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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I always read the () as “there’s nothing meaningful to stick in here, but I have to stick in something” so I don’t necessarily want the WrongPhase-thing.
There is very old commentary stating it would be lovely if someone could expose the PostTcType as a parameter of the AST-types, but that there are so many types and constructors, that it’s a boring chore to do. Actually, I was hoping haRe would come up to
speed to be able to do this. That being said, I think Simon’s idea to turn PostTcType into a type-family is a better way altogether; it also documents intent, i.e. () may not say so much, but PostTcType RdrName says quite a lot.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Simon commented that a lot of the internal structures aren’t trees, but cyclic graphs, e.g. the TyCon for Maybe references the DataCons for Just
and Nothing, which again refer to the TyCon for Maybe.</span><span lang="EN-US">
<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Alan & Kim Zimmerman [mailto:alan.zimm@gmail.com]
<br>
<b>Sent:</b> maandag 28 juli 2014 11:14<br>
<b>To:</b> Simon Peyton Jones<br>
<b>Cc:</b> Edward Kmett; Holzenspies, P.K.F. (EWI); 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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<p class="MsoNormal" style="margin-bottom:12.0pt">I have made a conceptual example of this here
<a href="http://lpaste.net/108262">http://lpaste.net/108262</a><o:p></o:p></p>
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<p class="MsoNormal">Alan<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><o:p> </o:p></p>
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<p class="MsoNormal">On Mon, Jul 28, 2014 at 9:50 AM, Alan & Kim Zimmerman <<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">What about creating a specific type with a single constructor for the "not relevant to this phase" type to be used instead of () above? That would also clearly document what was going on.<o:p></o:p></p>
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<p class="MsoNormal"><span class="hoenzb"><span style="color:#888888">Alan</span></span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><o:p> </o:p></p>
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<p class="MsoNormal">On Mon, Jul 28, 2014 at 9:14 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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<span lang="EN-GB">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></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><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-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">If the “hand grenades” are the PostTcTypes, etc, then I can explain why they are there.
</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">There simply is no sensible type you can put before the type checker runs. For example one of the constructors
in HsExpr is</span><span lang="EN-GB"><o:p></o:p></span></p>
<p><span lang="EN-GB"> | HsMultiIf PostTcType [LGRHS id (LHsExpr id)]<o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">After type checking we know what type the thing has, but before we have no clue.</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">We could get around this by saying</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> type PostTcType = Maybe TcType</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">but that would mean that every post-typechecking consumer would need a redundant pattern-match on a Just
that would always succeed.</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">It’s nothing deeper than that. Adding Maybes everywhere would be possible, just clunky.</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">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:</span><span lang="EN-GB"><o:p></o:p></span></p>
<p><span lang="EN-GB"> | HsMultiIf (PostTcType id) [LGRHS id (LHsExpr id)]<o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">and define PostTcType as a closed type family thus</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p><span lang="EN-GB"> type family PostTcType a where<o:p></o:p></span></p>
<p><span lang="EN-GB"> PostTcType Id = TcType<o:p></o:p></span></p>
<p><span lang="EN-GB"> PostTcType other = ()<o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">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.</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D">Simon</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="font-family:"Calibri","sans-serif";color:#1F497D"> </span><span lang="EN-GB"><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="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:<a href="mailto:ekmett@gmail.com" target="_blank">ekmett@gmail.com</a>]
<br>
<b>Sent:</b> 27 July 2014 18:27<br>
<b>To:</b> <a href="mailto:p.k.f.holzenspies@utwente.nl" target="_blank">p.k.f.holzenspies@utwente.nl</a><br>
<b>Cc:</b> <a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>; Simon Peyton Jones; ghc-devs</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-GB"><br>
<b>Subject:</b> Re: Broken Data.Data instances<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-GB"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB">Philip, Alan, <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB">If you need a hand, I'm happy to pitch in guidance. <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB">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></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB">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></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB">Simon,<o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB">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></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB">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></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB">-Edward<o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span lang="EN-GB"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB">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></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span lang="EN-GB" 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</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><b><span lang="EN-GB" style="font-family:"Tahoma","sans-serif";color:black">Van:</span></b><span lang="EN-GB" 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 lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB" style="color:black">By the way, I would be happy to attempt this task, if the concept is viable.</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span lang="EN-GB" style="color:black"> </span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB" 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:</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span lang="EN-GB" 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?</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span lang="EN-GB" 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</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span lang="EN-GB" style="color:black">And in addition HsCmdTop and ParStmtBlock are initialised with explicit 'undefined values.</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span lang="EN-GB" style="color:black">Perhaps use an initialiser that can have its panic turned off when called via the GHC API?</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB" style="color:black">Regards</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB" style="color:black"> Alan</span><span lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span lang="EN-GB" 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:</span><span lang="EN-GB"><o:p></o:p></span></p>
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<span lang="EN-GB" style="color:black">So... does anyone object to me changing these "broken" instances with the ones given by DeriveDataTypeable?</span><span lang="EN-GB"><o:p></o:p></span></p>
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<span lang="EN-GB" 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.</span><span lang="EN-GB"><o:p></o:p></span></p>
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<span lang="EN-GB" style="color:black"> </span><span lang="EN-GB"><o:p></o:p></span></p>
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<span lang="EN-GB" style="color:black">Simon</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="color:#1F497D"> </span><span lang="EN-GB"><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 lang="EN-GB"><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-GB" style="color:black"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span lang="EN-GB" 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>
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Regards,<br>
Philip</span><span lang="EN-GB"><o:p></o:p></span></p>
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<span lang="EN-GB" style="color:black"><a href="mailto:simonpj@microsoft.com" target="_blank"><b>Simon Peyton Jones</b></a></span><span lang="EN-GB"><o:p></o:p></span></p>
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<span lang="EN-GB" style="color:#9FA2A5">24 Jul 2014 18:22</span><span lang="EN-GB"><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-GB" style="color:#1F497D">GHC’s data structures are often mutually recursive. e.g.</span><span lang="EN-GB"><o:p></o:p></span></p>
<p><span lang="EN-GB" style="font-family:Symbol;color:#888888">·</span><span lang="EN-GB" style="font-size:7.0pt;color:#888888">
</span><span lang="EN-GB" style="color:#1F497D">The TyCon for Maybe contains the DataCon for Just</span><span lang="EN-GB"><o:p></o:p></span></p>
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</span><span lang="EN-GB" style="color:#1F497D">The DataCon For just contains Just’s type</span><span lang="EN-GB"><o:p></o:p></span></p>
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</span><span lang="EN-GB" style="color:#1F497D">Just’s type contains the TyCon for Maybe</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="color:#1F497D">So any attempt to recursively walk over all these structures, as you would a tree, will fail.
</span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" 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 lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" 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 lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="color:#1F497D"> </span><span lang="EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB" style="color:#1F497D">Simon</span><span lang="EN-GB"><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 lang="EN-GB"><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-GB" style="color:black"> </span><span lang="EN-GB"><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">Dear GHC-ers,</span><span lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><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 lang="EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span lang="EN-GB" style="color:black">_______________________________________________<br>
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_______________________________________________<br>
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