Re: Proposal: change Grob.id to Grob.output-properties

[Carl Sorensen]

On 9/15/16 6:37 PM, "lilypond-devel on behalf of Paul"
<address@hidden on behalf of
address@hidden> wrote:

>
>Looking at the code, the changes seem pretty straightforward to do.
>Assuming we agree this is a good improvement to make, the question I
>have is about migrating existing user files to the new property.
>Literal strings are easy enough to handle with convert-ly:  = "abc"
>converts to  = #'((id . "abc"))

Yes -- this looks good to me.

>
>The problem is when users have assigned a procedure (that returns a
>string) to this property, which is surely a common use case.  I don't
>see a reasonable way to handle that with convert-ly.  Maybe it is
>possible but seems like it would get ugly.

Just use a can't convert rule; have the user do the conversion.

>
>So we could make the new property's type be "string-or-list?". (We'd
>have to define that predicate.)  Then for strings, output a deprecation
>warning, but go ahead and handle the string as the user expects.  That
>would give users time to rewrite their code and then at some future
>point we change the type to just "list?" and only support alist values.

I think that using string-or-list? would make future code unnecessarily
complicated.  We should make the change once, rather than dragging it out
for years.

Let's just face the pain once.

Thanks,

Carl