LD Patch initial comments

Hi John,

first of all, thank you for your remarks. Just to add to Alexandre's reply


On Tue, Jul 15, 2014 at 6:51 PM, John Arwe <johnarwe@us.ibm.com
<javascript:_e(%7B%7D,'cvml','johnarwe@us.ibm.com');>> wrote:

> 7: 2.6 suggests (by omission) that it is impossible to delete a list,
> although (in later sections) it's possible to empty a list.  The asymmetry
> of being able to add a list, but not delete it again later, seems vaguely
> suspect although I have no concrete use cases requiring it.  At some level
> I think about it as: can I undo whatever havoc I cause?  ...and the answer
> here appears to be 'no'.
>
> It **is** possible to delete a list, although you are right, there is a
slight asymetry here. Indeed, you can add a list with a single statement :

  Add :s :p ( :john :paul :george :ringo ) .

while to remove it completely, you need two statements

  UpdateList :s :p 0> () .
  Delete :s :p rdf:nil .

I'm guessing you would have expected to be able to write

  Delete :s :p ( :john :paul :george :ringo ) .

and this is not allowed by the LD-Patch syntax in its current state. The
rationale for this decisions is that "( :john :paul :george :ringo )"
(whether it appears in an Add or UpdateList statement) is syntactic sugar
for a bunch or interlinked **fresh** blank nodes, that are to be added to
the graph.
In a Delete statement as the one suggested above, the blank nodes hidden in
the list expression would have a quite different semantics: they would be
variables that would need to be matched against the **actual** blank nodes
in the graph.

Now, I'm not saying that I would refuse to extend LD-Patch in that way. I'm
merely explaining why we prevented it in the first place. But I must say
that I find it relatively easy to understand, and quite elegant.

Furthermore, it would be a safer way to delete a list, as the
two-statements method that I gave above would remove the list regardless of
its values, while the one-statement "delete with a list as its object"
would only delete that particular list, or fail if the list contains
different values.

 best