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<p class="MsoNormal" style="margin-left:36.0pt">5) The first suspect turned out to be the culprit: I was using my plugin's by-fiat coercions in the most naive possible way, always simply `EvCast ev (fiatCoercion ty0 ty1)`. In particular, I was even doing that
to create new Given unlifted equality witnesses from existing Given unlifted equality witnesses when simplifying Given constraints (e.g. for example reducing a plugin-specific type family application on one side of an unlifted equality type ~#).<br>
<br>
<span style="font-size:12.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I don’t understand the issue here at all. Can you give a concrete example?<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>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Nicolas Frisby [mailto:nicolas.frisby@gmail.com]
<br>
<b>Sent:</b> 09 October 2017 03:50<br>
<b>To:</b> Richard Eisenberg <rae@cs.brynmawr.edu><br>
<b>Cc:</b> Simon Peyton Jones <simonpj@microsoft.com>; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Invariants about UnivCo?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p>Yep, that's the current question: why does preferring `EvCoercion (TransCo UnivCo (TransCo co UnivCo))` to `EvCast (EvCoercion co) UnivCo` seem to matter? In my scenario, `co` is the evidence for a Given equality type. And the coercion I'm building is also
a Given constraint's evidence -- I'm simplifying Givens.<o:p></o:p></p>
<p>The only hard indication I currently have of what "goes wrong" is the ASSERT failure described in the previous email.<o:p></o:p></p>
<p>I'm planning to spend some time investigating. I would appreciate any cycles you spend on it!<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Sun, Oct 8, 2017, 18:53 Richard Eisenberg <<a href="mailto:rae@cs.brynmawr.edu">rae@cs.brynmawr.edu</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Thanks for this status report. If I'm to boil it down to the question you seem to be asking: What does changing EvCast ... to EvCoercion ... fix the problem? I'm not sure of the answer at this point, but I want to make sure I understand
the question before I go digging for an answer. It's always possible a Note is wrong!<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks for this!<o:p></o:p></p>
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<p class="MsoNormal"><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"><o:p> </o:p></p>
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<p class="MsoNormal">On Oct 7, 2017, at 8:19 PM, Nicolas Frisby <<a href="mailto:nicolas.frisby@gmail.com" target="_blank">nicolas.frisby@gmail.com</a>> wrote:<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 happily report some progress: I'm seeing no more Core lint errors!<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">1) Thank you both Richard and Simon for your pointers -- <span style="color:#212121">-fprint-typechecker-elaboration</span> in particular was a revelation.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">2) Simon, I intend to match the spirit of the favor you requested, but not to the letter. My goal with this project is to write a typechecker plugin for achieving row types _without_ editing GHC's source code. I'm keeping an annotated bibliography
of things I've studied (papers, guide/wiki/blog, source Notes, etc). (It's nice to put a bunch of related notes in the same text file!) I'm also logging my epiphanies, which I do intend to write-up in some kind of document (probably on the dev wiki). I'm planning
a section for suggesting which Notes should be adjusted/expanded, but I don't anticipate feeling comfortable enough to actually edit the Notes myself. This is unfortunately just a hobby project. My intent is to offer you, Richard, and other experts the details
of what wasn't clear to me.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">3) I confirmed that the lack of cobox uniques in the dump output was indeed due to `ppr_co' deferring to `ppr @IfaceType'; it does that (at least) for every coercion with a head of `TyConAppCo'. With a tiny kludgy patch I was able to persist
those uniques just for debugging purposes.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">4) My top-level error is an "out of scope cobox" Lint error, but (once I patched the dumper) the output of <span style="color:#212121">-fprint-typechecker-elaboration</span> showed sufficient bindings for all of the cobox occurrences, even
the one that the Lint error was flagging! Stymied, I finally did a -DDEBUG build of the ghc-8.2.2-rc1 tag and used that. It ultimately lead to me finding my mistakes. (New wisdom: always use a DEBUG build when authoring a plugin. (... Duh.))<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">4a) ASSERT failures showed that I was invoking `substTy' without correctly initializing the `InScopeSet'. I also was ignorant that I should be using `extendTvSubstAndInScope' instead of just `extendTvSubst'. I don't think this was relevant
to my particular Lint error, but I fixed it if only to see further ASSERT failures.<br>
<br>
4b) Fixing my `InScopeSet's ASSERT failure revealed another: `extendIdSubst' was being called with a CoVar! That's something that my plugin code absolutely does not do, so at that point I knew that some higher-level operation I was doing was knocking the rest
