GHC 7.8 release status

Simon Peyton-Jones simonpj at
Mon Sep 9 14:51:40 UTC 2013


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?


From: Luite Stegeman [mailto:stegeman at]
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:
- updated hooksDemo:

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)

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