Re: Action of \unset

[Trevor Daniels]

Thanks Mats,

You wrote

> As far as I can see, it removes any settings of the property
> in the specified context (i.e. in the current context by default).

After I posted the query I hunted in the sources a bit and came to the same 
conclusion.  context.cc contains:

void
Context::unset_property (SCM sym)
{
 properties_dict ()->remove (sym);
}

which seems pretty definite.

> The reason that this often means that the setting is reverted
> to the default value is either
> 1. that the default is that the property is unset and that LilyPond
>    makes some suitable choice if the property is unset.
> 2. that the default setting is done at the Score level (or
>    at least higher up in the context hierarchy).
>
> The following example illustrates alternative 2:
> \relative{
> \set Staff.fontSize = #3
> c d e f
> \set Voice.fontSize = #1
> c d e f
> \unset Voice.fontSize
> c d e f
> \unset Staff.fontSize
> c d e f
> }
>
> \relative{
> \set fontSize = #3
> c d e f
> \set fontSize = #1
> c d e f
> \unset fontSize
> c d e f
> \unset fontSize
> c d e f
> }

It was an example like this that threw me, as I incorrectly generalised from 
this rather special action with fontSize in writing the section on \set in 
the LM originally.

This trick of setting a property at the Score level works for some other 
(but not all) context properties; for example, it works with clefOctavation 
but not with instrumentCueName.  I don't know what determines this.

> For a long time, I had the impression that \set and \unset did
> push and pop on a stack (if you know about these computer
> science concepts), but apparently I was wrong.

Me too.  I was fooled by the special action of fontSize.

> I'm not sure how to best explain these things, at least not in a
> Learning Manual.

I've had a go in LM 3.  It's in git as commit 
133986d102f82aa501a0e8592a80c3cd350ffdb1 -
GDP: Process comments on LM 3 (B).

As usual, git has added lots of changes which seem to do
nothing, but you can just about make out the changes I've
made.  Please let me know if you can suggest any improvements,
although it might be better to wait until the docs are
recompiled in GDP so you can see the changes in context.
(sorry! ;)

>   /Mats
Trevor

> Trevor Daniels wrote:
>
> > The description of \unset in the Learning Manual is currently incorrect, 
> > and I would like to replace that with an accurate description of its 
> > action, which seems to depend to some extent on the type of context 
> > property which is being unset.  Could someone please summarise exactly 
> > what \unset does. Thanks.
> >
> > Trevor
> >
> >
> > _______________________________________________
> > lilypond-devel mailing list
> > address@hidden
> > http://lists.gnu.org/mailman/listinfo/lilypond-devel
>
>
>
> --
> =============================================
> Mats Bengtsson
> Signal Processing
> Signals, Sensors and Systems
> Royal Institute of Technology
> SE-100 44  STOCKHOLM
> Sweden
> Phone: (+46) 8 790 8463 Fax:   (+46) 8 790 7260
> Email: address@hidden
> WWW: http://www.s3.kth.se/~mabe
> =============================================