David Kastrup <d...@gnu.org> writes:
> Dan Eble <d...@faithful.be> writes:
>
>> %{
>> NullVoice is unlike a Voice in this respect: trying to move it to a
>> different Staff with \change fails with this warning:
>>
>> Change_iterator::process (): NullVoice = `soprano':
>> cannot change `Staff' to `S': not changing to same context type: Staff
>>
>> Apart from the warning, this is inconvenient. The NullVoice remaining
>> in the original staff keeps the original staff alive, demanding manual
>> work to make it disappear.
>>
>> The NullVoice context definition contains both of the following:
>>
>> \alias "Staff"
>> \alias "Voice"
>>
>> My knowledge of contexts is not extensive, but I suggest that the test
>> that \change applies is too simple.
>
> The test is
>
> else if (it.get_outlet ()->is_alias (to_type))
> {
> // No enclosing context was found because the iterator's immediate
> // context is the kind that was sought.
> /* We could change the current translator's id, but that would make
> errors hard to catch.
>
> last->translator_id_string () = get_change
> ()->change_to_id_string (); */
> result = _f ("not changing to same context type: %s", ly_symbol2string
> (to_type).c_str ());
> }
>
> and that's indeed crapola. What the test should do is check whether the
> current context type would be accepted into the hierarchy of the
> proposed parent.
Well, that's a bit too simplistic because there is no "current context
type".
For something like
\change StaffGroup = "what"
the change iterator looks at the current context hierarchy upwards until
it finds a "StaffGroup" as a parent. The layer below that is the
context to move, and if it is significantly similar in specification to
the context that should be/become its parent, it might become hard to
move back and forth.
So it's likely a bit of work/thought to make sure one does not create
context trapdoors from which it is hard to escape. Hovercraft parent
contexts.
--
David Kastrup
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