title: How do I add a CPP defininition within cabal / stack?
body: <p>I'm looking at the following module: <a href="https://hackage.haskell.org/package/boxes-0.1.4/docs/src/Text-PrettyPrint-Boxes.html" rel="nofollow noreferrer">https://hackage.haskell.org/package/boxes-0.1.4/docs/src/Text-PrettyPrint-Boxes.html</a></p>
  
  <p>Which has contents of:</p>
  
  <pre><code>module Text.PrettyPrint.Boxes
      ( -- * Constructing boxes
  #ifdef TESTING
        Box(Box, content)
  #else
        Box
  #endif
  </code></pre>
  
  <p>How can I enable / define the <code>TESTING</code> value - for development purposes? Ideally I'd like to have this built / enabled within a GHCi session.</p>