prepend: fails for empty list

[czeildi]

purrr::prepend fails for empty list due to the checking of the before param though (partly based on the documentation) I would expect it to work as base::append by default. Is there a reason for this?

If you agree that it should work for empty lists as well I am happy to submit a PR.

purrr::prepend(list(), "a")
#> Error in purrr::prepend(list(), "a"): before > 0 && before <= n is not TRUE

append("a", list())
#> [[1]]
#> [1] "a"

purrr::prepend(list("b"), "a")
#> [[1]]
#> [1] "a"
#> 
#> [[2]]
#> [1] "b"
append("a", list("b"))
#> [[1]]
#> [1] "a"
#> 
#> [[2]]
#> [1] "b"

Created on 2019-02-10 by the reprex package (v0.2.0).

My first idea for implementation:

function (x, values, before = 1) 
{
    n <- length(x)
    stopifnot(n == 0 || (before > 0 && before <= n))
    if (before == 1) {
        c(values, x)
    }
    else {
        c(x[1:(before - 1)], values, x[before:n])
    }
}

[lionel-]

Maybe before should have NULL as default argument? Then it'd mean to prepend at the beginning, even in the 0 case? If 1 is passed instead, we'd still get an error as currently happens because this means the user assumptions regarding the input vector are not fulfilled.

A PR would be great!