title: "Cannot find pattern" when using a tagged union type
body: <p>I <a href="http://github.com/anicholson/elm-hangman" rel="nofollow">have an app</a> that spans multiple modules. In the first, I model my problem, creating several data types. In the second, I'm putting views.</p>
  
  <p>One of those types is a tagged union type:</p>
  
  <pre><code>type alias Letter = Char
  type GuessedLetter = Guessed Letter | Unguessed
  </code></pre>
  
  <p>In my View module, I have a function for displaying a letter:</p>
  
  <pre><code>guessToChar : GuessedLetter -&gt; Char
  guessToChar guess =
    case guess of
      Guessed l -&gt; l
      Unguessed -&gt; '_'
  </code></pre>
  
  <p>But when I try compiling these files, I get the following error:</p>
  
  <pre><code>## ERRORS in src/Views.elm #####################################################
  
  -- NAMING ERROR -------------------------------------------------- src/Views.elm
  
  Cannot find pattern `Guessed`
  
  21|       Guessed l -&gt; l
            ^^^^^^^^^
  
  -- NAMING ERROR -------------------------------------------------- src/Views.elm
  
  Cannot find pattern `Unguessed`
  
  22|       Unguessed -&gt; '_'
            ^^^^^^^^^
  
  Detected errors in 1 module.
  </code></pre>
  
  <p>I thought "Maybe I should export the tags as well as the type?", but neither <em>adding the tags to the module exports</em> nor <em>attempting to fully-qualify the tags</em> (<code>GuessedLetter.Guessed</code>) has resolved the issue.</p>
  
  <p>How do I fix this function?</p>