title: Using nest and purrr::map outside of mutate
body: <p>Lets say I want to split out mtcars into 3 csv files based on their cyl grouping. I can use mutate to do this, but it will create a <code>NULL</code> column in the output.</p>
  
  <pre><code>library(tidyverse)
  by_cyl = mtcars %&gt;% 
             group_by(cyl) %&gt;%
             nest()
  by_cyl %&gt;%
    mutate(unused = map2(data, cyl, function(x, y) write.csv(x, paste0(y, '.csv'))))
  </code></pre>
  
  <p>is there a way to do this on the by_cyl object without calling mutate?</p>