Re: strange behaviour of \layout block variable

[David Kastrup]

Janek Warchoł <address@hidden> writes:

> David,
>
> On Mon, May 21, 2012 at 7:20 AM, David Kastrup <address@hidden> wrote:
>> I don't see anything wrong in the _written_ assumptions.  Perhaps the
>> misunderstanding is that you consider context definitions as independent
>> from layout blocks, and just placed syntactically inside of them for
>> some unfathomable reason.  But each layout variable (including
>> $defaultlayout) has its own set of context definitions.  You can't
>> actually _change_ context definitions using LilyPond rather than Scheme
>> code.  If you write something like
>>
>>>   \context {
>>>     \Score
>>>     \override Hairpin #'thickness = 3
>>>   }
>>
>> This creates a _copy_ of all definitions in the current Score context
>> def, adds an override to it, and uses the result as a new context def.
>> This new context def is then used for Score because it contains a
>> \name "Score" definition, from the copy of the original Score context
>> def.
>
> Thanks for the explanation!  I think i now roughly understand how it
> works.  However, i find this design somewhat confusing, and i'm
> wondering whether it would be possible to change it (if changing it is
> a good idea at all).
>
> Going back to what we have, i thought that defining "layoutVariable"
> in this way will solve the problem:
>
> layoutVariable = {
>   \context {
>     \Score
>     \override Hairpin #'style = #'dashed-line
>   }
> }

Uh what?  That declares layoutVariable as sequential music.  This
sequential music starts with a \context missing a name and an undefined
command called \Score, containing an \override inside.

> Interestingly, it works but issues a syntax error.

Only one syntax error?

> Surprisingly,
>
> layoutVariable = {
>   \override Score.Hairpin #'style = #'dashed-line
> }
>
> (which is possible at all thanks to your work) doesn't issue any
> error, and does work.

But is anything but a layout variable.  It is an override inside of
sequential music.  You can leave off the braces so that it is only an
override, but it is still music.

> Does this mean that
>
>   \override Score.Hairpin #'style = #'dashed-line
>
> is "better" than
>
>   \context {
>     \Score
>     \override Hairpin #'style = #'dashed-line
>   }
>
> when used in \layout?

No.  It means that it is music rather than a context or layout
definition.  The "problem" when storing a context or layout definition
is that you store something absolute rather than a modification.  You
_can_ store context _modifications_ into variables as well, but they
don't carry a relation to a particular context definition.

There is no "layout modification", an incremental change to layouts.
The music has the advantage of containing just the wanted modification
_and_ the target of "Score".

> Until now i thought these two were equal.

They are.  And the second form is more direct.  The first form has the
advantage that it is easier to juggle around in variables.

-- 
David Kastrup