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copied!<p>This isn't possible with just Haskell's record system. What you want are <em>lenses</em>; this <a href="https://stackoverflow.com/questions/5767129/lenses-fclabels-data-accessor-which-library-for-structure-access-and-mutatio">previous Stack Overflow question</a> and its top answer are a good introduction. Personally, I use the <a href="http://hackage.haskell.org/package/data-lens" rel="nofollow noreferrer">data-lens</a> package talked about there. (See also <a href="http://hackage.haskell.org/package/data-lens-fd" rel="nofollow noreferrer">data-lens-fd</a> to use it with the <a href="http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Monad-State-Class.html#t:MonadState" rel="nofollow noreferrer">MonadState</a> class from mtl — if you don't know what that is, just know that you should probably use it whenever you want to use lenses in a <code>State</code> monad.)</p> <p>The <a href="http://hackage.haskell.org/package/data-lens-template" rel="nofollow noreferrer">data-lens-template</a> package might be the application of Template Haskell you want; it derives lens definitions for record fields.</p> <p>The other popular lens package is <a href="http://hackage.haskell.org/package/fclabels" rel="nofollow noreferrer">fclabels</a>. I prefer data-lens for its simplicity and speed; fclabels (as of version 1.0) is slightly more flexible, but you don't need that flexibility for what you want to do. (Note that due to its newly-increased flexibility, fclabels' <code>(:->)</code> type can no longer be directly translated to the simple definition of lenses, as mentioned in the Stack Overflow answer I linked.)</p>

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