of GHC's pipeline off the rails. (In particular, I traced this ASSERT callstack to extendIdSubst called from simpleOptExpr called from mkInlineUnfoldingWithArity called from DsBinds. I stopped there.)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">5) The first suspect turned out to be the culprit: I was using my plugin's by-fiat coercions in the most naive possible way, always simply `EvCast ev (fiatCoercion ty0 ty1)`. In particular, I was even doing that to create new Given unlifted
equality witnesses from existing Given unlifted equality witnesses when simplifying Given constraints (e.g. for example reducing a plugin-specific type family application on one side of an unlifted equality type ~#).<br>
<br>
In summary, I see no more ASSERT failures or Lint errors having now changed my plugin to prefer `EvCoercion (TransCo U (TransCo co U))` to `EvCast (EvCoercion co) U`. The actual diff excerpt is here: <a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fnfrisby%2Fcoxswain%2Fissues%2F3%23issuecomment-334972227&data=02%7C01%7Csimonpj%40microsoft.com%7C2b0edffd514b4da49abc08d50ec07ff9%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636431142301735979&sdata=Ng6o4ODbLXNJVlngL0qnBv1EQ5Sn9TJMPK79M99zGTg%3D&reserved=0" target="_blank">https://github.com/nfrisby/coxswain/issues/3#issuecomment-334972227</a><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 have not figured out exactly why that change matters, but it does seem a reasonable preference to require. In particular, Note [Coercion evidence terms] in TcEvidence.hs explicitly says that `EvCast (EvCoercion co1) co2` is a valid form
of evidence for ~#. So perhaps that Note deserves elaboration --- I'm guessing the missing part may be specific to Givens?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-Nick<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Thu, Sep 21, 2017 at 2:59 AM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Some thoughts</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:0cm;mso-list:l0 level1 lfo1">
<span style="font-size:12.0pt">Read Note [Coercion holes] in TyCoRep.</span><o:p></o:p></li></ul>
<p class="m637828321027685404m-3607345558727168865msolistparagraph"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:0cm;mso-list:l3 level1 lfo2">
<span style="font-size:12.0pt">As you’ll see, generally we don’t create value-bindings for (unboxed) coercions of type t1 ~# t2. (yes for boxed ones t1 ~ t2). Reasons in the Note. Exception: for superclasses of Givens we do create (co :: a ~# b) =
sc_sel1 d</span><o:p></o:p></li></ul>
<p class="m637828321027685404m-3607345558727168865msolistparagraph"><span style="font-size:12.0pt">where d is some dictionary with a superclass of type (a ~# b).</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-size:12.0pt">Side note: the use of “cobox” is wildly unhelpful. These Ids are specifically
<b>unboxed</b>! I’m going to change it to just “co”.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-size:12.0pt"> </span><o:p></o:p></p>
<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:0cm;mso-list:l2 level1 lfo3">
You appear to have bindings like[G] cobox_a67J = CO Sym cobox_a654. That is suspicious. Who is creating them? It may not actually be wrong but it’s suspicious. The time it’d be outright wrong is if you dropped the ev-binds on the floor.<o:p></o:p></li></ul>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
Ha! runTcSEqualites makes up an ev_binds_var, and solves the equalities – but it should be the case that no value bindings end up in the ev_binds_var. (reason: we are solving equalities in a type signature, so there is no place to put the evidence bindigns)
I suggest you add a DEBUG-only assertion to check this.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:0cm;mso-list:l1 level1 lfo4">
Do -ddump-tc -fprint-typechecker-elaboration; that should show you the evidence binds.<o:p></o:p></li></ul>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Can I ask you a favour? Separately from your branch, can you start a branch of small patches to GHC that include<o:p></o:p></p>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:0cm;mso-list:l4 level1 lfo5">
Extra assertions, such as that above<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:0cm;mso-list:l4 level1 lfo5">
<b><span style="font-family:"Courier New"">Notes</span></b> that explain things you wish you’d known earlier, with references to those Notes from the places you were studying when you that information would have been useful<o:p></o:p></li></ul>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Richard and I know too much! – your learning curve is very valuable and I don’t want to lose it.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Keeping this separate from your branch is useful : you can commit (via Phab) these updates right away, so they aren’t predicated on adding row types to GHC.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Simon<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs [mailto:<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@haskell.org</a>]
<b>On Behalf Of </b>Nicolas Frisby<br>
