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POExistentials and Scrap your Boilerplate
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  1. POExistentials and Scrap your Boilerplate
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    copied!<p>I'm writing a XML (de)serializer using Text.XML.Light and Scrap your Boilerplate (at <a href="http://github.com/finnsson/Text.XML.Generic" rel="nofollow noreferrer">http://github.com/finnsson/Text.XML.Generic</a>) and so far I got working code for "normal" ADTs but I'm stuck at deserializing existentials.</p> <p>I got the existential data type</p> <pre><code>data DataBox where DataBox :: (Show d, Eq d, Data d) =&gt; d -&gt; DataBox </code></pre> <p>and I'm trying to get this to compile</p> <pre><code>instance Data DataBox where gfoldl k z (DataBox d) = z DataBox `k` d gunfold k z c = k (z DataBox) -- not OK toConstr (DataBox d) = toConstr d dataTypeOf (DataBox d) = dataTypeOf d </code></pre> <p>but I can't figure out how to implement <code>gunfold</code> for <code>DataBox</code>.</p> <p>The error message is</p> <pre><code>Text/XML/Generic.hs:274:23: Ambiguous type variable `b' in the constraints: `Eq b' arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29 `Show b' arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29 `Data b' arising from a use of `k' at Text/XML/Generic.hs:274:18-30 Probable fix: add a type signature that fixes these type variable(s) </code></pre> <p>It's complaining about not being able to figure out the data type of <code>b</code>.</p> <p>I'm also trying to implement <code>dataCast1</code> and <code>dataCast2</code> but I think I can live without them (i.e. an incorrect implementation).</p> <p>I guess my questions are:</p> <ol> <li>Is it possible to combine existentials with Scrap your Boilerplate?</li> <li>If so: how do you implement gunfold for an existential data type?</li> </ol>
 

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