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<span style="mso-fareast-language:EN-US">> </span>. So apparently it is possible for a dictionary to be bottom somehow.<o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">That should not happen.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Except in the case of single-method dictionaries like<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"> class C a where op :: a -> a<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">In these cases the “dictionary” is represented by a newtype, like this<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"> newtype C a = MkC (a->a)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Then you could say<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"> instance C Int where<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"> op = bottom<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">and now a (C Int) dictionary is simply bottom.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">It would be easy to change this decision, and use a data constructor even for single-method classes. Some programs would become slightly less efficient, but things would be a bit more uniform.
If there was a real advantage to doing this, it’d definitely be worth measuring the perf cost (if any).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Glasgow-haskell-users <glasgow-haskell-users-bounces@haskell.org>
<b>On Behalf Of </b>Brandon Allbery<br>
<b>Sent:</b> 09 August 2021 16:32<br>
<b>To:</b> Tom Smeding <x@tomsmeding.com><br>
<b>Cc:</b> GHC users <glasgow-haskell-users@haskell.org>; sperber@deinprogramm.de<br>
<b>Subject:</b> Re: Avoiding construction of dead dictionaries<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
We haven't figured out what they did, but the other day we had someone in #haskell with an infinite loop evaluating a dictionary. So apparently it is possible for a dictionary to be bottom somehow.<o:p></o:p></p>
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<o:p> </o:p></p>
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On Mon, Aug 9, 2021 at 11:27 AM Tom Smeding <<a href="mailto:x@tomsmeding.com">x@tomsmeding.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Hi Mike,<o:p></o:p></p>
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<o:p> </o:p></p>
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> But wouldn't that imply that ghc can build dictionary-construction code<o:p></o:p></p>
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<o:p> </o:p></p>
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> that evaluates to bottom? Can that happen?<o:p></o:p></p>
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<o:p> </o:p></p>
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I assume no, but here the dictionary is embedded as a field in the GADT, right? So if the data value is bottom, there is not even a dictionary to be found, let alone not-bottom.<o:p></o:p></p>
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<o:p> </o:p></p>
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This assumes that the Dict in `Entail (Sub Dict)` is a GADT like<o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Dict :: Con b => Dict something<o:p></o:p></p>
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<o:p> </o:p></p>
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where the Con dictionary is contained in the GADT. Remember that in Core, dictionaries are values, and there is no difference between => and ->.<o:p></o:p></p>
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- Tom<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
-------- Original Message --------<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
On 9 Aug 2021, 15:24, Michael Sperber < <a href="mailto:sperber@deinprogramm.de" target="_blank">
sperber@deinprogramm.de</a>> wrote:<o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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Thanks for thinking about this one!<o:p></o:p></p>
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<o:p> </o:p></p>
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On Fri, Aug 06 2021, Tom Smeding <<a href="mailto:x@tomsmeding.com" target="_blank">x@tomsmeding.com</a>> wrote:<o:p></o:p></p>
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<o:p> </o:p></p>
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> Would it not be unsound for ghc to elide dictionary construction here?<o:p></o:p></p>
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<o:p> </o:p></p>
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> After all, the right-hand side might actually be a bottom<o:p></o:p></p>
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<o:p> </o:p></p>
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> (e.g. undefined) at run-time, in which case the pattern match cannot<o:p></o:p></p>
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<o:p> </o:p></p>
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> succeed according to the semantics of Haskell.<o:p></o:p></p>
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<o:p> </o:p></p>
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But wouldn't that imply that ghc can build dictionary-construction code<o:p></o:p></p>
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<o:p> </o:p></p>
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that evaluates to bottom? Can that happen?<o:p></o:p></p>
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<o:p> </o:p></p>
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> I suspect that if you make the pattern match lazy (i.e. ~(Entail (Sub<o:p></o:p></p>
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<o:p> </o:p></p>
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> Dict))) or ignore the argument altogether (i.e. _), dictionary<o:p></o:p></p>
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<o:p> </o:p></p>
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> construction will be elided.<o:p></o:p></p>
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<o:p> </o:p></p>
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Thanks for the hint! ghc gives me this unfortunately, implying that it<o:p></o:p></p>
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agreed with your first comment:<o:p></o:p></p>
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src/ConCat/Category.hs:190:29: error:<o:p></o:p></p>
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• Could not deduce: Con b arising from a use of ‘r’<o:p></o:p></p>
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from the context: Con a<o:p></o:p></p>
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bound by the type signature for:<o:p></o:p></p>
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(<+) :: forall a b r. Con a => (Con b => r) -> (a |- b) -> r<o:p></o:p></p>
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at src/ConCat/Category.hs:189:1-46<o:p></o:p></p>
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• In the expression: r<o:p></o:p></p>
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In an equation for ‘<+’: r <+ ~(Entail (Sub Dict)) = r<o:p></o:p></p>
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• Relevant bindings include<o:p></o:p></p>
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r :: Con b => r (bound at src/ConCat/Category.hs:190:1)<o:p></o:p></p>
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(<+) :: (Con b => r) -> (a |- b) -> r<o:p></o:p></p>
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(bound at src/ConCat/Category.hs:190:3)<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
|<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
190 | r <+ ~(Entail (Sub Dict)) = r<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
| ^<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Other ideas welcome!<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
--<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Regards,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Mike<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
_______________________________________________<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Glasgow-haskell-users mailing list<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<a href="mailto:Glasgow-haskell-users@haskell.org" target="_blank">Glasgow-haskell-users@haskell.org</a><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<a href="https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fglasgow-haskell-users&data=04%7C01%7Csimonpj%40microsoft.com%7C7bf3769704884ed6592e08d95b4aeb26%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637641199636853840%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=ix06XQPvpu%2B1PLzoc5rRQM6cMv8yPF6o87uVwD0sUwQ%3D&reserved=0" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users</a><o:p></o:p></p>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
_______________________________________________<br>
Glasgow-haskell-users mailing list<br>
<a href="mailto:Glasgow-haskell-users@haskell.org" target="_blank">Glasgow-haskell-users@haskell.org</a><br>
<a href="https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fglasgow-haskell-users&data=04%7C01%7Csimonpj%40microsoft.com%7C7bf3769704884ed6592e08d95b4aeb26%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637641199636863837%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=sQDTBnklNv7YLRvhiY5CEtbZgcZT8p7RR%2Bw57sCqFJk%3D&reserved=0" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users</a><o:p></o:p></p>
</blockquote>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<br clear="all">
<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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-- <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
brandon s allbery kf8nh<o:p></o:p></p>
</div>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<a href="mailto:allbery.b@gmail.com" target="_blank">allbery.b@gmail.com</a><o:p></o:p></p>
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