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 <[EMAIL PROTECTED]>
08/02/2007 23:39
To
Peter-M Schumacher/DMGGM/DMG UK/[EMAIL PROTECTED]
cc
r-devel@stat.math.ethz.ch, [EMAIL PROTECTED]
Subject
Re: [Rd] obscure error with subsetting as.list() of a function then
assigning that a (PR#9500)
[EMAIL PROTECTED] 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,
> [EMAIL PROTECTED]
>
>
> --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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> https://stat.ethz.ch/mailman/listinfo/r-devel
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907
---
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