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    <blockquote> <p>I was hoping GHC would be able to pull the <code>Typeable</code> constraint from the context on the <code>Wrap</code> constructor</p> </blockquote> <p>If it had a <code>Wrap</code> constructor, it could pull the <code>Typeable</code> constraint from it.</p> <p>But it doesn't have a <code>Wrap</code> constructor.</p> <p>The difference is that the <code>Eq</code> instance uses the value, so it's either a <code>Wrap something</code>, where the <code>Wrap</code> constructor makes the <code>Eq</code> dictionary for the wrapped type available, and everything is fine, or it's <code>⊥</code>, and then everything is fine too, evaluating <code>x == y</code> bottoms out.</p> <p>Note that the derived</p> <pre><code>instance Eq (Wrapper a) </code></pre> <p>does <em>not</em> have an <code>Eq</code> constraint on the type variable <code>a</code>.</p> <pre><code>Prelude DerivT&gt; (undefined :: Wrapper (Int -&gt; Int)) == undefined *** Exception: Prelude.undefined Prelude DerivT&gt; (undefined :: (Int -&gt; Int)) == undefined &lt;interactive&gt;:3:29: No instance for (Eq (Int -&gt; Int)) arising from a use of `==' Possible fix: add an instance declaration for (Eq (Int -&gt; Int)) In the expression: (undefined :: Int -&gt; Int) == undefined In an equation for `it': it = (undefined :: Int -&gt; Int) == undefined </code></pre> <p>But the <code>Typeable</code> instance must not make use of the value, so there's no bottoming out if the supplied value isn't a <code>Wrap something</code>.</p> <p>Thus the derived <code>instance Typeable1 Wrapper</code> supplies</p> <pre><code>instance Typeable t =&gt; Typeable (Wrapper t) </code></pre> <p>but not an unconstrained</p> <pre><code>instance Typeable (Wrapper t) </code></pre> <p>and that unconstrained instance cannot be derived by GHC.</p> <p>Hence you have to either provide a constrained</p> <pre><code>instance Typeable t =&gt; BothEqAndTypeable (Wrapper t) </code></pre> <p>or an unconstrained</p> <pre><code>instance Typeable (Wrapper t) </code></pre> <p>yourself.</p>
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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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    1. USDaniel Fischer
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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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    1. CORight, I keep forgetting about the finer points of `undefined` as I usually store something in my datatypes :-) I think I need some sort of `newtype` GADTs, then! And an additional question, if I may: is the unconstrained `Typeable (Wrapper t)` instance I wrote in any way bad, provided I don't care that `typeOf (error "aargh" :: Wrapper Bool)` bottoms out?
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    2. COWell, it's a slightly surprising instance since you can't use it with `undefined :: Wrapper Bool` like other `Typeable` instances. If you make that public, at least put a warning on it. But I don't think that should necessarily stop you. In future GHC releases, however - methinks from 7.8 onwards, all `Typeable` instances will be provided by GHC, and for all types, iirc. So you won't have to deal with such problems then.
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      1. USDaniel Fischer
    3. COLooking at the repository, you'll need `-XAutoDeriveTypeable`. Cool! I think I *might* just go and clone 7.8.1 over the weekend for that, but the last time I built GHC was in 2002 (early 6.x cycle?), so that's a bit scary... But thanks again!
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