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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt">
Can you elaborate? I don't understand where you are coming from yet.<o:p></o:p></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">In haste,<o:p></o:p></span></p>
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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US"><a href="https://gitlab.haskell.org/ghc/ghc/-/issues?label_name=FunctionalDependencies">Many open tickets about fundeps</a>… still open because
of lack of agreement about what the Right Thing is, not because the implementation is difficult<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">As originally proposed they are quite restrictive (the Coverage Condition). Many uses of fundeps use a more liberal coverage condition, currently
enabled by UndecidableInstances. I’m not sure if “Many” means “most” or just “a minority”; data needed.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">Fundeps affect unification in type inference, and have no effect in Givens, unlike type families. Nothing inherently wrong with that, but it
feels unsatisfying that we can know that t1~t2 (from a fundep between two givens) but can’t exploit that fact.<o:p></o:p></span></li></ul>
<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">They are undoubtedly useful. I’m not arguing for removal. They have just never felt as solid to me as other parts of our type system.<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"> Spiwack, Arnaud <arnaud.spiwack@tweag.io>
<br>
<b>Sent:</b> 04 December 2020 14:13<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> GHC Steering Committee <ghc-steering-committee@haskell.org><br>
<b>Subject:</b> Re: [ghc-steering-committee] #380 GHC2021: What's wrong with Functional dependencies<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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On Fri, Dec 4, 2020 at 3:07 PM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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But fundeps – because they don’t carry evidence, as they stand – feel less solidly rooted.<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">
Can you elaborate? I don't understand where you are coming from yet.<o:p></o:p></p>
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<o:p> </o:p></p>
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(for the record: I'm not applying just “guarded by their own syntax” as a criterion, for me, this criterion is a sufficient condition for “no surprising new errors”. I also believe (believed?) that functional dependencies were quite standard and expected them
to be here forever. I took TypeFamilyDependency to be a mild extension. I'm perfectly willing to be convinced otherwise.)<o:p></o:p></p>
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