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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Bartosz<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Well, I’m glad I wrote that Note. I think you are right on: it looks to me like an outright bug, correctly identified by the Note, that just so happens not to occur
in actual code.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">I’ve elaborated in this new ticket:
<a href="https://ghc.haskell.org/trac/ghc/ticket/11371">https://ghc.haskell.org/trac/ghc/ticket/11371</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Would you like to work on this?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Bartosz Nitka [mailto:niteria@gmail.com]
<br>
<b>Sent:</b> 06 January 2016 14:57<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> Edward Z. Yang <ezyang@mit.edu>; ghc-devs Devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: uniqAway and collisions<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Hello,<o:p></o:p></p>
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<o:p> </o:p></p>
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Thank you for the paper, it helped with my understanding of how it's supposed to work.<o:p></o:p></p>
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Simon, could my issue be related to your comment here: [1]?<o:p></o:p></p>
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<o:p> </o:p></p>
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-- Note [Generating the in-scope set for a substitution]<o:p></o:p></p>
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-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<o:p></o:p></p>
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-- If we want to substitute [a -> ty1, b -> ty2] I used to <o:p></o:p></p>
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-- think it was enough to generate an in-scope set that includes<o:p></o:p></p>
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-- fv(ty1,ty2). But that's not enough; we really should also take the<o:p></o:p></p>
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-- free vars of the type we are substituting into! Example:<o:p></o:p></p>
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-- (forall b. (a,b,x)) [a -> List b]<o:p></o:p></p>
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-- Then if we use the in-scope set {b}, there is a danger we will rename<o:p></o:p></p>
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-- the forall'd variable to 'x' by mistake, getting this:<o:p></o:p></p>
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-- (forall x. (List b, x, x)<o:p></o:p></p>
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-- Urk! This means looking at all the calls to mkOpenTvSubst....<o:p></o:p></p>
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<o:p> </o:p></p>
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Currently the InScope set only contains the free variables of the arguments when linting type application [2][3][4] and doesn't contain the free variables of the body that it's substituting in.<o:p></o:p></p>
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<o:p> </o:p></p>
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The definition of substTyWith is:<o:p></o:p></p>
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<o:p> </o:p></p>
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substTyWith :: [TyVar] -> [Type] -> Type -> Type<o:p></o:p></p>
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substTyWith tvs tys = ASSERT( length tvs == length tys )<o:p></o:p></p>
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substTy (zipOpenTCvSubst tvs tys)<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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When I changed it to include the free variables of the body my core lint error went away:<o:p></o:p></p>
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<o:p> </o:p></p>
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substTyWith :: [TyVar] -> [Type] -> Type -> Type<o:p></o:p></p>
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substTyWith tvs tys ty = ASSERT( length tvs == length tys )<o:p></o:p></p>
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substTy (extendTCvInScopeList (zipOpenTCvSubst tvs tys) (tyCoVarsOfTypeList ty)) ty<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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It seems unlikely to me that this is the issue, since this code is very old, but I don't have a better explanation for this and a second pair of eyes would help.<o:p></o:p></p>
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<o:p> </o:p></p>
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Thank you,<o:p></o:p></p>
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Bartosz<o:p></o:p></p>
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<o:p> </o:p></p>
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[1] <a href="https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/TyCoRep.hs;cac0795af33d622e4c6ebae6ae1f206969287088$1591-1601">
https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/TyCoRep.hs;cac0795af33d622e4c6ebae6ae1f206969287088$1591-1601</a><o:p></o:p></p>
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[2] <a href="https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/coreSyn/CoreLint.hs;cac0795af33d622e4c6ebae6ae1f206969287088$788">https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/coreSyn/CoreLint.hs;cac0795af33d622e4c6ebae6ae1f206969287088$788</a><o:p></o:p></p>
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[3] <a href="https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/TyCoRep.hs;cac0795af33d622e4c6ebae6ae1f206969287088$1756">https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/TyCoRep.hs;cac0795af33d622e4c6ebae6ae1f206969287088$1756</a><o:p></o:p></p>
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[4] <a href="https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/TyCoRep.hs;cac0795af33d622e4c6ebae6ae1f206969287088$1623">https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/TyCoRep.hs;cac0795af33d622e4c6ebae6ae1f206969287088$1623</a><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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2016-01-06 8:42 GMT+00:00 Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>>:<o:p></o:p></p>
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| I doubt there's a bug in uniqAway; it's more likely the in scope set<br>
| is not correct.<br>
<br>
I think Edward is probably right here.<o:p></o:p></p>
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