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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">
I've had to mangle a bunch of hand-written Data instances and push out patches to a dozen packages that used to be built this way before I convinced the authors to switch to safer versions of Data. Using virtual smart constructors like we do now in containers
and Text where needed can be used to preserve internal invariants, etc.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">If the “hand grenades” are the PostTcTypes, etc, then I can explain why they are there.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">There simply is no sensible type you can put before the type checker runs. For example one of the constructors in HsExpr is<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> | HsMultiIf PostTcType [LGRHS id (LHsExpr id)]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">After type checking we know what type the thing has, but before we have no clue.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">We could get around this by saying<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> type PostTcType = Maybe TcType<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">but that would mean that every post-typechecking consumer would need a redundant pattern-match on a Just that would always succeed.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">It’s nothing deeper than that. Adding Maybes everywhere would be possible, just clunky.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">However we now have type functions, and HsExpr is parameterised by an ‘id’ parameter, which changes from RdrName (after parsing) to Name (after renaming)
to Id (after typechecking). So we could do this:<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> | HsMultiIf (PostTcType id) [LGRHS id (LHsExpr id)]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">and define PostTcType as a closed type family thus<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> type family PostTcType a where<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> PostTcType Id = TcType<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> PostTcType other = ()<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">That would be better than filling it with bottoms. But it might not help with generic programming, because there’d be a component whose type wasn’t
fixed. I have no idea how generics and type functions interact.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;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""> Edward Kmett [mailto:ekmett@gmail.com]
<br>
<b>Sent:</b> 27 July 2014 18:27<br>
<b>To:</b> p.k.f.holzenspies@utwente.nl<br>
<b>Cc:</b> alan.zimm@gmail.com; Simon Peyton Jones; ghc-devs<br>
<b>Subject:</b> Re: Broken Data.Data instances<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Philip, Alan, <o:p></o:p></p>
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<o:p> </o:p></p>
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If you need a hand, I'm happy to pitch in guidance. <o:p></o:p></p>
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<o:p> </o:p></p>
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I've had to mangle a bunch of hand-written Data instances and push out patches to a dozen packages that used to be built this way before I convinced the authors to switch to safer versions of Data. Using virtual smart constructors like we do now in containers
and Text where needed can be used to preserve internal invariants, etc.<o:p></o:p></p>
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<o:p> </o:p></p>
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This works far better for users of the API than just randomly throwing them a live hand grenade. As I recall, these little grenades in generic programming over the GHC API have been a constant source of pain for libraries like haddock.<o:p></o:p></p>
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<o:p> </o:p></p>
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Simon,<o:p></o:p></p>
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<o:p> </o:p></p>
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It seems to me that regarding circular data structures, nothing prevents you from walking a circular data structure with Data.Data. You can generate a new one productively that looks just like the old with the contents swapped out, it is indistinguishable to
an observer if the fixed point is lost, and a clever observer can use observable sharing to get it back, supposing that they are allowed to try.<o:p></o:p></p>
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<o:p> </o:p></p>
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Alternately, we could use the 'virtual constructor' trick there to break the cycle and reintroduce it, but I'm less enthusiastic about that idea, even if it is simpler in many ways.<o:p></o:p></p>
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<o:p> </o:p></p>
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-Edward<o:p></o:p></p>
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<o:p> </o:p></p>
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On Sun, Jul 27, 2014 at 10:17 AM, <<a href="mailto:p.k.f.holzenspies@utwente.nl" target="_blank">p.k.f.holzenspies@utwente.nl</a>> wrote:<o:p></o:p></p>
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<span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:black">Alan,<br>
<br>
In that case, let's have a short feedback-loop between the two of us. It seems many of these files (Name.lhs, for example) are really stable through the repo-history. It would be nice to have one bigger refactoring all in one go (some of the code could use
a polish, a lot of code seems removable).<br>
<br>
Regards,<br>
Philip<o:p></o:p></span></p>
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<b><span style="font-family:"Tahoma","sans-serif";color:black">Van:</span></b><span style="font-family:"Tahoma","sans-serif";color:black"> Alan & Kim Zimmerman [<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>]<br>
<b>Verzonden:</b> vrijdag 25 juli 2014 13:44<br>
<b>Aan:</b> Simon Peyton Jones<br>
<b>CC:</b> Holzenspies, P.K.F. (EWI); <a href="mailto:ghc-devs@haskell.org" target="_blank">
ghc-devs@haskell.org</a><br>
<b>Onderwerp:</b> Re: Broken Data.Data instances</span><span style="color:black"><o:p></o:p></span></p>
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<span style="color:black">By the way, I would be happy to attempt this task, if the concept is viable.<o:p></o:p></span></p>
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<span style="color:black"><o:p> </o:p></span></p>
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<span style="color:black">On Thu, Jul 24, 2014 at 11:23 PM, Alan & Kim Zimmerman <<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>> wrote:<o:p></o:p></span></p>
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<span style="color:black">While we are talking about fixing traversals, how about getting rid of the phase specific panic initialisers for placeHolderType, placeHolderKind and friends?<o:p></o:p></span></p>
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<span style="color:black">In order to safely traverse with SYB, the following needs to be inserted into all the SYB schemes (see
<br>
<a href="https://github.com/alanz/HaRe/blob/master/src/Language/Haskell/Refact/Utils/GhcUtils.hs" target="_blank">https://github.com/alanz/HaRe/blob/master/src/Language/Haskell/Refact/Utils/GhcUtils.hs</a>)<br>
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-- Check the Typeable items<br>
