Broken Data.Data instances

["Philip K.F. Hölzenspies"]

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 <mailto:simonpj at 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-bounces at haskell.org] *On Behalf Of 
> *p.k.f.holzenspies at utwente.nl
> *Sent:* 24 July 2014 16:42
> *To:* ghc-devs at 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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