<b>Sent:</b> 19 September 2017 16:51<br>
<b>To:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Invariants about UnivCo?</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">[I summarize with some direct questions at the bottom of this email.]<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">I spent time last night trying to eliminate -dcore-lint errors from my record and variant library using the coxswain row types plugin. I made some progress, but I'm currently stuck, as
discussed on this github Issue.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fnfrisby%2Fcoxswain%2Fissues%2F3%23issuecomment-330577609&data=02%7C01%7Csimonpj%40microsoft.com%7Cde0675bbb584495a2f8008d4ff764c72%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636414330952932223&sdata=lPLcpIlb%2BhivQdCUoVOPUgYDHeEDaMX660NQS%2BQyyBw%3D&reserved=0" target="_blank">https://github.com/nfrisby/coxswain/issues/3#issuecomment-330577609</a><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">Here's the relevant bit:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:10.5pt;font-family:"Segoe UI",sans-serif;color:#24292E">The latest unresolved </span><code><span style="font-size:9.0pt;font-family:Consolas;color:#24292E">-dcore-lint</span></code><span style="font-size:10.5pt;font-family:"Segoe UI",sans-serif;color:#24292E"> error
is an out-of-scope </span><code><span style="font-size:9.0pt;font-family:Consolas;color:#24292E">cobox</span></code><span style="font-size:10.5pt;font-family:"Segoe UI",sans-serif;color:#24292E"> co var. I'm certainly not creating it <em><span style="font-family:"Segoe UI",sans-serif">directly</span></em> (there
are no </span><code><span style="font-size:9.0pt;font-family:Consolas;color:#24292E">U(plugin:coxswain,...</span></code><span style="font-size:10.5pt;font-family:"Segoe UI",sans-serif;color:#24292E"> in the Core Lint warning), but I have to wonder if my somewhat
loose use of </span><code><span style="font-size:9.0pt;font-family:Consolas;color:#24292E">UnivCo</span></code><span style="font-size:10.5pt;font-family:"Segoe UI",sans-serif;color:#24292E"> is violating some assumptions somewhere that's causing GHC to drop
the co var binding or overlook this occurrence of it on a renaming/subst pass. I checked </span><code><span style="font-size:9.0pt;font-family:Consolas;color:#24292E">UnivCo</span></code><span style="font-size:10.5pt;font-family:"Segoe UI",sans-serif;color:#24292E"> for
source comments looking for anything it should <em><span style="font-family:"Segoe UI",sans-serif">not</span></em> be used for, but I didn't find an obvious explanation along those lines.</span> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I haven't yet been able to effectively distill the test case.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I'm doing this all at -O0.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">With `-ddump-tc-trace`, I can see the offending cobox (cobox_a67M) is present in an "implication evbinds" listing after a "solveImplication end }" delimiter, but that's the last
obvious binding of it.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> [G] cobox_a67J = CO Sym cobox_a654,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> [G] cobox_a67M<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> = cobox_a67J `cast` U(plugin:coxswain,...)<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">cobox_a654 is introduced by a GADT pattern match. <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I'm also not seeing obvious occurrences of cobox_a67M, but I think the reason is that I'm seeing several (Sym cobox) with no uniques printed (even with `-dppr-debug`). Those are
probably the cobox in question, but I can't confirm.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Questions:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">1) Is there a robust way to ensure that covar's uniques are always printed? (Is the pprIface reuse with a free cobox part of the issue here?)<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">2) Is my plugin asking for this kind of trouble by using UnivCo to cast coboxes?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">3) If I spent the effort to create non-UnivCo coercions where possible, would that likely help? This is currently an "eventually" task, but I haven't seen an urgency for it yet.
I could bump its priority if it might help. E.G. I'm using UnivCo to cast entire givens when all I'm doing is reducing a type family application somewhere "deep" within the given's predtype. I could, with considerable effort, instead wrap a single, localized
UnivCo within a bunch of non-UnivCo "lifting" coercion constructors. Would that likely help?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">3) Is there a usual suspect for this kind of situation where a cobox binding is seemingly dropped (by the typechecker) even though there's an occurrence of it?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thank you for your time. -Nick<o:p></o:p></p>
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<p class="MsoNormal">_______________________________________________<o:p></o:p></p>
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<p class="MsoNormal"><br>
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