Broken Data.Data instances

Wed Aug 13 10:58:38 UTC 2014

Dear Alan,

I’ve had a look at the diffs on Phabricator. They’re looking good. I have a few comments / questions:

1) As you said, the renamer and typechecker are heavily interwoven, but when you *know* that you’re between renamer and typechecker (i.e. when things have ‘Name’s, but not ‘Id’s), isn’t it better to choose the PreTcType as argument? (Basically, look for any occurrence of “Name PostTcType” and replace with Pre.)

2) I saw your point about being able to distinguish PreTcType from () in SYB-traversals, but you have now defined PreTcType as a synonym for (). With an eye on the maximum line-width of 80 characters and these things being explicit everywhere as a type parameter (as opposed to a type family over the exposed id-parameter), how much added value is there still in having the names PreTcType and PostTcType? Would “()” and “Type” not be as clear? I ask, because when I started looking at GHC, I was overwhelmed with all the names for things in there, most of which then turn out to be different names for the same thing. The main reason to call the thing PostTcType in the first place was to give some kind of warning that there would be nothing there before TC.

3) The variable name “ptt” is a bit misleading to me. I would use “ty”.

4) In the cases of the types that have recently been parameterized in what they contain, is there a reason to have the ty-argument *after* the content-argument? E.g. why is it “LGRHS RdrName (LHsExpr RdrName PreTcType) PreTcType” instead of “LGRHS RdrName PreTcType (LHsExpr RdrName PreTcType)”? This may very well be a tiny stylistic thing, but it’s worth thinking about.

5) I much prefer deleting code over commenting it out. I understand the urge, but if you don’t remove these lines before your final commit, they will become noise in the long term. Versioning systems preserve the code for you. (Example: Convert.void)

Regards,
Philip

From: Alan & Kim Zimmerman [mailto:alan.zimm at gmail.com]
Sent: woensdag 13 augustus 2014 8:50
To: Holzenspies, P.K.F. (EWI)
Cc: Simon Peyton Jones; Edward Kmett; ghc-devs at haskell.org
Subject: Re: Broken Data.Data instances

And I dipped my toes into the phabricator water, and uploaded a diff to https://phabricator.haskell.org/D153
I left the lines long for now, so that it is clear that I simply added parameters to existing type signatures.

On Tue, Aug 12, 2014 at 10:51 PM, Alan & Kim Zimmerman <alan.zimm at gmail.com<mailto:alan.zimm at gmail.com>> wrote:
Status update
I have worked through a proof of concept update to the GHC AST whereby the type is provided as a parameter to each data type. This was basically a mechanical process of changing type signatures, and required very little actual code changes, being only to initialise the placeholder types.
The enabling types are

    type PostTcType = Type        -- Used for slots in the abstract syntax
                    -- where we want to keep slot for a type
                    -- to be added by the type checker...but
                    -- [before typechecking it's just bogus]
    type PreTcType = ()             -- used before typechecking

    class PlaceHolderType a where
      placeHolderType :: a

    instance PlaceHolderType PostTcType where

      placeHolderType  = panic "Evaluated the place holder for a PostTcType"
    instance PlaceHolderType PreTcType where
      placeHolderType = ()

These are used to replace all instances of PostTcType in the hsSyn types.

The change was applied against HEAD as of last friday, and can be found here

https://github.com/alanz/ghc/tree/wip/landmine-param
https://github.com/alanz/haddock/tree/wip/landmine-param
They pass 'sh validate' with GHC 7.6.3, and compile against GHC 7.8.3. I have not tried to validate that yet, have no reason to expect failure.

Can I please get some feedback as to whether this is a worthwhile change?

It is the first step to getting a generic traversal safe AST
Regards
  Alan

On Mon, Jul 28, 2014 at 5:45 PM, Alan & Kim Zimmerman <alan.zimm at gmail.com<mailto:alan.zimm at gmail.com>> wrote:
FYI I edited the paste at http://lpaste.net/108262 to show the problem

On Mon, Jul 28, 2014 at 5:41 PM, Alan & Kim Zimmerman <alan.zimm at gmail.com<mailto:alan.zimm at gmail.com>> wrote:
I already tried that, the syntax does not seem to allow it.

I suspect some higher form of sorcery will be required, as alluded to here http://stackoverflow.com/questions/14133121/can-i-constrain-a-type-family
Alan

On Mon, Jul 28, 2014 at 4:55 PM, <p.k.f.holzenspies at utwente.nl<mailto:p.k.f.holzenspies at utwente.nl>> wrote:
Dear Alan,

I would think you would want to constrain the result, i.e.

type family (Data (PostTcType a)) => PostTcType a where …

The Data-instance of ‘a’ doesn’t give you much if you have a ‘PostTcType a’.

