dput(as.list(function...)...) bug

On 23/03/2009 7:37 PM, Stavros Macrakis wrote:

 It appears to be the

zero-length name:

    is.name(ff$x) => TRUE
    as.character(ff$x) => ""

This may give you a hint:

 > y <- ff$x
 > y

Error: argument "y" is missing, with no default

It's a special internal thing that triggers the missing value error when 
evaluated.  It probably shouldn't be user visible at all.

Yes, it actually is the zero-length name that is being used for this, 
but that is not really useful knowledge because it is forbidden to 
create them as `` or as.name(""). We did briefly consider making the 
missing object a real R object, but the semantics are too weird:

Basically, you can only assign it once, next time you get errors:

 > x <- alist(a=)$a
 > missing(x)
[1] TRUE
 > y <- x
Error: argument "x" is missing, with no default
 > l <- alist(a=, b=2)
 > l$b <- x
Error: argument "x" is missing, with no default

And, as you think about it, you realize that you cannot disable these 
mechanisms, because _something_ has to trap use of missing arguments.

It does actually work to define a function mvi() which returns the 
missing value indicator and have things like

 > list(x= mvi(), b= quote(!x))
$x


$b
!x

work. I'd hate writing its help page, though.

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