[Fontconfig] Regularizing contains operator semantics

[Keith Packard]

Around 11 o'clock on Jul 12, Owen Taylor wrote:

>  when a key is referred
> to as having the value "foo,bar", there are three possible
> interpretations of that:
> 
>  * A string containing an embedded comma
>  * A pattern with multiple values with the same key
>  * A pattern with a single value with a composite type (LangSet)

and the winner is 2) -- foo,bar represents a pattern with multiple values 
for the same key.  LangSets and Charsets were designed to be a more compact 
representation of this idea for those specific kinds of values; I think 
there are some places where the fact that they are stored in a single 
entry are exposed to the user and I'd like to close those holes.

> If I do:
> 
>    fc-list times,courier
> 
> I assume that the resulting pattern has two FC_FAMILY elements, one
> for times, and one for courier. 

yes, that's correct -- commas separate multiple values with the same key.

> But then I don't see how your proposed changes section:
> 
> > 1)      Use a Contains-alike operator for LISTING which does exact
> >        matching for strings, permit Contains for EDITING to do
> >        substring matching
>
> (will result in a change ...) to the behavior described above.

I think I missed a step -- LISTING will require matches for all values of 
each key, so

	$ fc-list times,courier

will list only fonts with *both* family times and family courier (i.e. no 
fonts at all).  Yes, this is useless, but I want to make sure the meaning
of 

	$ fc-list :lang=en,de

means to list only fonts with *both* english and german support.  Having 
different meanings for different keys seems like a really bad idea, worse 
than defining the behaviour of 'fc-list times,courier' as useless.

Thanks for reading through this stuff; I'm hoping to get a chance to write 
down a specification for the library semantics from this discussion.

-keith