Musings on interfaces and properties

[address@hidden]

Hey all,

Working on eliminating the translate_axis call from axis-group-interface, I've 
found a less-than-ideal situation in the naming/using of interfaces and 
properties that I'd like to clean up, but it'd be a major, major change and 
would take time to implement.  So, before starting it as a project, I want to 
know if people think it is a good idea.

Currently, interfaces can have whatever properties they want, leading things 
like `padding' or `staff-padding' to mean different things in different 
interfaces (tuplet-bracket-interface and side-position-interface, for example). 
 This is bad when you try to mix functions from the two interfaces together 
that think these properties mean different things.

To avoid this, there are a few steps we could take:

1) Require that, for a grob to use a callback from an interface, it must 
implement that interface.
2) Require that, for a callback to lookup a property, the property must be 
implemented by the interface that the callback is part of (i.e. 
ly:side-position-interface::y-aligned-side can only look up properties from 
side-position-interface).
3) Have interface multiple inheritance so that, for example, all properties of 
grob-interface are accessible by side-position-interface.
4) In cases where a grob implements multiple interfaces that have the same 
property, the property must be specified as address@hidden (address@hidden).  
Otherwise, LilyPond issues a warning and sets all paddings to the value for all 
pertinent interfaces.
5) In define-grob-properties.scm, specify all interfaces for properties.

What do people think?

Cheers,
MS