From p.dalgaard at biostat.ku.dk  Sun Nov  1 12:05:22 2009
From: p.dalgaard at biostat.ku.dk (p.dalgaard at biostat.ku.dk)
Date: Sun,  1 Nov 2009 12:05:22 +0100 (CET)
Subject: [Rd] Internal error in 'ls' for pathological environments
	(PR#14035)
Message-ID: <20091101110522.94DFF282EF3D@mail.pubhealth.ku.dk>

macrakis at alum.mit.edu wrote:
> nchar(with(list(2),ls())) gives an internal error. This is of course
> a peculiar call (no names in the list), but the error is not caught
> cleanly.
> 
> It is not clear from the documentation whether with(list(2)...) is
> allowable; if it is not, it should presumably give an error. If it is, then
> ls
> shouldn't have problems with the resulting environment.
> 
>> qq <- with(list(2),ls())                         # An incorrect call (no
> names in list)
>> nchar(qq)
> Error in nchar(qq) : 'getEncChar' must be called on a CHARSXP  # ls returned
> a bad object
>> qq
> [1]Error: 'getEncChar' must be called on a CHARSXP
>> qq[1]
> [1]Error: 'getEncChar' must be called on a CHARSXP
>> qq[2]
> [1] NA
> 
> Apparently related:
> 
>> with(list(a=1,2),ls())
> Error in ls() : 'getEncChar' must be called on a CHARSXP

Thanks, yes, this looks wrong.

Also, closer to the root cause:

 > eval(quote(ls()),list(a=1,2))
Error in ls() : 'getEncChar' must be called on a CHARSXP


 > e <- evalq(environment(),list(2))
 > ls(e)
[1]Error: 'getEncChar' must be called on a CHARSXP

It is not quite clear that it should be allowed to have unnamed elements 
in lists used by eval (which is what with() uses internally). I suppose 
it should, since the intended semantics are unaffected in most cases. 
(I.e., there is nothing really wrong with eval(quote(a+b), 
list(a=1,b=2,3,4)), and people may have been using such code unwittingly 
all over.)

However, it IS a bug that we are creating ill-formed environments. The 
culprit seems to be that NewEnvironment (memory.c) is getting called in 
violation of its assumption that

" This definition allows
   the namelist argument to be shorter than the valuelist; in this
   case the remaining values must be named already.  (This is useful
   in cases where the entire valuelist is already named--namelist can
   then be R_NilValue.)
"

Removing the assumption from NewEnvironment looks like an efficiency 
sink, so I would suggest that we fix do_eval (eval.c) instead, 
effectively doing l <- l[names(l) != ""].  So in

     case LISTSXP:
	env = NewEnvironment(R_NilValue, duplicate(CADR(args)), encl);
	PROTECT(env);
	break;

we need to replace the duplicate() call with something that skips the 
unnamed elements.

The only side effect I can see from such a change is the resulting 
environments get a different length() than before. I'd say that if there 
are coder who actually rely on the length of an invalid environment, 
then they'd deserve what they'd get...

-- 
    O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
  (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907



[Rd] Internal error in 'ls' for pathological environments (PR#14035)
p.dalgaard at biostat.ku.dk