Several R vs S-Plus issues

On Thu, 4 Oct 2001, Duncan Murdoch wrote:

On Thu, 4 Oct 2001 18:14:48 +0200, Kurt Hornik wrote in message
<15292.35576.513722.492732@mithrandir.hornik.net>:

Agreed.  As Peter said, subscripting by NA should give NA but R cannot
distinguish a string NA from the string NA.

But it does make some sort of distinction, as my example contrasting
"NA" with paste(NA) shows.  In case you missed it:

 > is.na("NA")

 [1] TRUE

 > paste(NA)

 [1] "NA"

 > is.na(paste(NA))

 [1] FALSE

Yes.  is.na() of a string is true if that string is actually a reference
to R_NaString.  Parsing a literal "NA" will give R_NaString, as will
coercing NA from some other type.  However, paste() doesn't check whether
it produces "NA". Neither does toupper()

  > is.na(toupper("na"))

  [1] FALSE

  > toupper("na")

  [1] "NA"


So we're partway there already, in fact.  It looks like we basically
need to
1) stop the parser generating R_NaString from \"NA\"
2) Change PRINTNAME(R_NaString) to avoid ambiguity

x<-as.character(list(NA, "NA", paste(NA)))
x

is.na(x)

save(list="x", file="tmp.RData")
load("tmp.RData")
x

is.na(x)