[Rcpp-devel] coercion NULL to vector

[Dirk Eddelbuettel]

Serguei,

(Extra line breaks help readibility on text only mail readers)

On 18 April 2018 at 14:51, Serguei Sokol wrote:
| Le 18/04/2018 à 12:26, Georgi Boshnakov a écrit :
| > I may be missing something here but
| > it doesn't seem right to introduce silent coercion of NULL to vectors, etc.,
| > especially if it would become imposed on everybody using Rcpp/Armadillo.
| Nothing is imposed. Everyone will be free to use Nullable<T> mechanism as before.
| Actually, when NULL is provided to where a vector or a matrix is expected
| just an error is thrown. So we can hardly imagine that someone would have
| his program broken or will be misled if such coercion is introduced.
| 
| > Even the "convenience" of this  is questionable.
| Right. It's a matter of taste. But with a quick glance on SO
| I have seen a discussion here
| https://stackoverflow.com/questions/34718570/rcpp-pass-vector-of-length-0-null-to-cppfunction
| and on this list here (and follow-ups)
| http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2014-February/007215.html
| where some people are surprised not having such coercion and even
| expressed a wish like "I would be super happy if in the future the special value
| R_NilValue could be converted to an empty vector, it would make things
| very easy and coherent with R's behaviour."
| It's also my thought but I am ready to hear the arguments of those who
| think otherwise.

I am not religious about it. Adding the one line is something we might be
able to do. I did a partial rev.dep tests and it showed no issues.  But I
would like other Rcpp Core folks to weigh in too.

On 18 April 2018 at 15:09, Serguei Sokol wrote:
| Le 18/04/2018 à 13:32, Dirk Eddelbuettel a écrit :
| > 
| > On 18 April 2018 at 10:26, Georgi Boshnakov wrote:
| > | I may be missing something here but
| > | it doesn't seem right to introduce silent coercion of NULL to vectors, etc.,
| > | especially if it would become imposed on everybody using Rcpp/Armadillo.
| > | Even the "convenience" of this  is questionable.
| > 
| > Agreed. I am also not yet convinced by Serguei's argument.  It seems a little
| > invasive for an unclear (to me) use case.
| I can describe my own case which made me search for a such solution.
| I have an Rcpp/Armadillo function accepting in an optional argument a vector of uvec type.
| It can be called from R and I can call it from other C++ functions of the same package too.
| Nullable<IntegerVector> mechanism was cumbersome but sufficient (note that there is no possible Nullable<uvec>)

It needs to be as<>()/wrap() convertible to/from SEXP, yes.

| when only R calls were planned. But when it comes to calls from C++ I'll have to juggle with wrap() too.
| Both of Nullable<T> (as well as special code for dealing with NULL case) and wrap()
| become unnecessary with a neat declaration f(..., uvec v, ...). So that
| an R call with 'c()' can pass as well as C++ calls with an empty uvec without
| any additional hassle for treating NULL case.

Could you use XPtr ?

Dirk

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