Re: GHC 7.8 release status

Hi all, So I managed to remove 3 out of 4 of the -boot files. The one that remains, ironically, is the DsMonad.hs-boot. DsMonad has a (transitive) dependency on Hooks in at least two ways: once through Finder, which imports Packages, which imports Hooks; but that's easily solved, because Finder can import PackageState instead. However, it is less obvious to me how to resolve the following import cycle - DsMonad imports tcIfaceGlobal from TcIface - TcIface imports (loadWiredInHomeIface, loadInterface, loadDecls, findAndReadIface) from LoadIface - LoadIFace imports Hooks (There might be still others, this is the most direct one at the moment.) (Just to be clear, Hooks imports DsMonad because it needs the DsM type for the dsForeignsHook.) I'm sure this cycle can be broken somehow, but I'm not familiar enough with this part of the compiler to see if there is a natural point to do it. As things stand, we have a DsMonad.hs-boot which just exports the DsGblEnv, DsLclEnv, and DsM types. I don't know if this is something we should be worrying about or not? Just to summarize: the hooks patch as things stand now introduces the Hooks enumeration, rather than a separate type per hook so that we have a central and type checked list of all hooks; in order to do that, it moves some things around (some times moves to HscTypes), introduces a new module called PipelineMonad as per SPJ's suggestion, and introduces a single additional boot file for the DsMonad module. Edsko On Tue, Sep 10, 2013 at 12:40 PM, Simon Peyton-Jones wrote:

I do like the single record.

I would really really like a strong clear Note [blah] on the hooks::Dynamic field of DynFlags. It’s *so* non-obvious why it’s dynamic, and the reason is a really bad one, namely the windows DLL split nonsense. (Not our fault but still needs very clear signposting.)

I don’t understand why we need 4 new hs-boot files. Eg why DsMonad.hs-boot? It should be safely below Hooks.

Linker.hs-boot is solely because of LibrarySpec. It would be possible to push that into HscTypes. (Again with a comment to explain why.)

DriverPipeline is aleady 2,100 lines long, and could reasonably be split with CompPipeline in the PipelineMonad module, say.

In other words, a bit of refactoring might eliminate the loops *and* sometimes arguably improve the code.

I don’t feel terribly strongly about all this. It does feel a bit ad hoc… in a variety of places (eg deep in Linker.hs) there are calls to hooks, and it’s not clear to me why exactly those are the right places. But I suppose they are simply driven by what has been needed.

Anyway if you two are happy (no one else seems to mind either way) then go ahead.

Simon

From: Luite Stegeman [mailto:stegeman@gmail.com] Sent: 10 September 2013 08:37 To: Edsko de Vries Cc: Simon Peyton-Jones; ghc-devs; Edsko de Vries

Subject: Re: GHC 7.8 release status

Edsko has done some work of rearranging imports in DynFlags to make the DLL split work, and I've implemented the hooks on top of this, in a record, as discussed:

- https://github.com/ghcjs/ghcjs-build/blob/master/refs/patches/ghc-hooks-reco... (not final yet, but should be usable for testing)

- demo program: https://gist.github.com/luite/6506064

Some disadvantages:

- as long as the DLL split exists, more restructuring will be required if a new hook is added to something in a module on which DynFlags depends

- 4 new hs-boot files required, new hooks will often require additional hs-boot files (when module A has a hook (so A imports Hooks, this can't be a source import), the hook will often have some types defined by A, so Hooks will have to import A)

Advantages (over type families / Dynamic hooks):

- Hooks neatly defined together in a single record

I'm not so sure myself, but if everyone agrees that this is better than the older hooks I'll convert GHCJS to the new implementation later today and finalize the patch (comments are a bit out of date, and I'm not 100% sure yet that GHCJS doesn't need another hook for TH support in certain setups) and update the wiki.

luite

On Mon, Sep 9, 2013 at 4:55 PM, Edsko de Vries wrote:

Simon,

I talked to Luite this morning and I think we can come up with a design that includes the enumeration we prefer, with a single use of Dynamic in DynFlags -- it involves splitting off a PackageState module from Packages so that DynFlags doesn't depend on the entirely of Packages anymore (which would then, transitively, mean that it depends on Hooks and hence on a large part of ghc), but I think that should be doable. I'm working on that now.

Edsko

On Mon, Sep 9, 2013 at 3:51 PM, Simon Peyton-Jones wrote:

...

Edsko

I’m very short of time right now. I think you understand the issues here. Can you do a round or two with Luite and emerge with a design that you both think is best?

As I said earlier I’m uncomfortable with doing design work so late in the cycle, and I feel that I don’t have time to study the various alternatives properly in the next four days. But since you tell me it’s crucial for GHCJS, I suppose that a possible compromise is this. We release a GHC with some design for hooks, but specifically say that the hook design is evolving and may well change with the next version. And then you two, with Thomas and other interested parties, work together to evolve a design that everyone is happy with.

Does that sound ok?

Simon

From: Luite Stegeman [mailto:stegeman@gmail.com] Sent: 07 September 2013 22:04 To: Simon Peyton-Jones Cc: Thomas Schilling; Edsko de Vries; ghc-devs

Subject: Re: GHC 7.8 release status

· Why aren’t you using Data.Dynamic for the hook things? You are precisely doing dynamic typing after all. (Moreover I want to change Data.Dynamic so that it says

data Dynamic where Dyn :: Typeable a => a -> Dynamic and you want to take advantage of this.

Ah the goal is to avoid the Typeable constraint on the hooked function, and to identify what things are actually hooks. Wrapping it in a newtype also achieves the first goal and does make the design a bit simpler.

No need for these strange “data DsForeignsHook = DsForeignsHook” things, or for the Hook type family. Simple!

But it means that hooks are no longer recognisable by their type, they're just some Typeable (the type families approach would at least prevent users from accidentally inserting wrong hooks, even though they would still be able to make bogus instances on purpose)

· The design *must* list all the hooks that GHC uses and their types. There’s no point in adding a hook that GHC doesn’t call!

It appears to be difficult to define all hooks in one module or have them in one record because of dependencies and the DLL split on Windows. Re-exporting everything from a single module can be done, but would offer no guarantees about completeness.

With the type families design, everything that's an instance of Hook is a hook, although the definitions are scattered throughout the GHC source. The Dynamic design would just have to rely on a consistent naming convention. Would listing the hooks in comments (in the Hooks module) and on the wiki be a reasonable way to document them?

I've uploaded a new patch, using Dynamic, although I'm not sure if it's an improvement over the original one:

- patch:

https://github.com/ghcjs/ghcjs-build/blob/master/refs/patches/ghc-hooks-dyna...

- updated hooksDemo: https://gist.github.com/luite/6478973

It also adds hscParse' and tcRnModule' exports for Edsko's use case (I think that makes it somewhat more flexible than exporting another version of hscFileFrontend, since it allows users to write a hook that does something between parsing and typechecking or one that overrides one of these phases)

luite

...

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