exists function on list objects gives always a FALSE

SmoothData$span is not an object which can be checked by exists(), but
part of an object which can be checked by is.null().

  

is.null is unhelpful here, in that lists can contain NULL as a named
element, and retrieving a non-existent element returns NULL:

    foo = list(bar=NULL)
    is.null(foo$bar)
    # TRUE
    is.null(foo$foo)
    # TRUE

i must admit i find it surprising that ?'$' does not appropriately
explain what happens if a list is indexed with a name not included in
the list's names.  the closest is

" When extracting, a numerical, logical or character 'NA' index
     picks an unknown element and so returns 'NA' in the corresponding
     element of a logical, integer, numeric, complex or character
     result, and 'NULL' for a list. "

but it's valid for NAs in the index, and

"  If no match is found then 'NULL' is returned. "

but it's in the section on environments.


vQ