[R] recursive function, how to avoid list structure in return value

[Jenny Bryan]

I've written a recursive function to extract the members of an  
individual cluster within a hierarchical clustering.  I have something  
that works, but the return value has a list structure I don't like.  I  
know how to work around with 'unlist()' but I suspect the function  
could be fixed.  Can anyone show me show?

Thanks,
Jenny Bryan

Demo of my problem --

(Note: although my question has nothing to do with hierarchical  
clustering per se, my example does assume knowledge of the 'merge'  
object.)

## faking the key aspects of an hclust object
myClust <-
   list(merge = rbind(c(-1, -2),
          c(-3, -4),
          c(2, -5),
          c(1, 3),
          c(4, -6)),
        height = 1:5,
        order = 1:6)

## plot the example / fake tree
stats:::plot.hclust(myClust, hang = -1)

## recursive function to extract members of a cluster
## 'sapply' version
clMembFun1 <- function(x) {
   if(x < 0) {
     -x
   } else {
     sapply(1:2, function(j) clMembFun1(myClust$merge[x,j]))
   }
}

Here's a transcript of using clMembFun:

 > ## trivial case of cluster = 2 singletons is OK
 > clMembFun1(1)
[1] 1 2
 > str(clMembFun1(1))                      # num vector
  num [1:2] 1 2

 > ## case of cluster that contains a cluster --> list
 > clMembFun1(3)
[[1]]
[1] 3 4

[[2]]
[1] 5

 > str(clMembFun1(3))
List of 2
  $ : num [1:2] 3 4
  $ : num 5

 > ## now the list also has 2D matrix structure
 > clMembFun1(4)
      [,1] [,2]
[1,] 1    Numeric,2
[2,] 2    5
 > str(clMembFun1(4))
List of 4
  $ : num 1
  $ : num 2
  $ : num [1:2] 3 4
  $ : num 5
  - attr(*, "dim")= int [1:2] 2 2

 > ## and it just gets worse
 > clMembFun1(5)
[[1]]
      [,1] [,2]
[1,] 1    Numeric,2
[2,] 2    5

[[2]]
[1] 6

 > str(clMembFun1(5))
List of 2
  $ :List of 4
   ..$ : num 1
   ..$ : num 2
   ..$ : num [1:2] 3 4
   ..$ : num 5
   ..- attr(*, "dim")= int [1:2] 2 2
  $ : num 6

I know one workaround is to 'unlist' the return value:

 > ## post hoc fix
 > unlist(clMembFun1(3))
[1] 3 4 5
 > unlist(clMembFun1(4))
[1] 1 2 3 4 5
 > unlist(clMembFun1(5))
[1] 1 2 3 4 5 6

But can the function itself be fixed/improved?

I also tried using a 'for' loop instead of 'sapply' but that suffered  
from fatal problems (maybe I didn't implement correctly?).