title: Motivation behind Phantom Types?
body: <p>Don Stewart's <a href="http://web.archive.org/web/20150129012424/http://code.haskell.org/~dons/talks/dons-google-2015-01-27.pdf" rel="noreferrer">Haskell in the Large</a>'s presentation mentioned <em>Phantom Types</em>:</p>
  
  <pre><code>data Ratio n = Ratio Double
  1.234 :: Ratio D3
  
  data Ask ccy = Ask Double
  Ask 1.5123 :: Ask GBP
  </code></pre>
  
  <p>I read over his bullet points about them, but I did not understand them. In addition, I read the <a href="https://wiki.haskell.org/Phantom_type" rel="noreferrer">Haskell Wiki</a> on the topic. Yet I still am missing their point.</p>
  
  <p>What's the motivation to use a phantom type?</p>