R devel - Feb 2013 - quote() vs quote(expr=)

Hi all,

I think there's a small buglet in quote:

str(quote())
# Error in quote() : 0 arguments passed to 'quote' which requires 1
str(quote(expr = ))
# symbol

I bring this up because this seems like the most natural way of
capturing the "missing" symbol with pure R code, compared to
substitute() or bquote() or formals(plot)$x

Hadley

-- 
Chief Scientist, RStudio
http://had.co.nz/

On Feb 18, 2013, at 16:20 , Hadley Wickham wrote:
> Hi all,
> 
> I think there's a small buglet in quote:
> 
> str(quote())
> # Error in quote() : 0 arguments passed to 'quote' which requires 1
> str(quote(expr = ))
> # symbol
> 
If there is a bug here, I'd say that it is in str(), revealing the
implementation of the missing value as the symbol ``, which we otherwise try not
to disclose to R code, e.g.
> as.symbol("")
Error in as.symbol("") : attempt to use zero-length variable name

There's a difference between passing 0 arguments and passing a missing
argument and I see nothing particularly wrong with quote(expr=) returning
missing. It is, for instance, reasonably consistent that
> eval(quote(expr=))
Error in eval(expr, envir, enclos) : argument is missing, with no default
> I bring this up because this seems like the most natural way of
> capturing the "missing" symbol with pure R code, compared to
> substitute() or bquote() or formals(plot)$x
Are you sure you want to do that? I tend to think that it belongs in the
"if it breaks, you get to keep both pieces" category.

-- 
Peter Dalgaard, Professor
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

I wouldn't count on any way of capturing this thing being reliable in
the long term.  As I recall what I do in codetools and the compiler is
use features of missing() to test for it, but try to abstract those
uses into one or two places only so I can easily change them if
missing()'s behavior changes. Basically this internal thing
_shouldn't_ be visible at R level, and if we ever figure out how to
make that happen it will.

Best,

luke

On Mon, 18 Feb 2013, Hadley Wickham wrote:
> Hi all,
>
> I think there's a small buglet in quote:
>
> str(quote())
> # Error in quote() : 0 arguments passed to 'quote' which requires 1
> str(quote(expr = ))
> # symbol
>
> I bring this up because this seems like the most natural way of
> capturing the "missing" symbol with pure R code, compared to
> substitute() or bquote() or formals(plot)$x
>
> Hadley
>
>
-- 
Luke Tierney
Chair, Statistics and Actuarial Science
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu