How do I preserve list names when working with nested lists?

Question

The advantage of using plyr's llply over lapply is that it preserves list names. See ?llply for an explanation. I love that feature but cannot seem to get it to work in the case of a nested list. Example:

library(plyr) 
m <- as.list(1:2)
names(m) <- c('M1', 'M2')
foo <- list(m, m)
names(foo) <- paste0("R", 1:2)


result <- ldply(foo, function(x){
        ldply(x, function(z) { data.frame(a=z, b= z^2)})
})
> result
  .id a b
1  M1 1 1
2  M2 2 4
3  M1 1 1
4  M2 2 4

# if I don't operate on the inner list, I can preserve the outer list's names
result2 <- ldply(foo, function(x){
     data.frame(a = x[[1]], b = x[[1]]^2)
    })
> result2
  .id a b
1  R1 1 1
2  R2 1 1

Note that result does not contain R1 and R2 (which would get added in as .id if I didn't operate on the nested list inside each element of foo as is the case with result2). How do I make sure I get the outer list names added in when working on nested lists?

Answer 1

It seems that the column name is the problem:

result <- ldply(foo, function(x){
    df <- ldply(x, function(z) { data.frame(a=z, b= z^2)})
    names(df)[1] <- ".id2"; df
})
result
  .id .id2 a b
1  R1   M1 1 1
2  R1   M2 2 4
3  R2   M1 1 1
4  R2   M2 2 4