title: rapply to nested list of data frames in R
body: <p>i have a nested list whose fundamental element is data frames, and i want to traverse this list recursively to do some computation of each data frame, finally to get a nested list of results in the same structure as the input. I know "rapply" is exactly for such kind of task, but i met a problem that, rapply actually goes even deeper than i want, i.e. it decomposes every data frame and applies to each column instead (because a data frame itself is a list in R). </p>
  
  <p>One workaround i can think about is to convert each data frame to matrix, but it will force to uniform the data types, so i don't like it really. I want to know if there is any way to control the recursive depth of rapply. Any idea? Thanks.</p>