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] 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
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