[Rd] Two ALTREP questions

[Dirk Eddelbuettel]

On 21 November 2020 at 20:56, Jiefei Wang wrote:
| Hello,
| 
| I have two related ALTREP questions. It seems like there is no way to
| assign attributes to an ALTREP vector without using C++ code. To be more
| specifically, I want to make an ALTREP matrix, I have tried the following R
| code but none of them work.
| ```
| .Internal(inspect(1:6))
| .Internal(inspect(matrix(1:6, 2,3)))
| .Internal(inspect(as.matrix(1:6)))
| .Internal(inspect(structure(1:6, dim = c(2L,3L))))
| .Internal(inspect({x <- 1:6;attr(x, "dim") <- c(2L,3L);x}))
| .Internal(inspect({x <- 1:6;attributes(x)<- list(dim = c(2L,3L));x}))
| ```
| 
| The only way to make an ALTREP matrix is to use the C level function
| ```
| attachAttrib <- inline::cxxfunction( signature(x = "SEXP", attr = "SEXP" )
| , '
| SET_ATTRIB(x,attr);
| return(R_NilValue);
| ')
| x <- 1:6
| attachAttrib(x, pairlist(dim = c(2L, 3L)))
| .Internal(inspect(x))
| ```

(That's C code. The confusion here is partly our fault. When Romain and I
extended the inline package with 'cxxfunction' to support the then-young but
active Rcpp package, we picked C++. Strictly speaking that isn't required;
you are only in C++ here because ... "it made sense to us 10 years ago" and
it could generalized to C++ or C.  All ALTREP is, of course, purely C as it
is an R API.)
 
| Since the matrix, or adding attributes, is a common need for the object
| operation, I wonder if this missing feature is intended? This also brings
| my second question, it seems like the ALTREP coercion function does not
| handle attributes correctly.  After the coercion, the ALTREP object will
| lose its attributes.
| ```
| coerceFunc <- inline::cxxfunction( signature(x = "SEXP", attr = "SEXP" ) , '
| SET_ATTRIB(x,attr);
| return(Rf_coerceVector(x, REALSXP));
| ')
| > coerceFunc(1:6, pairlist(dim = c(2L, 3L)))
| [1] 1 2 3 4 5 6
| > coerceFunc(1:6 + 0L, pairlist(dim = c(2L, 3L)))
|      [,1] [,2] [,3]
| [1,]    1    3    5
| [2,]    2    4    6
| ```
| The problem is that the coercion function is directly dispatched to the
| user-defined ALTREP coercion function, so the user is responsible to attach
| the attributes after the coercion. If he forgets to do so, then the result
| is a plain vector. Similar to the `Duplicate` and `DuplicateEX` functions
| where the former one will attach the attributes by default, I feel that the
| `Coerce` function should only return a plain vector and there should be a
| `CoerceEx` function to do the attribute assignment, so the logic in the
| no-EX ALTREP functions can be consistent. I do not know how dramastic the
| change would be, so maybe this is too hard to do.
| 
| BTW, is there any way to contribute to the R source? I know R has a limited
| resouces, so if possible, I will be happy to fix the matrix issue myself
| and make some minor contributions to the R community.

Yes. The generally recommended way is via a bug report at bugs.r-project.org
(for which you need an account there, and I always forget who the account
creation gatekeeper is ...) preferably with a patch. You effectively need an
R Core "sponsor" as someone has to actually put the code into R base.  So for
"everything else" the recommendation generally is to go via a package. Maybe
the easier route for you is to bundle a few ALTREP helper functions first?

Dirk

-- 
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