checkItemStage1 :: (Typeable a) => SYB.Stage -> a -> Bool<br>
checkItemStage1 stage x = (const False `SYB.extQ` postTcType `SYB.extQ` fixity `SYB.extQ` nameSet) x<br>
where nameSet = const (stage `elem` [SYB.Parser,SYB.TypeChecker]) :: GHC.NameSet -> Bool<br>
postTcType = const (stage < SYB.TypeChecker ) :: GHC.PostTcType -> Bool<br>
fixity = const (stage < SYB.Renamer ) :: GHC.Fixity -> Bool<o:p></o:p></span></p>
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<span style="color:black">And in addition HsCmdTop and ParStmtBlock are initialised with explicit 'undefined values.<o:p></o:p></span></p>
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<span style="color:black">Perhaps use an initialiser that can have its panic turned off when called via the GHC API?<o:p></o:p></span></p>
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<span style="color:black">Regards<o:p></o:p></span></p>
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<span style="color:black"> Alan<o:p></o:p></span></p>
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<span style="color:black"><o:p> </o:p></span></p>
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<span style="color:black"><o:p> </o:p></span></p>
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<span style="color:black">On Thu, Jul 24, 2014 at 11:06 PM, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<o:p></o:p></span></p>
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<span style="color:black">So... does anyone object to me changing these "broken" instances with the ones given by DeriveDataTypeable?<o:p></o:p></span></p>
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<span style="color:black">That’s fine with me provided (a) the default behaviour is not immediate divergence (which it might well be), and (b) the pitfalls are documented.<o:p></o:p></span></p>
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<span style="color:black"> <o:p></o:p></span></p>
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<span style="color:black">Simon<o:p></o:p></span></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"> "Philip K.F. Hölzenspies" [mailto:<a href="mailto:p.k.f.holzenspies@utwente.nl" target="_blank">p.k.f.holzenspies@utwente.nl</a>]
<br>
<b>Sent:</b> 24 July 2014 18:42<br>
<b>To:</b> Simon Peyton Jones<br>
<b>Cc:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Re: Broken Data.Data instances</span><span style="color:black"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span style="color:black">Dear Simon, et al,<br>
<br>
These are very good points to make for people writing such traversals and queries. I would be more than happy to write a page on the pitfalls etc. on the wiki, but in my experience so far, exploring the innards of GHC is tremendously helped by trying small
things out and showing (bits of) the intermediate structures. For me, personally, this has always been hindered by the absence of good instances of Data and/or Show (not having to bring DynFlags and not just visualising with the pretty printer are very helpful).<br>
<br>
So... does anyone object to me changing these "broken" instances with the ones given by DeriveDataTypeable?<br>
<br>
Also, many of these internal data structures could be provided with useful lenses to improve such traversals further. Anyone ever go at that? Would be people be interested?<br>
<br>
Regards,<br>
Philip<br>
<br>
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<span style="color:black"><a href="mailto:simonpj@microsoft.com" target="_blank"><b>Simon Peyton Jones</b></a><o:p></o:p></span></p>
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<span style="color:#9FA2A5">24 Jul 2014 18:22</span><span style="color:black"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">GHC’s data structures are often mutually recursive. e.g.</span><span style="color:black"><o:p></o:p></span></p>
<p><span style="font-family:Symbol;color:#888888">·</span><span style="font-size:7.0pt;color:#888888">
</span><span style="color:#1F497D">The TyCon for Maybe contains the DataCon for Just</span><span style="color:black"><o:p></o:p></span></p>
<p><span style="font-family:Symbol;color:#888888">·</span><span style="font-size:7.0pt;color:#888888">
</span><span style="color:#1F497D">The DataCon For just contains Just’s type</span><span style="color:black"><o:p></o:p></span></p>
<p><span style="font-family:Symbol;color:#888888">·</span><span style="font-size:7.0pt;color:#888888">
</span><span style="color:#1F497D">Just’s type contains the TyCon for Maybe</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">So any attempt to recursively walk over all these structures, as you would a tree, will fail.
</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Also there’s a lot of sharing. For example, every occurrence of ‘map’ is a Var, and inside that Var is map’s type, its strictness, its rewrite RULE,
etc etc. In walking over a term you may not want to walk over all that stuff at every occurrence of map.</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Maybe that’s it; I’m not certain since I did not write the Data instances for any of GHC’s types</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Simon</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><span style="color:black"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span lang="EN-US" style="color:black">From:</span></b><span lang="EN-US" style="color:black"> ghc-devs [<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">mailto:ghc-devs-bounces@haskell.org</a>]
<b>On Behalf Of </b><a href="mailto:p.k.f.holzenspies@utwente.nl" target="_blank">p.k.f.holzenspies@utwente.nl</a><br>
<b>Sent:</b> 24 July 2014 16:42<br>
<b>To:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Broken Data.Data instances</span><span style="color:black"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">Dear GHC-ers,</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">Is there a reason for explicitly broken Data.Data instances? Case in point:</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> instance Data Var where</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> -- don't traverse?</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> toConstr _ = abstractConstr "Var"</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> gunfold _ _ = error "gunfold"</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> dataTypeOf _ = mkNoRepType "Var"</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">I understand (vaguely) arguments about abstract data types, but this also excludes convenient queries that can, e.g. extract all types from
a CoreExpr. I had hoped to do stuff like this:</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> collect :: (Typeable b, Data a, MonadPlus m) => a -> m b</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> collect = everything mplus $ mkQ mzero return</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> allTypes :: CoreExpr -> [Type]</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">> allTypes = collect</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">Especially when still exploring (parts of) the GHC API, being able to extract things in this fashion is very helpful. SYB’s “everything” being
broken by these instances, not so much.</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">Would a patch “fixing” these instances be acceptable?</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">Regards,</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black">Philip</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="color:black"> </span><span style="color:black"><o:p></o:p></span></p>
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