R: relisting a flat list - r

Question

This question has a nice solution of flattening lists while preserving their data types (which unlist does not):
flatten = function(x, unlist.vectors=F) {
while(any(vapply(x, is.list, logical(1)))) {
if (! unlist.vectors)
x = lapply(x, function(x) if(is.list(x)) x else list(x))
x = unlist(x, recursive=F)
}
x
}
If I give it the following list, it behaves as expected:
> a = list(c(1,2,3), list(52, 561), "a")
> flatten(a)
[[1]]
[1] 1 2 3
[[2]]
[1] 52
[[3]]
[1] 561
[[4]]
[1] "a"
Now I'd like to restructure the flat list like a. relist fails miserably:
> relist(flatten(a), skeleton=a)
[[1]]
[[1]][[1]]
[1] 1 2 3
[[1]][[2]]
[1] 52
[[1]][[3]]
[1] 561
[[2]]
[[2]][[1]]
[[2]][[1]][[1]]
[1] "a"
[[2]][[2]]
[[2]][[2]][[1]]
NULL
[[3]]
[[3]][[1]]
NULL
Now, I could of course do relist(unlist(b), a) but that loses data types again. What is a good way to restructure a flat list?
Bonus points if it handles the analogous attribute to unlist.vectors correctly.

Answer 1

One way to do it is:
relist2 = function(x, like, relist.vectors=F) {
if (! relist.vectors)
like = rapply(a, function(f) NA, how='replace')
lapply(relist(x, skeleton=like), function(e) unlist(e, recursive=F))
}
This retains the classes and distinguishes between lists and vectors:
> relist2(flatten(a), like=a)
[[1]]
[1] 1 2 3
[[2]]
[[2]][[1]]
[1] 52
[[2]][[2]]
[1] 561
[[3]]
[1] "a"
> relist2(flatten(a, unlist.vectors=T), like=a, relist.vectors=T)
[[1]]
[1] 1 2 3
[[2]]
[[2]][[1]]
[1] 52
[[2]][[2]]
[1] 561
[[3]]
[1] "a"