remove_s4_classes() respecting inheritance

[famuvie]

prevents derived classes to remain orphan
Fixes #848

[famuvie]

Mmm... contrary to #848, with this reverse removing of classes the error arises when the derived class is named 'splines'. What is it special with this name?

[famuvie]

Ok. There is nothing particular with the class name splines except for the fact that alphabetically comes after my_class. The current devtools version removes classes by alphabetical order which happen to coincide with inheritance hierarchy (i.e. my_class is extended by splines).

I needed to write a small routine to sort the classes defined in the package, so that extended classes are before their respective extensions. With this approach I think the problem is solved.

[hadley]

@wch happen to remember an elegant way to do this?

[hadley]

Ideally you'd do a topological sort on the S4 dependency graph before deleting

[famuvie]

Right. My solution worked for me, but not necessarily in general.
Implemented a topological sorting. Not very fast in R, but graphs of classes should not be huge.

[hadley]

Could you please add some unit tests for sort_s4classes()?

[famuvie]

sure... in a couple of weeks.

[famuvie]

Done, with some modifications thanks to the testing.
Incidentally, this fixes one previously failing check that was commented out in test-s4-unload.R.

[hadley]

Please indent arguments by only two spaces

[hadley]

Could you please also add a bullet to NEWS?

[wch]

I think it would be best to just take out the License field here. Or it should be the same license as devtools itself.

[famuvie]

Done.

[hadley]

Thanks!