Re: [Rd] PROTECT and OCaml GC.

From: Simon Urbanek <simon.urbanek_at_r-project.org>
Date: Mon, 30 Nov 2009 16:25:59 -0500

On Nov 30, 2009, at 16:07 , Guillaume Yziquel wrote:

> Simon Urbanek a écrit :

>>>
>>> And it goes then to my other question: How can you pass to eval a  
>>> LANGSXP where the CAR is an *anonymous* function, no SYMSXP  
>>> involved?
>> 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);
>> }
>> > .Call("myEval",function(x) x + 1, 10)
>> [1] 11
>

> In the eval function in eval.c, you have:
>
>>    case LANGSXP:
>>        if (TYPEOF(CAR(e)) == SYMSXP)
>>            /* This will throw an error if the function is not found  
>> */
>>            PROTECT(op = findFun(CAR(e), rho));
>>        else
>>            PROTECT(op = eval(CAR(e), rho));
>

> So imagine you have a LANGSXP whose CAR is a CLOSXP, the execution
> goes into the last line of the code snippet above.
> And re-entring eval with a CLOSXP, the code goes into
>
>>    tmp = R_NilValue;           /* -Wall */
>> #ifdef Win32
>>    /* This is an inlined version of Rwin_fpreset (src/gnuwin/extra.c)
>>       and resets the precision, rounding and exception modes of a  
>> ix86
>>       fpu.
>>     */
>>    __asm__ ( "fninit" );
>> #endif
>>    R_Visible = TRUE;
>>    switch (TYPEOF(e)) {
>>    case NILSXP:
>>    case LISTSXP:
>>    case LGLSXP:
>>    case INTSXP:
>>    case REALSXP:
>>    case STRSXP:
>>    case CPLXSXP:
>>    case RAWSXP:
>>    case S4SXP:
>>    case SPECIALSXP:
>>    case BUILTINSXP:
>>    case ENVSXP:
>>    case CLOSXP:
>>    case VECSXP:
>>    case EXTPTRSXP:
>

> so PROTECT(op = eval(CAR(e), rho)) evaluates to R_NilValue.
>

Nope, it simply evaluates to itself (with ref count increased) - see tmp = e; ..; return(tmp).

> I figured out that's why evaluating a LANGSXP with CAR a CLOSXP
> simply fails.
>

Wrong ;). A closure is a constant like any other object so it evaluates to itself.

Cheers,
Simon

> I'll have a look at the code snippet you gave, since I do not
> understand why it doesn't fail the same way mine does.

>

> Thanks a lot.
>

> --
> Guillaume Yziquel
> http://yziquel.homelinux.org/
>
>

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