[Rd] Is NULL a vector?

[Hadley Wickham]

Hi all,

Would you generally consider NULL to be a vector? Base R functions are
a little inconsistent:

## In favour

``` r
identical(as.vector(NULL), NULL)
#> [1] TRUE

identical(as(NULL, "vector"), NULL)
#> [1] TRUE

# supports key vector vector generics
length(NULL)
#> [1] 0
NULL[c(3, 4, 5)]
#> NULL
NULL[[1]]
#> NULL
```

## Against

``` r
is.vector(NULL)
#> [1] FALSE

is(NULL, "vector")
#> [1] FALSE
```

## Abstentions

``` r
is.atomic(NULL)
#> [1] TRUE
# documentation states "returns NULL if x is of an atomic type (or NULL)"
# is "or" exclusive or inclusive?
```

Hadley

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