title: F#: warning FS0020: This expression should have type 'unit', but has type 'bool'
body: <p>I'm trying to learn F# by going through some of the Euler problems and I found an issue I haven't been able to figure out. This is my naive solution.</p>
  
  <pre><code>let compute =
      let mutable f = false
      let mutable nr = 0
      while f = false do
          nr &lt;- nr + 20
          f = checkMod nr
      nr
  </code></pre>
  
  <p>When i do this I get the error message warning FS0020: This expression should have type 'unit', but has type 'bool' on the expression "nr &lt;- nr +20". I've tried rewriting and moving the expressions around and I always get that error on the line below the while statement.</p>
  
  <p>I'm writing this using VS2010 Beta.</p>