Assigning a zero length vector to a list (PR#8157)

Duncan Murdoch <murdoch at stats.uwo.ca> writes:

After foo<-list(), foo$bar is NULL, so we can simplify this.

Here's a simpler version:

# These work, which is a bit of a surprise, but there is some 
inconsistency:   one x becomes a list, the other is numeric:

 > x <- NULL
 > x[[1]] <- 1:10
 > x

[[1]]
  [1]  1  2  3  4  5  6  7  8  9 10

 > x <- NULL
 > x[[1]] <- 1
 > x

[1] 1


# This generates the same bug as the above:

 > x <- NULL
 > x[[1]] <- numeric(0)
 > x

[1] 4.250083e-314

It looks like we're trying to be too clever with handling assignments to 
components of NULL.  Wouldn't it make more sense for those to generate 
an error?

Once upon a time, we had pairlists, and NULL was the empty list. This
looks like it might be a relic. If so, it likely also predates
consistent handling of zero-length vectors, so something is getting
confused. I think it would be reasonable to expect similar results to
this: 

x<-list()
x[[1]] <- numeric(0)
x

[[1]]
numeric(0)

 

S-PLUS also tries to handle NULL as a zero length list, with some
anomalies:

x <- NULL
x[[1]] <- numeric(0)
x

$value:
numeric(0)

x <- list()
x[[1]] <- numeric(0)
x

[[1]]:
numeric(0)