Empty pairlist entries (argument lists)

[flying-sheep]

Getting the formals of a function results in pairlists with only named entries, but sometimes empty values:

> formals(bquote)
$expr

$where
parent.frame()
> formals(bquote)[[1L]]

> str(formals(bquote)[[1L]])
symbol 
> as.character(formals(bquote)[[1L]])
""

Looks like this is the empty symbol, but I don’t seem to be able to construct it:

> as.symbol('')
Error in as.symbol(""): attempt to use zero-length variable name
> Rcpp::evalCpp('Rcpp::Symbol("")')
Error in fun(): attempt to use zero-length variable name

How can I programmatically construct a pairlist with empty values like this?

[flying-sheep]

Found it:

The error comes from Rf_install. There’s an internal function mkSYMSXP, redefined to Rf_mkSYMSXP in the internal header Defn.h.

There seem to be a bunch of marker symbols exported from Rinternals.h, among them … R_MissingArg.

[eddelbuettel]

Right. When I saw this (before I went out to run) I wondered what this could conceivably have to do with Rcpp. Guess my hunch was correct...

[flying-sheep]

I don’t know the C API for pairlists and couldn’t find any example on how to construct an Rcpp::Pairlist with Named("foo", R_MissingArg). Maybe we should add this as an example for #987.

[eddelbuettel]

Please stop debating base R APIs here. We don't have the staff and bandwidth, and you know where to find the r-devel mailing list.

[eddelbuettel]

Also FWIW and IIRC the few times pair lists came up on the r-devel or alike lists, I do recall R Core members stating just about each and every time that they are either deprecated or not meant to be seen at the API. Maybe try want you want to do with a simple list instead?