title: Count number of Instances of Char in a given String Haskell
body: <p>Simple problem.</p>
  
  <p>Given a String, I want to be able to enter a letter and then have my function count the number of times that letter appears in a string.</p>
  
  <pre><code>countLetters :: String -&gt; Char -&gt; Int
  </code></pre>
  
  <p>How can this be done?</p>