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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">Like I say I am DEEPLY suspicious of
</span>ForallXImplicitBndrs. I can’t make head or tail of it. Is see in you patch you define
<o:p></o:p></li></ul>
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<p class="MsoListParagraph" style="text-indent:36.0pt">type ForallXPat (c :: * -> Constraint) (x :: *) =…<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> What is this? Why do we need it? What goes wrong if we remove it altogether?<o:p></o:p></p>
<p class="MsoListParagraph"><o:p> </o:p></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">Likewise `forall thing. Data thing` makes no sense to me as a constraint, even with quantified context. Shayan and I discussed this at ICFP and agreed it made no sense. The hoped-for
quantified-context extension is NOT a solution.<o:p></o:p></li></ul>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Overall I’ve lost track of these enormous constraint tuples that seem to be associated with Data instances. Can you give a small artificial (ie not full GHC) example of why they are necessary? Perhaps it’s this<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">data T p = T1 (XT1 p) Int<br>
| T1 (XT2 p) (IdP p)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I suppose that a Data instance would need to have<o:p></o:p></p>
<p class="MsoNormal"> instance (Data (XT1 p), Data (XT2p), Data (IdP p)) => Data IT p) where…<o:p></o:p></p>
<p class="MsoNormal">Is that right? In which case why do you have all this PostRn stuff in the DataId type? (And why is it called DataId?)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-top:6.0pt;margin-bottom:6.0pt;margin-left:0cm;mso-list:l0 level1 lfo1">
“The first approach leads to a spread of the constraint throughout the AST, which gets very messy.” I don’t understand what the first approach is, or why it gets messy. Could you be more concrete?<o:p></o:p></li><li class="MsoListParagraph" style="margin-top:6.0pt;margin-bottom:6.0pt;margin-left:0cm;mso-list:l0 level1 lfo1">
“Perhaps the simplest way forward is to get rid of the `thing` parameter completely, and introduce the three or so ImplicitBinders variants that are used.” I don’t’ think it could possibly make anything simpler to have three separate data types. Can you
illustrate concretely? <o:p></o:p></li></ul>
<p class="MsoNormal">It’d be great to explore these issues with small, concrete examples, rather than the full glory of GHC, both for our own benefit and the benefit of those who will review the patch and (in future) understand the code.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Sorry to be slow<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></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"> Alan & Kim Zimmerman [mailto:alan.zimm@gmail.com]
<br>
<b>Sent:</b> 23 October 2017 22:36<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Re: forall in constraint<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:12.0pt;margin-left:0cm">
In Shayan's implementation he has [1]<br>
<br>
data ImplicitBndrs x thing<br>
= IB<br>
(XIB x thing)<br>
thing<br>
<br>
| NewImplicitBndrs<br>
(XNewImplicitBndrs x thing)<br>
<br>
type family XIB x thing<br>
type family XNewImplicitBndrs x thing<br>
<br>
type ForallXImplicitBndrs c x thing =<br>
( c (XIB x thing)<br>
, c (XNewImplicitBndrs x thing)<br>
)<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:12.0pt;margin-left:0cm">
This gets used, in the same file as <br>
<br>
type LSigType x = ImplicitBndrs x (LType x)<o:p></o:p></p>
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where `thing` is resolved to a specific type.<o:p></o:p></p>
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<o:p> </o:p></p>
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Because it is all in the same file, there is no problem making a <o:p></o:p></p>
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constraint on anything using LSigType, that mentions LHsType.<o:p></o:p></p>
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<o:p> </o:p></p>
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But in the approach I am taking[2], the type families are defined in<br>
HsExtension, which is compiled early in the cycle, and imported by<br>
HsTypes, HsBinds, HsDecl etc.<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">
In order to derive a Data instance for anything using `LSigType x`, we<o:p></o:p></p>
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need to be able to specify that a Data instance exists for `LHsType x`.<o:p></o:p></p>
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<o:p> </o:p></p>
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So we can either do that directly in HsBinds, or try to add it to the existing<o:p></o:p></p>
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DataId constraint in HsExtension.<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">
The first approach leads to a spread of the constraint throughout the AST,<o:p></o:p></p>
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which gets very messy.<o:p></o:p></p>
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The second approach requires being able to specify a<o:p></o:p></p>
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`forall thing. Data thing` constraint in HsExtension.<o:p></o:p></p>
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<o:p> </o:p></p>
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I tried an intermediate approach, introducing a constraint in HsDecls[3] to capture this,<o:p></o:p></p>
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but it eventually runs into needing it in the HsExpr.hs-boot file, which means I need<o:p></o:p></p>
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LHsType in that file.<o:p></o:p></p>
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<o:p> </o:p></p>
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Perhaps the simplest way forward is to get rid of the `thing` parameter completely,<br>
and introduce the three or so ImplicitBinders variants that are used.<o:p></o:p></p>
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I hope this does not just confuse things even more.<o:p></o:p></p>
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<o:p> </o:p></p>
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Alan<o:p></o:p></p>
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<br>
[1] <a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc%2Fghc%2Fblob%2Fwip%2FGrowableAST%2Fcompiler%2FhsSyn%2FAST.hs%23L475&data=02%7C01%7Csimonpj%40microsoft.com%7Cd950b40099a34322c49a08d51a5e1892%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636443913792981424&sdata=ZGS3PmcI2nIw%2FRQLDs%2FKWk51LPR4Gmtv9rQHzY1Jre8%3D&reserved=0">
https://github.com/ghc/ghc/blob/wip/GrowableAST/compiler/hsSyn/AST.hs#L475</a><o:p></o:p></p>
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[2] <a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc%2Fghc%2Ftree%2Fwip%2Fttg%2F2017-10-21&data=02%7C01%7Csimonpj%40microsoft.com%7Cd950b40099a34322c49a08d51a5e1892%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636443913792981424&sdata=WyNfWykERAEKvhfXdSacyvEukkLtEuwvHNgwrbPnkEw%3D&reserved=0">
https://github.com/ghc/ghc/tree/wip/ttg/2017-10-21</a><o:p></o:p></p>
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[3] <a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc%2Fghc%2Fcommit%2F22812296818fe955752fa4762cf72250abd09bf9%23diff-7cfa6eef12e44d312aca9ef4af0081b3R1439&data=02%7C01%7Csimonpj%40microsoft.com%7Cd950b40099a34322c49a08d51a5e1892%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636443913792981424&sdata=S7kXIL7P5drGmbjcS%2Bd2kNhhHCoCTQStV%2FWqu8CK7i8%3D&reserved=0">
https://github.com/ghc/ghc/commit/22812296818fe955752fa4762cf72250abd09bf9#diff-7cfa6eef12e44d312aca9ef4af0081b3R1439</a><o:p></o:p></p>
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On 23 October 2017 at 23:04, 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">I’m lost. Could you give me a bit more context?<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">I’m deeply suspicious about that ForallXImplicitBndrs thing with strange higher kinded parameters. Smells all wrong to me.<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>
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