R devel - Feb 2007 - obscure error with subsetting as.list() of a function then assigning that a (PR#9500)

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.

Regards,
Peter-M.Schumacher at db.com


--please do not edit the information below--

Version:
 platform = i386-pc-mingw32
 arch = i386
 os = mingw32
 system = i386, mingw32
 status  major = 2
 minor = 4.1
 year = 2006
 month = 12
 day = 18
 svn rev = 40228
 language = R
 version.string = R version 2.4.1 (2006-12-18)

Windows XP Professional (build 2600) Service Pack 2.0

Locale:
LC_COLLATE=English_United Kingdom.1252;LC_CTYPE=English_United
Kingdom.1252;LC_MONETARY=English_United
Kingdom.1252;LC_NUMERIC=C;LC_TIME=English_United Kingdom.1252

Search Path:
 .GlobalEnv, file:c:/schupl/R/myRLib/.RData, package:stats, package:graphics,
package:grDevices, package:utils, package:datasets, package:methods, Autoloads,
package:base
---

This e-mail may contain confidential and/or privileged infor...{{dropped}}

Also note:

   missing(a) # TRUE


On 2/8/07, peter-m.schumacher at db.com <peter-m.schumacher at db.com>
wrote:
>
>
> 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.
>
> Regards,
> Peter-M.Schumacher at db.com
>
>
> --please do not edit the information below--
>
> Version:
>  platform = i386-pc-mingw32
>  arch = i386
>  os = mingw32
>  system = i386, mingw32
>  status >  major = 2
>  minor = 4.1
>  year = 2006
>  month = 12
>  day = 18
>  svn rev = 40228
>  language = R
>  version.string = R version 2.4.1 (2006-12-18)
>
> Windows XP Professional (build 2600) Service Pack 2.0
>
> Locale:
> LC_COLLATE=English_United Kingdom.1252;LC_CTYPE=English_United
Kingdom.1252;LC_MONETARY=English_United
Kingdom.1252;LC_NUMERIC=C;LC_TIME=English_United Kingdom.1252
>
> Search Path:
>  .GlobalEnv, file:c:/schupl/R/myRLib/.RData, package:stats,
package:graphics, package:grDevices, package:utils, package:datasets,
package:methods, Autoloads, package:base
> ---
>
> This e-mail may contain confidential and/or privileged infor...{{dropped}}
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

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 = i386-pc-mingw32
>  arch = i386
>  os = mingw32
>  system = i386, mingw32
>  status >  major = 2
>  minor = 4.1
>  year = 2006
>  month = 12
>  day = 18
>  svn rev = 40228
>  language = R
>  version.string = R version 2.4.1 (2006-12-18)
>
> Windows XP Professional (build 2600) Service Pack 2.0
>
> Locale:
> LC_COLLATE=English_United Kingdom.1252;LC_CTYPE=English_United
Kingdom.1252;LC_MONETARY=English_United
Kingdom.1252;LC_NUMERIC=C;LC_TIME=English_United Kingdom.1252
>
> Search Path:
>  .GlobalEnv, file:c:/schupl/R/myRLib/.RData, package:stats,
package:graphics, package:grDevices, package:utils, package:datasets,
package:methods, Autoloads, package:base
> ---
>
> This e-mail may contain confidential and/or privileged infor...{{dropped}}
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
-- 
   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
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