Add layout changing command \layout-from for getting context defs from music

Add layout changing command \layout-from for getting context defs from music

[janek, 2012-02-23 00:13:34]

LGTM

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly
File ly/context-mods-init.ly (right):

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly#newcode45
ly/context-mods-init.ly:45: contexts.
I like this description.

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly#newcode49
ly/context-mods-init.ly:49: to @code{'Voice}.
I don't get what these Bottoms are.

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly#newcode55
ly/context-mods-init.ly:55: ;; Scheme.
I don't understand this.  What happens?

[dak, 2012-02-23 00:53:57]

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly
File ly/context-mods-init.ly (right):

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly#newcode49
ly/context-mods-init.ly:49: to @code{'Voice}.
On 2012/02/23 00:13:34, janek wrote:
> I don't get what these Bottoms are.

\set whatever = value is the same as \set Bottom.whatever = value.  Bottom is an
alias for Voice most of the time, but in some contexts (like TabStaff and below)
also for TabVoice, or for Lyrics or some other things.

How should I write this instead?

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly#newcode55
ly/context-mods-init.ly:55: ;; Scheme.
On 2012/02/23 00:13:34, janek wrote:
> I don't understand this.  What happens?
The parser turns all sets, overrides etc into something wrapped in
ContextSpeccedMusic.  If you ever get a set, override etc that is not wrapped in
ContextSpeccedMusic, the user has created it in Scheme himself.  In that case,
the code will try to use #f as a context modification resulting in an error. 
It's a situation that can only be triggered when you are messing in experienced
areas, so I don't bother providing a different behavior.

One could let "mods" be an empty context modification instead: in that case, the
bad commands outside of ContextSpeccedMusic would just silently get eaten.  I
prefer an explicit error, even if it is a Scheme execution error.  It is close
enough to the cause to be informative.

Proposal for a comment expressing that better?

[janek, 2012-02-23 05:52:32]

HTH,
Janek

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly
File ly/context-mods-init.ly (right):

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly#newcode49
ly/context-mods-init.ly:49: to @code{'Voice}.
On 2012/02/23 00:53:58, dak wrote:
> On 2012/02/23 00:13:34, janek wrote:
> > I don't get what these Bottoms are.
> 
> \set whatever = value is the same as \set Bottom.whatever = value.  Bottom is
an
> alias for Voice most of the time, but in some contexts (like TabStaff and
below)
> also for TabVoice, or for Lyrics or some other things.
> 
> How should I write this instead?

maybe
"if you specify optional @var{bottom} argument, all instructions that apply to
bottom-level contexts (for example Voice) will be applied to the context you
specified.  For example, you can tell layout-from to apply \override #'font-size
= #2 to TabVoice or Lyrics context instead of default Voice context."
(if i understood it correctly)

http://codereview.appspot.com/5686046/diff/1/ly/context-mods-init.ly#newcode55
ly/context-mods-init.ly:55: ;; Scheme.
On 2012/02/23 00:53:58, dak wrote:
> The parser turns all sets, overrides etc into something wrapped in
> ContextSpeccedMusic.  If you ever get a set, override etc that is not wrapped
in
> ContextSpeccedMusic, the user has created it in Scheme himself.  In that case,
> the code will try to use #f as a context modification resulting in an error. 
> It's a situation that can only be triggered when you are messing in
experienced
> areas, so I don't bother providing a different behavior.
> 
> One could let "mods" be an empty context modification instead: in that case,
the
> bad commands outside of ContextSpeccedMusic would just silently get eaten.  I
> prefer an explicit error, even if it is a Scheme execution error.  It is close
> enough to the cause to be informative.
> 
> Proposal for a comment expressing that better?

Something like
"All standard layout instruction events occur inside ContextSpeccedMusic.  We
initialize the context with #f and overwrite it using ContextSpeccedMusic - this
will fail and produce an error in case of special instructions written by user
in Scheme."
perhaps.
Or you could simply use the first paragraph of your explanation (The parser
turns...).  It's nice & clear to me.