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<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Good question, and I’m afraid it’s quite awkward.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Stuff that is wired in to the compiler indeed avoids mentioning classes (or even complicated TyCons) precisely because they are complicated
structures. It would not be impossible to wire them in too, but I’d prefer not to.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">My guess is that the unfolding you desire
<b>also</b> mentions hard-to-wire-in classes or types.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">So here’s an alternative.
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:39.15pt;text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="font-size:14.0pt;font-family:Symbol;color:#1F497D;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">In MkId add a new wired-in Id magicNatSingI :: forall a. a<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:39.15pt;text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="font-size:14.0pt;font-family:Symbol;color:#1F497D;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">In your Sing module define<br>
natSingI :: Sing a -> (Sing I a => b) -> b<br>
natSingI = magicNatSingI<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:39.15pt;text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="font-size:14.0pt;font-family:Symbol;color:#1F497D;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">In prelude/PrelRules you’ll find ‘builtinRules’ which gives RULES that can’t be expressed in Haskell. Add a RU<a name="_MailEndCompose">LE
for magicNatSingI that rewrites (natSingI tau) to the right Core. The tau you’ll get is precisely the type of natSingI.<o:p></o:p></a></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">So in effect magicNatSingI is a place-holder for the Core.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Not beautiful, but I think it’s a recipe that should work.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif""> ghc-devs-bounces@haskell.org [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Iavor Diatchki<br>
<b>Sent:</b> 21 May 2013 02:55<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Question about wired-in class TyCons<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hello,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I was trying to add a new compulsory-unfoldable primitive (i.e., one like `<span style="font-family:"Courier New"">unsafeCoerce</span>` in `<span style="font-family:"Courier New"">basicTypes/MkId.hs`</span>), but I got stuck while trying
to construct its type. The primitive needs to have this type:<o:p></o:p></p>
</div>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> natSingI :: Sing a -> (SingI a => b) -> b</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">`<span style="font-family:"Courier New"">Sing</span>` is a kind polymorphic data family.<o:p></o:p></p>
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<div>
<p class="MsoNormal">`<span style="font-family:"Courier New"">SingI</span>` is a kind polymorphic class.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I am using these at kind `<span style="font-family:"Courier New"">Nat</span>` so, I believe, the fully-explicit type that I need to construct is actually this:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal"><span style="font-family:"Courier New""> natSingI :: forall (a :: Nat) (b :: *). Sing Nat a -> (SingI Nat a -> b) -> b</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I can almost create this type, but I got stuck on making the `<span style="font-family:"Courier New"">SingI</span>` type constructor. While there are many examples of making `<span style="font-family:"Courier New"">Name</span>`s for classes,
I couldn't find one where we make a `<span style="font-family:"Courier New"">TyCon</span>` for a class. In particular, it looks like to make a class `<span style="font-family:"Courier New"">TyCon</span>`, I need to provide a `<span style="font-family:"Courier New"">Class</span>`
value, which contains a whole lot of information that would normally be constructed by the compiler itself (class `<span style="font-family:"Courier New"">SingI</span>` is an ordinary class defined in source code).<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">So I was wondering if there might be a way to obtain this information in some way or if, indeed, I need to define explicitly the entire class as a wired-in thing.<o:p></o:p></p>
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<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">Any advice would be most appreciated!<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">-Iavor<o:p></o:p></p>
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<div>
<p class="MsoNormal"><o:p> </o:p></p>
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