There are two issues here, what it does and what it should do..
One issue is what is meant by 'expression': is it an expressions vector of
type "expression" or is it 'expression' as documented as an element of
such a vector in ?expression? For body<- it is the latter.
Then there are a couple of odd exceptions.
1) If 'value' is of type expression, the first element is taken, silently.
2) If 'value' is a length-one list, its first element is taken.
If 'value' is a named list of length greater than one. the last element is
taken as the value and the remaining elements as additional arguments.
The first is by design, the second seems to be a bug (and in the case of
formals<- it definitely is a bug which I have just corrected). The issue
is in
base::`body<-`
function (fun, envir = environment(fun), value)
{
if (is.expression(value))
value <- value[[1]]
as.function(c(formals(fun), value), envir)
}
formals(fun) is a normally a pairlist so c() is concatenating 'value' to a
list, and will do an implicit as.list(). However, in the case of no
arguments it fails (another bug I have just corrected).
I think we want this to be
as.function(c(as.list(formals(fun)), list(value)), envir)
but that does change the behaviour for a length-one list 'value'. It
seems a fair guess that no one is intentionally relying on the latter, but
you never know ....
On Fri, 21 Nov 2008, Peter Dalgaard wrote:
fabian.sche...@stat.uni-muenchen.de wrote:
Full_Name: Fabian Scheipl
Version: 2.8.0
OS: Windows, Linux
Submission from: (NULL) (138.246.7.150)
It seems to me that the documentation for body<- is wrong.
The help file for
body(fun, envir = environment(fun)) <- value
says that:
value can be an expression or a list of R expressions.
This produces errors however:
################################
f <- function(x){}
body(f) <- list(expression(res <- x^2),expression(return(x)))
Fehler in as.function.default(c(formals(fun), value), envir) : ungültiges
formales Argument für "function"
################################
The only way to assign multiple statements to a function body
that I could get to work is to put all the statements into a single
expression,
(separated by ';' and in curly brackets):
################################
body(f) <- expression({res <- x^2; return(x)})
################################
I think the words 'or a list of R expressions.' should be removed.
It's slightly trickier than that. The value _can_ be a list, but only one of
length 1. The same actually goes for expressions, but in that case there is
explicit code to take only the first element. Also, the distinction between
objects of mode "expression" and unevaluated expressions is unclear (here and
elsewhere).
body(f) <- expression(x,y,z)
f
function ()
x
body(f) <- list(quote(2+2))
f
function ()
2 + 2
body(f) <- list(expression(2+2))
f
function ()
expression(2 + 2)
BTW: Don't use 'wishlist' on things that are real errors.
--
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
~~~~~~~~~~ - (p.dalga...@biostat.ku.dk) FAX: (+45) 35327907
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--
Brian D. Ripley, rip...@stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
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