title: Create summary string of row contents by group using dplyr
body: <p>I'm doing a <code>group_by</code> + <code>summarize()</code> operation on a large dataframe for a lot of groups and across a lot of rows. On the aggregated dataset (each line a group), I would like to concatenate all the row entries in one string per line.</p>
  <pre><code>library(dplyr)
  
  df &lt;- tribble(
    ~country, ~var,     
    &quot;France&quot;, &quot;Apple&quot;,  
    &quot;France&quot;, &quot;Pear&quot;,   
    &quot;France&quot;, &quot;Orange&quot;, 
    &quot;France&quot;, &quot;Apple&quot;,  
    &quot;USA&quot;, &quot;Pear&quot;,   
    &quot;USA&quot;, &quot;Pear&quot;,   
    &quot;USA&quot;, &quot;Orange&quot; 
  )
  </code></pre>
  <p>The output that should result is:</p>
  <pre><code>df_summarized &lt;- tribble(
     ~country, ~summary,
    &quot;France&quot;, &quot;Apple, Pear, Orange, Apple&quot;, 
    &quot;USA&quot;, &quot;Pear, Pear, Orange&quot;
  )
  </code></pre>