys
Subject: reading properties
Date: Thu, 4 Apr 2002 16:59:49 +0200

address@hidden writes:
> I consider it very hard to read properties from within lily.
> The problem is that you need to pass a pointer to some sort of musical
> expression - and I don't know how to get such a pointer.
> I.e. when thinking about the ottava-implementation it would be nice to
> replace
>   \property Staff.centralCPosition = #-13
> with
>   \property Staff.centralCPosition = #(- (ly-get-context-property
> "Staff" "centralCPosition") 7)
> But the "get-translator-property"-function needs to get a pointer to the
> Staff rather than just the "Staff"-string - and hence the above does not
> work...
> Is this something like this currently possible at all? Othervise this
> would be something nice to allow before 1.6, imho.

The proper solution would be to have a special new type of music
production, something like

            \applycontext SCHEME-FUNCTION

which would be a music expression/event, that when executed does

      (apply SCHEME-FUNCTION current-translator)

then what you want could be done using

     \context Staff
        \applycontext #(lambda (trans)
              (ly-set-trans-property trans centralCPosition
               (+ (ly-get-trans-property trans centralCPosition) 7)))

We could even use this mechanism as a basic way to express various
other operations, such as barchecks, \property, staff changes,
etc. Implementing it shouldn't be that hard. Basically, you have to
implement a new music-iterator that does the gh_call ().  Loads of fun
if you've never written such a beast before :-)


Han-Wen Nienhuys   |   address@hidden    |

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