title: Functor declaration with multiple types?
body: <p>I've got the following data type:</p>
  
  <pre><code>data Users id height weight = User id height weight
  
  instance Functor Users where
  fmap f (User id height weight) = User(f id height weight)
  </code></pre>
  
  <p>Yet this won't compile? </p>
  
  <p>It works fine when I use a type with a single parameter, such as:</p>
  
  <pre><code>data Users id = User id
  instance Functor Users where
  fmap f (User id) = User (f id)
  </code></pre>
  
  <p>Why isn't my first example working?</p>