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POBest way to implement ad-hoc polymorphism in Haskell?
 

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  1. POBest way to implement ad-hoc polymorphism in Haskell?
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    <p>I have a polymorphic function like: </p> <pre><code>convert :: (Show a) =&gt; a -&gt; String convert = " [label=" ++ (show a) ++ "]" </code></pre> <p>But sometimes I want to pass it a Data.Map and do some more fancy key value conversion. I know I can't pattern match here because Data.Map is an abstract data type (according to <a href="https://stackoverflow.com/questions/3327532/haskell-pattern-matching-on-the-empty-set">this similar SO question</a>), but I have been unsuccessful using guards to this end, and I'm not sure if ViewPatterns would help here (and would rather avoid them for portability). </p> <p>This is more what I want:</p> <pre><code>import qualified Data.Map as M convert :: (Show a) =&gt; a -&gt; String convert a | M.size \=0 = processMap2FancyKVString a -- Heres a Data.Map | otherwise = " [label=" ++ (show a) ++ "]" -- Probably a string </code></pre> <p>But this doesn't work because M.size can't take anything other than a Data.Map.</p> <p>Specifically, I am trying to modify the <a href="http://hackage.haskell.org/packages/archive/fgl/5.4.2.3/doc/html/src/Data-Graph-Inductive-Graphviz.html#graphviz%27" rel="nofollow noreferrer">sl utility function in the Functional Graph Library</a> in order to handle coloring and other attributes of edges in GraphViz output. </p> <p><strong>Update</strong></p> <p>I wish I could accept all three answers by TomMD, Antal S-Z, and luqui to this question as they all understood what I really was asking. I would say:</p> <ul> <li>Antal S-Z gave the most 'elegant' solution as applied to the FGL but would also require the most rewriting and rethinking to implement in personal problem.</li> <li>TomMD gave a great answer that lies somewhere between Antal S-Z's and luqui's in terms of applicability vs. correctness. It also is direct and to the point which I appreciate greatly and why I chose his answer.</li> <li>luqui gave the best 'get it working quickly' answer which I will probably be using in practice (as I'm a grad student, and this is just some throwaway code to test some ideas). The reason I didn't accept was because TomMD's answer will probably help other people in more general situations better.</li> </ul> <p>With that said, they are all excellent answers and the above classification is a gross simplification. I've also updated the question title to better represent my question (Thanks Thanks again for broadening my horizons everyone!</p>
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