Your point about SYB-recognition of WrongPhase is, of course, a good one ;)

Regards,
Philip

From: Alan & Kim Zimmerman [mailto:alan.zimm at gmail.com<mailto:alan.zimm at gmail.com>]
Sent: maandag 28 juli 2014 14:10
To: Holzenspies, P.K.F. (EWI)
Cc: Simon Peyton Jones; Edward Kmett; ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>

Subject: Re: Broken Data.Data instances

Philip
I think the main reason for the WrongPhase thing is to have something that explicitly has a Data and Typeable instance, to allow generic (SYB) traversal. If we can get by without this so much the better.
On a related note, is there any way to constrain the 'a' in

type family PostTcType a where
  PostTcType Id    = TcType
  PostTcType other = WrongPhaseTyp
to have an instance of Data?
I am experimenting with traversals over my earlier paste, and got stuck here (which is the reason the Show instances were commentet out in the original).
Alan

On Mon, Jul 28, 2014 at 12:30 PM, <p.k.f.holzenspies at utwente.nl<mailto:p.k.f.holzenspies at utwente.nl>> wrote:
Sorry about that… I’m having it out with my terminal server and the server seems to be winning. Here’s another go:

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.

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. I was wondering whether it would be possible to make stateful lenses for this. Of course, for specific cases, we could do this, but I wonder if it is also possible to have lenses remember the things they visited and not visit them twice. Any ideas on this, Edward?

Regards,
Philip

From: Alan & Kim Zimmerman [mailto:alan.zimm at gmail.com<mailto:alan.zimm at gmail.com>]
Sent: maandag 28 juli 2014 11:14
To: Simon Peyton Jones
Cc: Edward Kmett; Holzenspies, P.K.F. (EWI); ghc-devs

Subject: Re: Broken Data.Data instances

I have made a conceptual example of this here http://lpaste.net/108262
Alan

On Mon, Jul 28, 2014 at 9:50 AM, Alan & Kim Zimmerman <alan.zimm at gmail.com<mailto:alan.zimm at gmail.com>> wrote:
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.
Alan

On Mon, Jul 28, 2014 at 9:14 AM, Simon Peyton Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
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.

If the “hand grenades” are the PostTcTypes, etc, then I can explain why they are there.

There simply is no sensible type you can put before the type checker runs.  For example one of the constructors  in HsExpr is

  | HsMultiIf   PostTcType [LGRHS id (LHsExpr id)]
After type checking we know what type the thing has, but before we have no clue.

We could get around this by saying
            type PostTcType = Maybe TcType
but that would mean that every post-typechecking consumer would need a redundant pattern-match on a Just that would always succeed.

It’s nothing deeper than that.  Adding Maybes everywhere would be possible, just clunky.

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:

  | HsMultiIf   (PostTcType id) [LGRHS id (LHsExpr id)]
and define PostTcType as a closed type family thus

     type family PostTcType a where

          PostTcType Id = TcType

          PostTcType other = ()

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.

Simon

From: Edward Kmett [mailto:ekmett at gmail.com<mailto:ekmett at gmail.com>]
Sent: 27 July 2014 18:27
To: p.k.f.holzenspies at utwente.nl<mailto:p.k.f.holzenspies at utwente.nl>
Cc: alan.zimm at gmail.com<mailto:alan.zimm at gmail.com>; Simon Peyton Jones; ghc-devs

Subject: Re: Broken Data.Data instances

Philip, Alan,

If you need a hand, I'm happy to pitch in guidance.

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.

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.

Simon,

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.

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.

-Edward

On Sun, Jul 27, 2014 at 10:17 AM, <p.k.f.holzenspies at utwente.nl<mailto:p.k.f.holzenspies at utwente.nl>> wrote:
Alan,

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).

Regards,
Philip
________________________________
Van: Alan & Kim Zimmerman [alan.zimm at gmail.com<mailto:alan.zimm at gmail.com>]
Verzonden: vrijdag 25 juli 2014 13:44
Aan: Simon Peyton Jones
CC: Holzenspies, P.K.F. (EWI); ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>
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 gmail.com<mailto:alan.zimm at 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 <simonpj at microsoft.com<mailto:simonpj at 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.holzenspies at utwente.nl<mailto:p.k.f.holzenspies at utwente.nl>]
Sent: 24 July 2014 18:42
To: Simon Peyton Jones
Cc: ghc-devs at haskell.org<mailto:ghc-devs at 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
[cid:image001.jpg at 01CFB6F2.177F3FF0]
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<mailto:p.k.f.holzenspies at utwente.nl>
Sent: 24 July 2014 16:42
To: ghc-devs at haskell.org<mailto: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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