title: Formula interface for glmnet
body: <p>In the last few months I've worked on a number of projects where I've used the <code>glmnet</code> package to fit elastic net models. It's great, but the interface is rather bare-bones compared to most R modelling functions. In particular, rather than specifying a formula and data frame, you have to give a response vector and predictor matrix. You also lose out on many quality-of-life things that the regular interface provides, eg sensible (?) treatment of factors, missing values, putting variables into the correct order, etc.</p>
  
  <p>So I've generally ended up writing my own code to recreate the formula/data frame interface. Due to client confidentiality issues, I've also ended up leaving this code behind and having to write it again for the next project. I figured I might as well bite the bullet and create an actual package to do this. However, a couple of questions before I do so:</p>
  
  <ul>
  <li>Are there any issues that complicate using the formula/data frame interface with elastic net models? (I'm aware of <a href="https://stackoverflow.com/questions/17887747/how-does-glmnets-standardize-argument-handle-dummy-variables">standardisation and dummy variables</a>, and wide datasets maybe requiring sparse model matrices.)</li>
  <li>Is there any existing package that does this?</li>
  </ul>