Re: [Rd] PROTECT and OCaml GC.

From: Simon Urbanek <simon.urbanek_at_r-project.org>
Date: Tue, 01 Dec 2009 20:49:04 -0500

On Dec 1, 2009, at 8:32 PM, Guillaume Yziquel wrote:

> Simon Urbanek a écrit :

>> You just pass it as value of the call. I suspect the reason it doesn't work is in your code, not in the facility (note that the link above is useless since the construction is mystery - if you were constructing it right, it would work ;)).
>> Small example:
>> SEXP myEval(SEXP FN, SEXP first_arg) {
>>  return eval(LCONS(FN, CONS(first_arg, R_NilValue)), R_GlobalEnv);
>> }
>> ... or reading R-ext:
>> "There are a series of small macros/functions to help construct pairlists and language objects (whose internal structures just differ by SEXPTYPE. Function CONS(u, v) is the basic building block: is constructs a pairlist from u followed by v (which is a pairlist or R_NilValue). LCONS is a variant that constructs a language object. Functions list1 to list4 construct a pairlist from one to four items, andlang1 to lang4 do the same for a language object (a function to call plus zero to three arguments). Function elt and lastElt find the ith element and the last element of a pairlist, and nthcdr returns a pointer to the nth position in the pairlist (whose CAR is the nth item)."

>
> Reading R-exts,
>
> -1- I believe it would be a good idea to put an example with CONS and LCONS in section 5.11.
>
> -2- Building a LANGSXP list with tags from arguments needs invocations of CONS LCONS SET_TAG and install. While this is not exactly to my taste, using install is really not to my taste since it checks the symbol table and eventually creates a symbol. Isn't there a way to create a tag without using install over and over? A macro that simply sets the tag to whatever CHARSXP might be useful?
>

No, because you cannot use CHARSXP as a TAG. TAGs are always symbols. Therefore, logically, you must register it first in (or obtain from) the symbol table using install.

Cheers,
Simon



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