title: writing methods in f#
body: <p>I have a question:</p>
  
  <p>There is such a method in C:</p>
  
  <pre><code>inline void ColorSet(int face, int pos,int col)
  {
    color[face*9+pos]=col;
  }
  </code></pre>
  
  <p>I've tried to write it in F#;</p>
  
  <pre class="lang-ml prettyprint-override"><code>type ColorSet =
      member this.ColorSet (face: int, pos: int, col: int) = 
          color.[face*9+pos] = col
  </code></pre>
  
  <p>but I encountered with such an error:</p>
  
  <blockquote>
    <p>The operator <code>'expr.[idx]'</code> has been used on an object of indeterminate type based on information prior to this program point. Consider adding further type constraints...</p>
  </blockquote>
  
  <p>Could you help me to write the exact method?</p>