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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">I’m a bit dubious about whether it’s worth the effort of making this an extension that requires GHC support. Does the gain justify the (maybe-small but eternal) pain<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
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<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 [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Richard Eisenberg<br>
<b>Sent:</b> 25 January 2016 14:06<br>
<b>To:</b> David Feuer <david.feuer@gmail.com><br>
<b>Cc:</b> Haskell Libraries <libraries@haskell.org>; ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: Type class for sanity<o:p></o:p></span></p>
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<p class="MsoNormal">Might be nice to have whnf too, while we're at it. Perhaps whnf is enough for someone and going all the way to nf would be less efficient / impossible.<o:p></o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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<p class="MsoNormal">On Jan 25, 2016, at 8:44 AM, David Feuer <<a href="mailto:david.feuer@gmail.com">david.feuer@gmail.com</a>> wrote:<o:p></o:p></p>
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<p>I don't care about the name at all. Unstuck? Would we want to distinguish between whnf (e.g., Proxy Any) and nf, or is only nf sufficiently useful?<o:p></o:p></p>
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<p class="MsoNormal">On Jan 25, 2016 7:34 AM, "Richard Eisenberg" <<a href="mailto:eir@cis.upenn.edu">eir@cis.upenn.edu</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">+1<br>
<br>
This would be very easy to implement, too.<br>
<br>
But I suggest a different name. Ground? Terminating? NormalForm? Irreducible? ValueType? I don't love any of these, but I love Sane less.<br>
<br>
On Jan 24, 2016, at 4:24 PM, David Feuer <<a href="mailto:david.feuer@gmail.com">david.feuer@gmail.com</a>> wrote:<br>
<br>
> Since type families can be stuck, it's sometimes useful to restrict<br>
> things to sane types. At present, the most convenient way I can see to<br>
> do this in general is with Typeable:<br>
><br>
> type family Foo x where<br>
> Foo 'True = Int<br>
><br>
> class Typeable (Foo x) => Bar x where<br>
> blah :: proxy x -> Foo x<br>
><br>
> This will prevent anyone from producing the bogus instance<br>
><br>
> instance Bar 'False where<br>
> blah _ = undefined<br>
><br>
> Unfortunately, the Typeable constraint carries runtime overhead. One<br>
> possible way around this, I think, is with a class that does just<br>
> sanity checking and nothing more:<br>
><br>
> class Sane (a :: k)<br>
> instance Sane Int<br>
> instance Sane Char<br>
> instance Sane 'False<br>
> instance Sane 'True<br>
> instance Sane '[]<br>
> instance Sane '(:)<br>
> instance Sane (->)<br>
> instance Sane 'Just<br>
> instance Sane 'Nothing<br>
> instance (Sane f, Sane x) => Sane (f x)<br>
><br>
> To really do its job properly, Sane would need to have instances for<br>
> all sane types and no more. An example of an insane instance of Sane<br>
> would be<br>
><br>
> instance Sane (a :: MyKind)<br>
><br>
> which would include stuck types of kind MyKind.<br>
><br>
> Would it be useful to add such an automatic-only class to GHC?<br>
><br>
> David<br>
> _______________________________________________<br>
> Libraries mailing list<br>
> <a href="mailto:Libraries@haskell.org">Libraries@haskell.org</a><br>
> <a href="https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.haskell.org%2fcgi-bin%2fmailman%2flistinfo%2flibraries&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7cdd2a26ab307a45b5f56808d32590ab5b%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=cEc6WoMyDqd3ulJ6UenlCm%2bpeYdQ4z2FMSNcXkg8Qpc%3d" target="_blank">
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries</a><o:p></o:p></p>
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