title: Add new single-character variables to list of dataframes in R
body: <p>I need to merge a long list of dataframes in R, and I'd like to create an identifier variable for each using  <code>purrr::map</code> and <code>dplyr::mutate</code>. The result will be a new variable for each dataframe that is unique and a single string. </p>
  
  <p>Here is a reprex of the error I'm getting. How can I fix this?</p>
  
  <pre><code>library(tidyverse)
  
  df_1 &lt;- mtcars %&gt;% 
      as_tibble() %&gt;% 
      select(1:3)
  
  df_2 &lt;- mtcars %&gt;% 
      as_tibble() %&gt;% 
      select(4:6)
  
  df_ls &lt;- list(df_1, df_2)
  
  
  new_vary &lt;- c('first dataframe', 'second dataframe')
  
  
  map2(df_ls, new_vary, function(x, y){
      x %&gt;% 
          mutate(new_variable = new_vary)
  })
  #&gt; Error: Column `new_variable` must be length 32 (the number of rows) or one, not 2
  </code></pre>