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<p class="MsoNormal"><span style="font-size:12.0pt">Mathias<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I suspect substImplication was removed simply because it was never called, but I’m not certain.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">The entire cloning and substitution business is very heavyweight. Here’s an alternative idea:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-size:12.0pt">Gather ‘fuvs’, the set of all free unification variables of the constraint you are about to solve.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-size:12.0pt">Solve the constraint<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-size:12.0pt">For each variable in ‘fuvs’, set its IORef to Flexi; that is, simply undo any unification that has taken place.<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">That seems simple, very non-invasive, and robust.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><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"> Matthías Páll Gissurarson <pallm@student.chalmers.se>
<br>
<b>Sent:</b> 10 April 2018 00:57<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Subject:</b> Nested Implications<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Greetings Simon,<br>
<br>
I've been working on implementing the valid substitutions for typed holes in terms of nested implications, and I think I've worked out how to do it. But one thing bothers me. To use the implications that are passed along with the ReportErrCtxt, I have to clone
the type variables within, to avoid having side-effects that could impact later searches. I clone the type variables and create a substitution from the old variables to the new, which I'd like to apply to the implication. When I was looking around for how
to do that, I came across this code in GHC 7.6.3<br>
<br>
<a href="https://na01.safelinks.protection.outlook.com/?url=https:%2F%2Fdownloads.haskell.org%2F~ghc%2F7.6-latest%2Fdocs%2Fhtml%2Flibraries%2Fghc-7.6.3%2Fsrc%2FInst.html&data=02%7C01%7Csimonpj%40microsoft.com%7Cebcb3f452d22449ccd4d08d59e75a6d7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636589150506893746&sdata=y188p6jNMD4%2Bd1eHVhxBhIdSq2RDgrytpfNmkGG8WUk%3D&reserved=0">https://downloads.haskell.org/~ghc/7.6-latest/docs/html/libraries/ghc-7.6.3/src/Inst.html</a> <br>
<br>
The `substImplication` function does precisely what I want, i.e. applies a substitution to an implication. However, that code is no longer present in GHC 8.5! The git history doesn't go far enough back for me to find an explanation, so I wanted to ask: why
was this function removed? Is there something I should be wary of if I re-implement it?<br>
<br>
Please correct me if my approach is all wrong, I'd like to hear what you had in mind.<o:p></o:p></p>
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-- <o:p></o:p></p>
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-<br>
<br>
Matthías Páll Gissurarson<o:p></o:p></p>
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