Hello Bulat,<br><div><br>You are indeed correct. However I fail to see how there is any undecideability. If instead one specified it as follows, it would be fine:<br><br>class Locatable a b | a -> b, b -> a where<br>
value :: a -> b
<br> wrap :: b -> a<br><br>instance Locatatable (Located a) a where</div><div><span class="q"><br> value (L a) = a<br> wrap a = L a<br><br></span></div><div>It compiles fine, yet this is semantically the same.
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On 6/7/06, <b class="gmail_sendername">Bulat Ziganshin</b> <<a href="mailto:bulat.ziganshin@gmail.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">bulat.ziganshin@gmail.com</a>> wrote:</span>
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Hello Christophe,<br><br>Wednesday, June 7, 2006, 12:27:22 PM, you wrote:<br><br>> global instance of a typeclass (so there is no overlapping), GHC will<br>> still complain about overlapping. Is there any reason this is so or is
<br><br>GHC compains here about undecidability, not overlapping :)<br><br>> -- Illegal instance declaration for `Locatable a'<br>> -- (There must be at least one non-type-variable in the instance head<br>> -- Use -fallow-undecidable-instances to permit this)
<br>> -- In the instance declaration for `Locatable a'<br><br>> Christophe(vincenz)<br><br><br><br><br>--<br>Best regards,<br> Bulat mailto:<a href="mailto:Bulat.Ziganshin@gmail.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">
Bulat.Ziganshin@gmail.com
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