title: If SML.NET had functors why can't F#?
body: <p>This question started out from  </p>
  
  <ol>
  <li>My translating of "<a href="http://www.cl.cam.ac.uk/~lp15/MLbook/" rel="noreferrer">ML for the Working Programmer</a>" (<a href="http://www.worldcat.org/oclc/26013714" rel="noreferrer">WorldCat</a>) by L. C. PAULSON to F# which uses functors for the examples.   </li>
  <li>Eventual desire to translate "<a href="https://rads.stackoverflow.com/amzn/click/com/0521663504" rel="noreferrer" rel="nofollow noreferrer">Purely Functional Data Structures</a>" (<a href="http://www.worldcat.org/oclc/38521530" rel="noreferrer">WorldCat</a>) by Chris Okasaki which uses functors.  </li>
  <li>Reading "<a href="http://www.di.ens.fr/users/longo/files/CategTypesStructures/book.pdf" rel="noreferrer">CATEGORIES TYPES AND STRUCTURES - An Introduction to Category Theory for the working   computer scientist</a>" (<a href="http://www.worldcat.org/oclc/20755899" rel="noreferrer">WorldCat</a>) by Andrea Asperti and Giuseppe Longo.   </li>
  <li>Not understanding it all, mostly the category theory.</li>
  </ol>
  
  <p><a href="http://www.cl.cam.ac.uk/research/tsg/SMLNET/" rel="noreferrer">SML.NET</a> can do functors and worked with Microsoft .NET.<br>
      * See: <a href="http://www.cl.cam.ac.uk/research/tsg/SMLNET/smlnet.pdf" rel="noreferrer">SML.NET User Guide</a> Section 4.8.2 Class types and functors?</p>
  
  <p>I keep seeing that F# cannot do true functors because of some limitation in Microsoft .NET.<br>
      * <a href="https://stackoverflow.com/q/1426239/1243762">Can ML functors be fully encoded in .NET (C#/F#)?</a><br>
      * <a href="http://cs.hubfs.net/topic/None/57222" rel="noreferrer">Any workaround for functor?</a> </p>
  
  <p>So if SML.NET could do functors on .NET then why can't F#? What did SML.NET do that F# can't?</p>
  
  <p>The more I learn about functors coming from category theory, the more I see the beauty of them and desire to have them in F#.</p>
  
  <p>EDIT</p>
  
  <p>In a pursuit to better understand the relation between category theory and functional programming see these <a href="https://cs.stackexchange.com/questions/tagged/category-theory+functional-programming">Q&amp;A</a> at <a href="https://cs.stackexchange.com/">CS:StackExchange</a>.</p>