title: disabling fields in formlets/digestive-functors?
body: <p>With formlets/digestive-functors, I'm trying to figure out how selectively disable fields at run-time. Disabling a field would disable both the showing of the field as well as validation.</p>
  
  <p>Contrived example: Suppose our basic form data type looks like:</p>
  
  <pre><code>data Info =
    Info {
       favcolor :: String,
       deptId   :: Int,
       company  :: String,
       agree    :: Boolean
    }
  </code></pre>
  
  <p>but the <code>deptId</code> field should only be shown if the user is an employee, otherwise the <code>company</code> field should be shown. And the <code>agree</code> field should be shown if the user hasn't already clicked the "Agree" box. </p>
  
  <p>I don't want to create a different form type for each possible variation: (favcolor, deptId, agree), (favcolor, deptId), (favcolor, company, agree), (favcolor, company). What else can I do?</p>