title: F# "this rule will never be matched" expression in a NOT recursive function
body: <p>I have such a function:</p>
  
  <pre><code>let RotateFace (face: int, turns: int) =
      match face with
          | upperface -&gt; 
              TiltCube(2)
              TwistCube(turns)
              TiltCube(2)
          | leftface -&gt; 
              TiltCube(3)
              TwistCube(turns)
              TiltCube(1)
          | frontface -&gt; 
              TurnCube(1)
              TiltCube(3)
              TwistCube(turns)
              TiltCube(1)
              TurnCube(3)
          | rightface -&gt; 
              TiltCube(1)
              TwistCube(turns)
              TiltCube(3)
          | backface -&gt; 
              TiltCube(3)
              TwistCube(turns)
              TiltCube(1)
          | downface -&gt; 
              TurnCube(3)
              TiltCube(3)
              TwistCube(turns)
              TiltCube(1)
              TurnCube(1)
          | _ -&gt; ()
  </code></pre>
  
  <p>I have a "this rule will never be matched" problem on my cases "'upperface', 'leftface', 'frontface', 'rightface', 'backface', 'downface', and '_'.".</p>
  
  <p>I've looked this link;</p>
  
  <p><a href="https://stackoverflow.com/questions/18984535/f-match-expression-rule-will-never-be-matched">f# match expression - &quot;rule will never be matched&quot;</a></p>
  
  <p>but to be honest I didn't understand what should I do with my case.</p>