Possible bug with list_transpose

I came across a bug in paws.common that does something similar to list_transpose. paws-r/paws#791 (comment).

When transposing a list the first element is taken to create the template (if one isn't already provided)

Line 78 in 870696c

template <- template %||% vec_index(x[[1]])

If the first element is empty then list_transpose will return an empty list. Would taking the maximum length be more appropriate to avoid this?

# paws.common original implementation:
transpose_orig <- function (x) {
    if (any(found <- lengths(x) == 0)) {
        x[found] <- list(rep(list(), length.out = length(x[[1]])))
    }
    .mapply(list, x, NULL)
}

# paws.common new implementation:
transpose_new <- function (x) {
    lens <- lengths(x)
    if (any(found <- lens == 0)) {
        x[found] <- list(rep(list(), length.out = max(lens)))
    }
    .mapply(list, x, NULL)
}

obj <- list(
    var1 = list(),
    var2 = c(1,2,3)
)

transpose_orig(obj)
#> list()

transpose_new(obj)
#> [[1]]
#> [[1]]$var1
#> NULL
#> 
#> [[1]]$var2
#> [1] 1
#> 
#> 
#> [[2]]
#> [[2]]$var1
#> NULL
#> 
#> [[2]]$var2
#> [1] 2
#> 
#> 
#> [[3]]
#> [[3]]$var1
#> NULL
#> 
#> [[3]]$var2
#> [1] 3

purrr::list_transpose(obj, simplify = F)
#> list()

^{Created on 2024-07-02 with reprex v2.1.0}

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I like this idea: it's a simple way to generally improve the default performance of list_transpose().

If someone tackles this for TDD, it'll need some documentation updates and a test.