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Exposing a function that returns an array of tuples fails to compile

Cerrado
  • ¿Algo no está bien?
#1,329 1 comentario 0 reacciones 0 asignados Ver en GitHub

Los mantenedores suelen responder en 1 día

Ya se ha fusionado un pull request relacionado.

  • #1330 de @jgabry — fusionado

Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
30/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
cpp, r
Área
build-system, compilers

Línea de trabajo

Start by reproducing the failure with the snippet in the issue, which calls $expose_functions() on a function that returns an array of tuples. The header inst/include/rcpp_tuple_interop.hpp defines the tuple wrap overload, and compile_functions() in R/expose.R sets the include order for the generated code. Done means the tuple-array return compiles and exposes, and the existing tuple-argument behaviour still works.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

bug

A Stan function that returns an array of tuples can't be exposed with $expose_functions(), and because every function in the block is compiled together, that means no functions from the model can be exposed.

library(cmdstanr)
mod <- cmdstan_model(write_stan_file("
functions {
  array[] tuple(real, real) f(array[] tuple(real, real) x) { return x; }
}
"))
mod$expose_functions()

The only message R shows is "Error 1 occurred building shared library". The compiler output is actually

Rcpp/internal/wrap.h:528:23: error: static assertion failed due to
requirement '!sizeof(std::tuple<double, double>)': cannot convert type
to SEXP

I did some investigating with Claude. Here's its diagnosis, although I haven't verified myself yet that this is correct:

The cause is in inst/include/rcpp_tuple_interop.hpp. It defines Rcpp::wrap for std::tuple, which works when the tuple is the return type itself (and for nested tuples, where the overload calls itself).
For std::vector<std::tuple<...>> Rcpp's own container wrap takes over and calls ::Rcpp::wrap per element from inside the Rcpp headers (wrap.h:200, range_wrap_dispatch___generic), where our overload isn't visible yet, so the generic fallback fires its static assertion. It's the same visibility problem that keeps Rcpp::List::create(args...) from compiling for nested tuples (see the commit message of #1310).

Taking a tuple array as an argument is fine, the Exporter side handles it. Only returning one fails.

Rcpp's documented way to extend wrap for a new type is to include RcppCommon.h, declare the overload, then include Rcpp.h, so the declaration is visible to the templates in the Rcpp headers. The
generated code includes Rcpp.h first through the other interop headers, so this would need the include order in compile_functions() (R/expose.R) rearranged. Untested.

Tested on macOS with CmdStan 2.40.0, Rcpp 1.1.2, on the v1.0 branch with the fix for #1001 (#1310) in. Pre-existing: the old tuple header failed the same way.

Lenguaje dominante
R
Estrellas
162
Forks
69
Merge medio
1 d 9 h
PR fusionados (30 d)
48

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  3. Haz un fork del repositorio y trabaja en una rama.
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