Re: keyAlterationOrder

[Jean Abou Samra]

Hi,

> > > I know that the alist cannot
> > >               be read with assoc, at least in its simple
> > >             
> > > form.
> > >
> > > Why not?
> > >
> > >           
> >           assoc would read the first key of a given value none of
> >             the others keys of
> >           
> > the same value would be read in the chain.
> >         
>         It is at best a stretch to call this property an association
>           list to begin with. An alist is a list of key-value
>           pairs; but this property is a list of
>           step-alteration pairs with no associative semantics. To use this
>           list, you need only filter it to the items you are interested
>           in, and the resulting list will have the items in
>           the desired order. Consider this contrived example:
>         
> %%%%
> \version "2.20.0"
>
> \new Voice {
>   \applyContext
>   #(lambda (context)
>     (let ((keyAlterationOrder
>            (ly:context-property context 'keyAlterationOrder))
>           (pitches #{ \fixed c' { cis dis ees fis gis aes bes } #}))
>      (set! pitches (map (lambda (pitch)
>                          (cons (ly:pitch-steps pitch)
>                                (ly:pitch-alteration pitch)))
>                         (music-pitches pitches)))
>      (format #t "\n  Unordered: ~s" pitches)
>      (format #t "\n    Ordered: ~s"
>       (filter
>        (lambda (elem) (member elem pitches))
>        keyAlterationOrder))))
> }
> %%%%
> ====
> Parsing...
> Interpreting music...
>          Unordered: ((0 . 1/2) (1 . 1/2) (2 . -1/2) (3 . 1/2) (4 .
>           1/2) (5 . -1/2) (6 . -1/2))
>          Ordered: ((6 . -1/2) (2 . -1/2) (5 . -1/2) (3 . 1/2)
>           (0 . 1/2) (4 . 1/2) (1 . 1/2))
>         
> ====
>
>         Converted from (step . alteration) back to pitch, the final
>           order is: Bb Eb Ab F# C# G# D#. This should make
>           sense given the standard convention of how flats and
>           sharps are arranged.
>

You are right indeed, it cannot really be called an alist
and assoc is of little use. Thanks for the example.

I think you misunderstood Carl. The order of the alist is the
critical piece of information. This property controls the order
in which alterations are printed in the key. For example:

I originally thought it did; but i can not see the logic by studying the two examples above

Aaron's reply should answer that.

> > One question though,   Does the key signature affect the midi output?
> >         
> Yes, Key_performer generates a key signature meta event (FF 59).
>

Absolutely. Note that in MIDI, key signatures are limited to
a number of flats or sharps and a boolean representing major
or minor scale [1]. So the MIDI output is unaffected by
keyAlterationOrder.

Regards,
Jean

[1] http://www.somascape.org/midi/tech/mfile.html#meta