R中递归连接列表的列表

Question

Is there a concise way to create a list of recursively concatenated lists in R?

Here's a concrete illustration of the type of problem I'd like to solve: Given two lists, such as

u <- list("A", list("B", list("C", "D")), "E")
v <- c("x", "y")

I would like to (recursively) produce the following list of character vectors

w <- list(c("A", "x"), c("A", "y"),
          c("B", "C", "x"), c("B", "C", "y"),
          c("B", "D", "x"), c("B", "D", "y"),
          c("E", "x"), c("E", "y"))

By “concise” I mean in an R-idiomatic manner with a minimal number of explicit for-loops.

What I've tried : I first tried solving the more basic problem of pairwise concatenation, that is, the problem of forming a list like

r <- list(c("A", "x"), c("A", "y"), c("B", "x"), c("B", "y"),
          c("C", "x"), c("C", "y"))

from two lists like

p <- list("A", "B", "C")
q <- list("x", "y")

However, performing the seemingly sensible operation

outer(p, q, FUN = "c")

threw the exception

dims [product 6] do not match the length of object [12]

What gives?

Answer 1

One option might be

 w1 <- do.call(c,lapply(u, function(x) {
        x1 <- if(!is.list(x)) outer(x,v, FUN=paste) 
        else t(outer(do.call(paste, x), v, FUN= paste))
        strsplit(x1, " ")}))
identical(w, w1)
#[1] TRUE