title: can the value.var in dcast be a list or have multiple value variables?
body: <p>In the help files for <code>dcast.data.table</code>, there is a note stating that a new feature has been implemented: "dcast.data.table allows value.var column to be of type list"</p>
  
  <p>I take this to mean that one can have multiple value variables within a list, i.e. in this format:</p>
  
  <pre><code>dcast.data.table(dt, x1~x2, value.var=list('var1','var2','var3'))
  </code></pre>
  
  <p>But we get an error: <code>'value.var' must be a character vector of length 1.</code></p>
  
  <p>Is there such a feature, and if not, what would be other one-liner alternatives?</p>
  
  <p><strong>EDIT: In reply to the comments below</strong> </p>
  
  <p>There are situations where you have multiple variables that you want to treat as the <code>value.var</code>. Imagine for example that x2 consists of 3 different weeks, and you have 2 value variables such as salt and sugar consumption and you want to cast those variables across the different weeks. Sure, you can 'melt' the 2 value variables into a single column, but why do something using two functions, when you can do it in one function like <code>reshape</code> does? </p>
  
  <p>(Note: I've also noticed that <code>reshape</code> cannot treat multiple variables as the time variable as <code>dcast</code> does.)</p>
  
  <p>So my point is that I don't understand why these functions don't allow for the flexibility to include multiple variables within the <code>value.var</code> or the <code>time.var</code> just as we allow for multiple variables for the <code>id.var</code>.</p>