title: How to use mutate on list?
body: <p>What is the correct way to mutate a list? In this specific case the list is being returned by <code>split</code></p>
  
  <pre><code>library(dplyr)
  csv&lt;-data.frame(participant_number=c(1,1,1,2,2),SelfEsteem=c(3,4,2,1,3))
  csv&lt;-csv%&gt;%split(.,.$participant_number)%&gt;%mutate(.,var(.$SelfEsteem))
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Error in UseMethod("mutate_") : 
    no applicable method for 'mutate_' applied to an object of class "list"
  </code></pre>
  
  <p><strong>UPDATE In answer to comments</strong>: I intend to calculate the variance of <code>SelfEsteem</code> each group (the grouping var is <code>participant_number</code>). If I try <code>group_by</code> I do not get the expected result....Why? </p>
  
  <pre><code>library(dplyr)
  
  #Please note I changed the dataframe to make my point about variance differences more obvious
  csv&lt;-data.frame(participant_number=c(1,1,1,2,2),SelfEsteem=c(3,4,2,1,3))
  
  csv&lt;-csv%&gt;%group_by(participant_number)%&gt;%mutate(.,SE_variance=var(.$SelfEsteem))
  
  #var(c(3,4,2)) #1
  #var(c(1,3))  #2
  </code></pre>
  
  <p>Expected:</p>
  
  <pre><code>        participant_number SelfEsteem SE_variance
  (dbl)            (dbl)       (dbl)
  1                  1          3         1
  2                  1          4         1
  3                  1          2         1
  4                  2          1         2
  5                  2          3         2
  </code></pre>