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POHow can I get GHC to generate instances of Data.Typeable for GADTs with Typeable in the context?
 

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  1. POHow can I get GHC to generate instances of Data.Typeable for GADTs with Typeable in the context?
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    <p>Suppose I have the following code:</p> <pre><code>{-# LANGUAGE GADTs, DeriveDataTypeable, StandaloneDeriving #-} import Data.Typeable class Eq t =&gt; OnlyEq t class (Eq t, Typeable t) =&gt; BothEqAndTypeable t data Wrapper a where Wrap :: BothEqAndTypeable a =&gt; a -&gt; Wrapper a deriving instance Eq (Wrapper a) deriving instance Typeable1 Wrapper </code></pre> <p>Then, the following instance declaration works, <em>without</em> a constraint on <code>t</code>:</p> <pre><code>instance OnlyEq (Wrapper t) </code></pre> <p>and does what I expect it to do.</p> <hr> <p>But the following instance declaration doesn't work:</p> <pre><code>instance BothEqAndTypeable (Wrapper t) </code></pre> <p>since GHC - I'm using 7.6.1 - complains that:</p> <pre><code>No instance for (Typeable t) arising from the superclasses of an instance declaration Possible fix: add (Typeable t) to the context of the instance declaration In the instance declaration for `BothEqAndTypeable (Wrapper t)' </code></pre> <p>Adding <code>Typeable t</code> to the context works, of course. But so does adding the following instance:</p> <pre><code>instance Typeable (Wrapper t) where typeOf (Wrap x) = typeOf1 (Wrap x) `mkAppTy` typeOf x </code></pre> <p>Is there a way to get GHC to write this latter instance for me? If so, how? If not, why not? </p> <p>I was hoping GHC would be able to pull the <code>Typeable</code> constraint from the context on the <code>Wrap</code> constructor, just as it did with the <code>Eq</code> constraint. I think that my problems boils down to the fact that GHC explicitly disallows writing <code>deriving instance Typeable (Wrapper t)</code>, and the standard <code>(Typeable1 s, Typeable a) =&gt; Typeable (s a)</code> instance can't 'look inside' <code>s a</code> to find a <code>Typeable a</code> dictionary.</p>
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