Broken Data.Data instances

p.k.f.holzenspies at p.k.f.holzenspies at
Sun Jul 27 14:17:27 UTC 2014


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).


Van: Alan & Kim Zimmerman [alan.zimm at]
Verzonden: vrijdag 25 juli 2014 13:44
Aan: Simon Peyton Jones
CC: Holzenspies, P.K.F. (EWI); ghc-devs at
Onderwerp: Re: Broken Data.Data instances

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.zimm at<mailto:alan.zimm at>> 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

-- 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?


On Thu, Jul 24, 2014 at 11:06 PM, Simon Peyton Jones <simonpj at<mailto:simonpj at>> 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.


From: "Philip K.F. Hölzenspies" [mailto:p.k.f.holzenspies at<mailto:p.k.f.holzenspies at>]
Sent: 24 July 2014 18:42
To: Simon Peyton Jones
Cc: ghc-devs at<mailto:ghc-devs at>
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?


[cid:image001.jpg at 01CFA78B.7D356DE0]
Simon Peyton Jones<mailto:simonpj at>
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


From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of p.k.f.holzenspies at<mailto:p.k.f.holzenspies at>
Sent: 24 July 2014 16:42
To: ghc-devs at<mailto:ghc-devs at>
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?


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