R devel - Apr 2004 - Re: [R] More user-friendly error message needed.

Perhaps one could create a utility function 

	has.element <- function(list, name) name %in% names(list)

and then have $ generate a warning (not an error!) when the named element
does not exist.

This would be helpful in debugging code.  Yesterday I spent quite some time
tracking down an error that turned out to be $ returning a NULL because the
data file I read in had a variable mis-named.   A warning message would have
made everything clear right away.

-G
> -----Original Message-----
> From: r-devel-bounces@stat.math.ethz.ch
> [mailto:r-devel-bounces@stat.math.ethz.ch]On Behalf Of Duncan Murdoch
> Sent: Wednesday, April 07, 2004 4:16 PM
> To: Shin; r-devel@stat.math.ethz.ch
> Subject: Re: [Rd] Re: [R] More user-friendly error message needed.
> 
> 
> On Wed, 07 Apr 2004 15:38:54 -0400, Duncan Murdoch
<dmurdoch@pair.com>
> wrote :
> 
> >There are several places this could be fixed.  When you use x$z, the
> >code for $ could give an error message or a warning; instead it
> >returns NULL with no error or warning.  Changing this would probably
> >be dangerous:  I'd guess there's code out there that relies 
> on getting
> >a NULL back from a construction like that.  But maybe we 
> should change
> >that in 2.0?
> 
> No, this would be a bad idea.
> 
> A standard test for the existence of a list element is 
> 
> if (is.null(x$z)) ....
> 
> Those would all need some other kind of test if this were changed.
> Not a good idea at all!
> 
> Duncan Murdoch
> 
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"Warnes, Gregory R" <gregory_r_warnes@groton.pfizer.com> writes:
> Perhaps one could create a utility function 
> 
> 	has.element <- function(list, name) name %in% names(list)
> 
> and then have $ generate a warning (not an error!) when the named element
> does not exist.
Because of partial matching, list$name can return a result when 
your has.element function returns FALSE.
> lst = list(foo = 1:3, bar = LETTERS[1:3])
> lst$f
[1] 1 2 3
> has.element <- function(list, name) name %in% names(list)
> has.element(lst, "f")
[1] FALSE

Before we go too far in speculating about what the $ operator should
or should not return when the name is not matched, let's all remember
that the semantics of the $ operator explicitly state that it should
return NULL and, as Duncan wrote, there are many, many places in the
base code for R that depend upon this behavior.  Trust me - you really
don't want to change this.