By the way, I would be happy to attempt this task, if the concept is viable.
On Thu, Jul 24, 2014 at 11:23 PM, Alan & Kim Zimmerman <alan.z...@gmail.com> wrote: > While we are talking about fixing traversals, how about getting rid of the > phase specific panic initialisers for placeHolderType, placeHolderKind and > friends? > > In order to safely traverse with SYB, the following needs to be inserted > into all the SYB schemes (see > > https://github.com/alanz/HaRe/blob/master/src/Language/Haskell/Refact/Utils/GhcUtils.hs > ) > > -- Check the Typeable items > checkItemStage1 :: (Typeable a) => SYB.Stage -> a -> Bool > checkItemStage1 stage x = (const False `SYB.extQ` postTcType `SYB.extQ` > fixity `SYB.extQ` nameSet) x > where nameSet = const (stage `elem` [SYB.Parser,SYB.TypeChecker]) :: > GHC.NameSet -> Bool > postTcType = const (stage < SYB.TypeChecker ) :: > GHC.PostTcType -> Bool > fixity = const (stage < SYB.Renamer ) :: > GHC.Fixity -> Bool > > And in addition HsCmdTop and ParStmtBlock are initialised with explicit > 'undefined values. > > Perhaps use an initialiser that can have its panic turned off when called > via the GHC API? > > Regards > Alan > > > > On Thu, Jul 24, 2014 at 11:06 PM, Simon Peyton Jones < > simo...@microsoft.com> wrote: > >> So... does anyone object to me changing these "broken" instances with >> the ones given by DeriveDataTypeable? >> >> 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. >> >> >> >> Simon >> >> >> >> *From:* "Philip K.F. Hölzenspies" [mailto:p.k.f.holzensp...@utwente.nl] >> *Sent:* 24 July 2014 18:42 >> *To:* Simon Peyton Jones >> *Cc:* ghc-devs@haskell.org >> *Subject:* Re: Broken Data.Data instances >> >> >> >> Dear Simon, et al, >> >> 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). >> >> So... does anyone object to me changing these "broken" instances with the >> ones given by DeriveDataTypeable? >> >> 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? >> >> Regards, >> Philip >> >> >> *Simon Peyton Jones* <simo...@microsoft.com> >> >> 24 Jul 2014 18:22 >> >> GHC’s data structures are often mutually recursive. e.g. >> >> · The TyCon for Maybe contains the DataCon for Just >> >> · The DataCon For just contains Just’s type >> >> · Just’s type contains the TyCon for Maybe >> >> >> >> So any attempt to recursively walk over all these structures, as you >> would a tree, will fail. >> >> >> >> 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. >> >> >> >> Maybe that’s it; I’m not certain since I did not write the Data instances >> for any of GHC’s types >> >> >> >> Simon >> >> >> >> *From:* ghc-devs [mailto:ghc-devs-boun...@haskell.org >> <ghc-devs-boun...@haskell.org>] *On Behalf Of * >> p.k.f.holzensp...@utwente.nl >> *Sent:* 24 July 2014 16:42 >> *To:* ghc-devs@haskell.org >> *Subject:* Broken Data.Data instances >> >> >> >> Dear GHC-ers, >> >> >> >> Is there a reason for explicitly broken Data.Data instances? Case in >> point: >> >> >> >> > instance Data Var where >> >> > -- don't traverse? >> >> > toConstr _ = abstractConstr "Var" >> >> > gunfold _ _ = error "gunfold" >> >> > dataTypeOf _ = mkNoRepType "Var" >> >> >> >> 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: >> >> >> >> > collect :: (Typeable b, Data a, MonadPlus m) => a -> m b >> >> > collect = everything mplus $ mkQ mzero return >> >> > >> >> > allTypes :: CoreExpr -> [Type] >> >> > allTypes = collect >> >> >> >> 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. >> >> >> >> Would a patch “fixing” these instances be acceptable? >> >> >> >> Regards, >> >> Philip >> >> >> >> >> >> >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs@haskell.org >> http://www.haskell.org/mailman/listinfo/ghc-devs >> >> >
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