Issue 2717: Implement \single for converting overrides to tweaks

Issue 2717: Implement \single for converting overrides to tweaks

\single, like \once, can be applied to music consisting of one or more
overrides.  It translates such an override set, for example
\easyHeadsOn, into a tweak on the following music item, so you can use
it like

{ < \single\easyHeadsOn c' g' >2 q }

inside of a chord and have it only affect a single notehead of the
chord when \once\easyHeadsOn would affect all noteheads.

In that manner, most overrides defined in ly/property-init.ly
are made available as tweaks.

[Keith, 2012-09-24 06:34:00]

Works well for me.

You mis-typed something in the description because 

> so you can use things like \single\voiceOne c4 .

is not true.  I can apply \single to simple sets of overrides, though, like
\single\voiceOneStyle

http://codereview.appspot.com/6495135/diff/2001/ly/music-functions-init.ly
File ly/music-functions-init.ly (right):

http://codereview.appspot.com/6495135/diff/2001/ly/music-functions-init.ly#ne...
ly/music-functions-init.ly:677: #{ \override $grob #'stencil = ##f #})
I also tried this with #'stencil = #point-stencil  
which avoids some programming-error messages when we \omit NoteHead.  

It is more useful as you have it, with #'stencil = ##f
because the point stencil affects spacing of other things that are padded away
from the point-stencil.  I can no longer find any cases that crash.

[dak, 2012-09-24 08:44:31]

On 2012/09/24 06:34:00, Keith wrote:
> Works well for me.
> 
> You mis-typed something in the description because 
> 
> > so you can use things like \single\voiceOne c4 .
> 
> is not true.

But it is _intended_ to be true.  It is just that LilyPond is an inconsistent
heap of garbage that does not bother generating the same kind of data for Scheme
generated overrides as it does for manually generated overrides.

And then the iterator takes a look and tries figuring out what kind of data
structure was actually used.  \single failed to do the same.

New patch up, but this will likely afflict other override-reinterpreting
commands as well.

Thanks for checking up on my over-optimistic statements.

[Reinhold, 2012-09-26 09:05:18]

is there any particular reason why you created a new command (\single) and
didn't reuse \once. IMO, using \once as an indicator that the next command
applies only to the next item would make it more consistent.

[dak, 2012-09-26 09:26:36]

On 2012/09/26 09:05:18, Reinhold wrote:
> is there any particular reason why you created a new command (\single) and
> didn't reuse \once. IMO, using \once as an indicator that the next command
> applies only to the next item would make it more consistent.

\once does something entirely different.  It does not turn an override into a
tweak but rather marks it at being active only at the current timestep.  \once
applies at a single time, \single applies on a single item.

My first reaction was "how can one not understand this?" but then I reread the
description, and it says
"\single can, like \once, translate larger override sets like \voiceOne into a
tweak, so you can use things like \single\voiceOne c4".

Uh, ouch.  This is a rather sense-altering distortion/contraction of
"\single, like \once, can be applied to music consisting of one or more
overrides.  It translates such an override set, for example \voiceOne, into a
tweak on the following music item, so you can use things like \single\voiceOne
c4 inside of a chord and have it only affect a single notehead of the chord". 
Which does not make all that much sense musically since \voiceOne mostly applies
to stuff like stems that are per-chord anyway.  So I probably should rather use
something like \single\easyHeadsOn for illustration.

I'll alter the description accordingly.

[reinhold_kainhofer.com, 2012-09-26 13:08:29]

On 26/09/2012 11:26, dak@gnu.org wrote:
> \once does something entirely different.  It does not turn an override
> into a tweak but rather marks it at being active only at the current
> timestep.  \once applies at a single time, \single applies on a single
> item.

To me as a user, \once\override is used to change the next note (or
items attached to it) and \single\tweak is used to change the next
item.

That \once\override applies to the next timestep, and the
following note incidentally happens at that timestep is an internal
detail when I'm writing a score.

From a developer's POV there is a different concept involved, but I
doubt that a normal user really cares about this.
To me it's basically the same as the \set / \override distinction. It
might be a different concept to anyone who knows the internals,
but to a user both are used to change the default.

Cheers,
Reinhold

-- 
------------------------------------------------------------------
Reinhold Kainhofer, reinhold@kainhofer.com, http://www.kainhofer.com
 * Financial & Actuarial Math., Vienna Univ. of Technology, Austria
 * http://www.fam.tuwien.ac.at/, DVR: 0005886
 * Edition Kainhofer, Music Publisher, http://www.edition-kainhofer.com

[dak, 2012-09-26 13:32:19]

Reinhold Kainhofer <reinhold@kainhofer.com> writes:

> On 26/09/2012 11:26, dak@gnu.org wrote:
>> \once does something entirely different.  It does not turn an override
>> into a tweak but rather marks it at being active only at the current
>> timestep.  \once applies at a single time, \single applies on a single
>> item.
>
> To me as a user, \once\override is used to change the next note (or
> items attached to it) and \single\tweak is used to change the next
> item.

Both are only roughly equivalent when there is only one item per
timestep.  \once\override affects _everything_ happening at the same
time.  Your complaint amounts to "I don't want to know about the
difference between an override and a tweak", but that's not a problem
that can be cured by not providing a method of converting one into the
other.

> That \once\override applies to the next timestep, and the
> following note incidentally happens at that timestep is an internal
> detail when I'm writing a score.
>
>>From a developer's POV there is a different concept involved, but I
>>doubt that a normal user really cares about this.

Shrug.  Feel free to ignore the distinction until things break for you,
but it is not like _I_ am responsible for it merely because I provide an
additional tool making it easier to deal with that difference.

> To me it's basically the same as the \set / \override distinction. It
> might be a different concept to anyone who knows the internals, but to
> a user both are used to change the default.

Code reviews are not the place for _unrelated_ LilyPond-bashing.  I get blamed
for enough things every day without the need to invent extra causes.

[reinhold_fam.tuwien.ac.at, 2012-09-26 22:52:02]

On 2012-09-26 15:32, dak@gnu.org wrote:
> Reinhold Kainhofer <reinhold@kainhofer.com> writes:
>
>> On 26/09/2012 11:26, dak@gnu.org wrote:
>>> \once does something entirely different.  It does not turn an
> override
>>> into a tweak but rather marks it at being active only at the current
>>> timestep.  \once applies at a single time, \single applies on a
> single
>>> item.
>
>> To me as a user, \once\override is used to change the next note (or
>> items attached to it) and \single\tweak is used to change the next
>> item.
>
> Both are only roughly equivalent when there is only one item per
> timestep.  \once\override affects _everything_ happening at the same
> time.

Ah, thanks for the explanation. I didn't understand that \single is 
meant to fix the case where \once applies to multiple items at the same 
timestep!

Cheers,
Reinhold

-- 
------------------------------------------------------------------
Reinhold Kainhofer, reinhold@kainhofer.com, http://www.kainhofer.com
  * Financial & Actuarial Math., Vienna Univ. of Technology, Austria
  * http://www.fam.tuwien.ac.at/, DVR: 0005886
  * Edition Kainhofer, Music Publisher, http://www.edition-kainhofer.com

[janek, 2012-09-28 05:00:53]

i don't feel competent to speak about scheme code, but the idea LGTM.