title: Haskell: map one element of a tuple
body: <p>I'm new to Haskell, and wondering what would be an idiomatic way to convert a tuple <code>(a,b)</code> to <code>(c,b)</code>. In other words, only transform the first value, leaving the second unchanged.</p>
  
  <p>I can, of course, use <code>fst</code> and <code>snd</code>, but if I can <code>map</code> over lists, why not tuples?</p>