title: unique elements in a haskell list
body: <p>okay, this is probably going to be in the prelude, but: is there a standard library function for finding the unique elements in a list? my (re)implementation, for clarification, is:</p>
  
  <pre><code>has :: (Eq a) =&gt; [a] -&gt; a -&gt; Bool
  has [] _ = False
  has (x:xs) a
    | x == a    = True
    | otherwise = has xs a
  
  unique :: (Eq a) =&gt; [a] -&gt; [a]
  unique [] = []
  unique (x:xs)
    | has xs x  = unique xs
    | otherwise = x : unique xs
  </code></pre>