GHC 7.8 release status

Wed Sep 11 14:32:54 UTC 2013

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
<simonpj at microsoft.com> 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 at 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-record.patch
> (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 <edskodevries at gmail.com>
> 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
> <simonpj at microsoft.com> 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 at 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-dynamic.patch
>>
>> - 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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>>
>
>