Dear all,<div><br></div><div>    I&#39;d like to write a function &quot;maybeShow :: a -&gt; Maybe String&quot;, which runs &quot;show&quot; if its argument is of class Show.</div><div><br></div><div>    The context and motivation for this are as follows. I have a GADT type which encapsulates abstract-value computation (or constants or error codes), a snippet of which is below.</div>

<div><br></div><div><div>data AV t where</div><div>    AVLeft :: AV a -&gt; AV (Either a b)</div></div><div><br></div><div>    This is used to implement an arrow transformer, and due to Arrows mapping all Haskell functions, I cannot put some kind of qualification on the constructor, like &quot;AVLeft :: Show a =&gt; ...&quot;.</div>

<div><br></div><div>    Of course any replies are welcome, but I do need something implemented and stable. If there are GHC-compatible hacks, even an &quot;unsafeShow :: a -&gt; String&quot;, that&#39;d be great. I&#39;d also prefer not to branch on all types which could possibly be maybeShow&#39;s argument.</div>

<div><br></div><div><br></div><div><br></div><div>    (Concretely, if I have &quot;newtype AVFunctor a b c = AVF (a (AV b) (AV c))&quot;, then the Arrow class declaration forces all types, c.f. variable b, to be potential variables of type AV),</div>

<div><br></div><div><div>class (Category a) =&gt; Arrow a where</div><div>  arr :: (b -&gt; c) -&gt; a b c</div></div><div><div><br></div><div><br></div><div>Thanks very much,<br clear="all">Nicholas — <a href="https://ntung.com" target="_blank">https://ntung.com</a> — CS major @ UC Berkeley<br>

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</div></div><div>p.s. I posted this question on StackOverflow if you care to get brownie points there, <a href="http://goo.gl/PrmYW">http://goo.gl/PrmYW</a></div><div><br></div><div>p.s. 2 -- if there is a general &quot;dump var&quot; function in ghci, which does more than &quot;:info&quot;, I&#39;d love to know :)</div>