[Rd] Suggestion: Dimension-sensitive attributes

[Laurent Gautier]

Starting by working on an interface for such object(s) is probably the 
first step toward a unified solution, and this before about if and how R 
attributes are used.

It would also help to ensure a smooth transition from the existing 
classes implementing a similar solution (first the interface is added to 
those classes, then after a grace period the classes are eventually 
refactored).

Dimension-level is what seems to the be most needed... but I am not 
convinced of the practicality of the object-level, and cell-level scheme 
s proposed:

- Object-level, if not linked to any dimension-attribute is such saying 
that one want to attach anything to any object. That's what attr() is 
already doing.

- Cell-level, is may be out-of-scope for one first trial (but may be I 
missed the use-cases for it)



If starting with behaviour, it seems to boil to having "["/"[<-" and 
"dimmeta()"/"dimmeta<-()", :

- extract "[" / replace "[<-" :

   * keeps working the way it already does

   * extracts a subset of the object as well as a subset of the 
dimension-associated metadata.

   * departing too much from the way "[" is working and add 
behind-the-curtain name matching will only compromise the chances of 
adoption.

   * forget about the bit about which metadata is kept and which one 
isn't when using "[". Make a function "unmeta()" (similar behavior to 
"unname()") to drop them all, or work it out with something like
 > dimmeta(x, 1) <- NULL # drop the metadata associated with dimension 1

- access the dimension-associated metadata:

   * may be a function called "dimmeta()" (for consistency with 
"dimnames()") ? The signature could be dimmeta(x, i), with x the object, 
and i the dimension requested. A replace function "dimmeta<-"(x, i, 
value) would be provided.


In the abstract the "names" associated with a given dimension is just 
one of possible metadata, but I'd keep away from meddling with it for a 
start.


It would seem natural that metadata associated with one dimension:
would a table-like object (data.frame seems natural in R, and 
unfortunately there is no data.frame-like structure in R).



L.