Naming callback generators for grobs

[David Kastrup]

Sorry for being a few days late with this call for comments.

Please take a look at
<URL:https://code.google.com/p/lilypond/issues/detail?id=4045>.  The
issue is that we have a number of grob callback generators that create a
callback for some grob property.

Issue 4045 is about renaming one of them.  You may want to take a look
at the proposal in the code review.  Basically I am waffling back and
forth over several proposals.

Here are, as far as I can see, the current relevant callback generators:

(define ((grob::calc-property-by-copy prop) grob)
  (ly:event-property (event-cause grob) prop))

So calling grob::calc-property-by-copy with the name of a property
returns a function which, when called with a grob as argument, will look
up the event-cause of the grob and take the named property from that
event.

Then there is
(define ((grob::inherit-parent-property axis property . default) grob)
  "@var{grob} callback generator for inheriting a @var{property} from
an @var{axis} parent, defaulting to @var{default} if there is no
parent or the parent has no setting."

Some uses are in scm/define-grobs.scm, stuff like
scm/define-grobs.scm:        (text . ,(grob::calc-property-by-copy 'text))
scm/define-grobs.scm:        (X-offset . ,(grob::calc-property-by-copy 
'X-offset))
scm/define-grobs.scm:        (Y-offset . ,(grob::calc-property-by-copy 
'Y-offset))
scm/define-grobs.scm:        (transparent . ,(grob::inherit-parent-property
scm/define-grobs.scm-                         X 'transparent))
scm/define-grobs.scm:        (break-visibility . ,(grob::inherit-parent-property
scm/define-grobs.scm-                              X 'break-visibility))
scm/define-grobs.scm:        (color . ,(grob::inherit-parent-property
scm/define-grobs.scm-                   X 'color))

There is no particular reason that the referenced property and the grob
property calculated via a callback needs to be the same, though it will
in practice be more often than not the case.

For grob::calc-property-by-copy a few names were bounced around in the
Rietveld discussion.  My last proposal regarding
grob::calc-property-by-copy were grob::from-event-property and
grob::event-property-getter.  An intermediate proposal was just
grob::event-property but that is less likely to be expected returning a
callback procedure rather than a property.

What do you think?

Obviously, the same naming would want to get applied to
grob::inherit-parent-property which currently is named completely
differently.

-- 
David Kastrup