From p.dalgaard at biostat.ku.dk  Fri Feb  9 00:39:45 2007
From: p.dalgaard at biostat.ku.dk (Peter Dalgaard)
Date: Fri, 09 Feb 2007 00:39:45 +0100
Subject: [Rd] obscure error with subsetting as.list() of a function then
 assigning that a (PR#9500)
In-Reply-To: <20070208180825.7B3D75A842@slim.kubism.ku.dk> (peter-m
	schumacher's message of "Thu,  8 Feb 2007 19:08:25 +0100 (CET)")
References: <20070208180825.7B3D75A842@slim.kubism.ku.dk>
Message-ID: <x2mz3ousla.fsf@viggo.kubism.ku.dk>

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
> ---
>
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>
> ______________________________________________
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-- 
   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] obscure error with subsetting as.list() of a function then assigning that a (PR#9500)
Peter Dalgaard