R devel - Feb 2007 - obscure error with subsetting as.list() of a function then (PR#9506)

Ok, thanks for clearing it up. But:
> It's not a bug things work in ways that confuse users when they pry
> into things they were not expected to pry into.... Do you have a good
> reason to call this a bug?
Well if it's intended to work that way then it's not a bug.
As I said, I was computing on the language, in particular writing code that
processes the parse tree of a function. But I guess I need to avoid certain
gotchas like this one.

Regards,
Peter




             Peter Dalgaard <p.dalgaard at biostat.ku.dk>

             08/02/2007 23:39                                                   
To
                                                                                
Peter-M Schumacher/DMGGM/DMG UK/DeuBa at DBEMEA
                                                                                
cc
                                                                                
r-devel at stat.math.ethz.ch, R-bugs at biostat.ku.dk
                                                                                
Subject
                                                                                
Re: [Rd] obscure error with subsetting as.list() of a function then assigning
that a (PR#9500)










peter-m.schumacher at db.com writes:
> Hello. I was writing some code that computes on the language and came
across
> this. I can work around it, but thought you might like to know about it.
>
>> f <- function(x) { NULL }
>> a <- as.list(f)[[1]]
>> a # ie print(a)
> Error: argument "a" is missing, with no default
>
> Note it says *argument* "a", which is strange. In fact, and
unsurprisingly, the bug lies
> with the object itself, not with print():
>
>> typeof(a)
> Error in typeof(a) : argument "a" is missing, with no default
>> deparse(a)
> Error in deparse(a) : argument "a" is missing, with no default
>
> However, this does work:
>> as.list(f)[[1]]
>
> It prints nothing, which is correct, and there is no error. So it seems the
bug lies with
> assigning a name to as.list(f)[[1]] as above, then trying to work with that
new object.

It's not a bug things work in ways that confuse users when they pry
into things they were not expected to pry into.... Do you have a good
reason to call this a bug?

What you're seeing is R's "missing argument object", via the
default
value of the formal argument x. A slightly cleaner way to get your
result is
> formals(f)
$x

> a <-formals(f)$x
> a
Error: argument "a" is missing, with no default

Technically, the missing argument object is a zero-length variable
name:
> mode(formals(f)$x)
[1] "name"
> as.character(formals(f)$x)
[1] ""


Except for direct meddling with the formals(f), the only way to assign
the missing argument object is via parameter passing - any other
attempt to access it gives an error. So the common case is that the
object is indeed a function argument.


> Regards,
> Peter-M.Schumacher at db.com
>
>
> --please do not edit the information below--
>
> Version:
>  platform =3D i386-pc-mingw32
>  arch =3D i386
>  os =3D mingw32
>  system =3D i386, mingw32
>  status =3D
>  major =3D 2
>  minor =3D 4.1
>  year =3D 2006
>  month =3D 12
>  day =3D 18
>  svn rev =3D 40228
>  language =3D R
>  version.string =3D R version 2.4.1 (2006-12-18)
>
> Windows XP Professional (build 2600) Service Pack 2.0
>
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Kingdom.1252;LC_MONETARY=3DEnglish_United
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>
> Search Path:
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--
   O__  ---- Peter Dalgaard             =D8ster 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



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