exists function on list objects gives always a FALSE

You might think that you can check names(xxx) to see if the slot has been
explicitly set, but it depends on *how* you have explicitly set the slot to
NULL:

   > xxx$hello <- 3
   > xxx$hello <- NULL
   > names(xxx)

   character(0)                  # no names -- assigning to NULL kills slot
  

kills indeed:

    foo = list(bar=1)

    with(foo, bar)
    # 1

    foo$bar = NULL
    with(foo, bar)
    # error: object 'bar' not found

   > xxx <- list(hello=NULL)
   > names(xxx)

   [1] "hello"                    # 1 name -- constructing with NULL-valued
slot
  

but:

    # cleanup -- don't do it in mission critical session
    rm(list=ls())

    foo
    # error: object 'foo' not found

    foo = NULL
    foo
    # NULL

that is, foo$bar = NULL kills bar within foo (even though NULL is a
valid component of lists), but foo = NULL does *not* kill foo.

Welcome to R!
  

... and its zemanticks.

vQ