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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I don’t think it’s all that complicated or fragile. <o:p></o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>To resolve the constraint (C T1 T2), use the appropriate
instance declaration to express it in terms of (hopefully simpler) constraints.
Keep doing that. If you terminate, GHC should. <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Example: to resolve Eq [Int], use the instance declaration<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> instance Eq a => Eq [a]<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>That gives rise to the new constraint Eq Int. Use the instance
declaration<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> instance Eq Int<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>That gives rise to no new instances. Done.<o:p></o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>If you terminate and GHC does not, write down your reasoning
(ie how you resolved the instance) and send it in.<o:p></o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>[NB: There is a wrinkle for “recursive dictionaries”, described
in the SYB3 paper.]<o:p></o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Simon<o:p></o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal>Well, the reasoning for the "devil" camp (which I
admit to being<br>
firmly in[1]) is that such proofs must rely on the algorithm the<br>
compiler uses to resolve instances. You might be able to prove it,<br>
but the proof is necessarily only valid for (possibly current versions<br>
of) GHC. The typeclass resolution algorithm is not in the report, and<br>
there are several conceivable ways of of going about it.<br>
<br>
<br>
<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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