GHC 7.8 release status

Simon Peyton-Jones simonpj at
Thu Sep 5 10:36:34 UTC 2013

Thank you for the wiki page. This is great. For the first time I have something I can get my teeth into.  Some comments

·         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.

·         I don't understand the need for the "key" types and the Hook type to get to the real type.  Why not do this:

o   As now, DynFlags has a map from TypeRep -> Dynamic, with the invariant that if you look up a typerep tr, then the Dynamic you get has typerep tr.

o   When you want to add a new pass, say for DsForeigns, you say
   newtype DsForeignsHook = DSF ([LForeignDecl Id ] -> DsM (ForegnStubs, OrdList Binding)) deriving Typeable

o   To invoke the hook, GHC looks up the type DsForeignsHook in the hook-map, and gets back a newtype value of type DsForeignsHook, which it unwraps and applies.

o   To install the hook GHC provides a function
   insertHook : Typeable a => a -> Hooks -> Hooks

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

·         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!


From: Luite Stegeman [mailto:stegeman at]
Sent: 05 September 2013 02:10
To: Thomas Schilling
Cc: Simon Peyton-Jones; Edsko de Vries; ghc-devs
Subject: Re: GHC 7.8 release status

I've updated the wiki page, expanding the descriptions and adding code from the actual implementation:
An demo program that uses all hooks here:
(I've listed the users (or proposers) of every hook, and how they use it, Thomas / Edsko, can you check that they indeed do what you need?)
The patch is here:

In addition to defining the heterogeneous map and the hooks themselves, extra exports have been added to make it possible for users to actually make a hook implementation without copying most of the module's source code. The demo program implements all hooks to check this.

Also the GHCJS patch is here:

It adds the following:
- in DynFlags an extra WayCustom constructor to add a custom 'tag' to generated files (GHCJS builds multiple architectures to support Template Haskell among other things, one with the 'js' tag)
- in ForeignCall the JavaScriptCallConv calling convention
- in Platform an ArchJavaScript architecture
- `foreign import javascript' support in the parser and lexer
- The JavaScriptFFI extension that enables the `foreign import javascript' syntax, only supported on ArchJavaScript (So using it on a regular GHC would always result in a compile error saying that it's unsupported on the user's platform)


On Thu, Sep 5, 2013 at 12:17 AM, Thomas Schilling <nominolo at<mailto:nominolo at>> wrote:
I started a wiki page at:

Luite: could you please fill in the hooks that your latest patch defines?

On 4 Sep 2013, at 19:40, Simon Peyton-Jones <simonpj at<mailto:simonpj at>> wrote:

> I do need more than a patch, please, please.  A wiki page explaining the design, as seen by the user (of the GHC API), the problems it solves, and the use-cases it enables, would be most helpful.
> Simon
> | -----Original Message-----
> | From: Thomas Schilling [mailto:nominolo at<mailto:nominolo at>]
> | Sent: 04 September 2013 17:26
> | To: Simon Peyton-Jones; Luite Stegeman
> | Cc: Nicolas Frisby; "Pedro Magalhães (dreixel at<mailto:dreixel at>)"; Richard
> | Eisenberg (eir at<mailto:eir at>); Geoffrey Mainland
> | (mainland at<mailto:mainland at>); Iavor Diatchki; Austin Seipp; Edsko de Vries;
> | Ryan Newton (rrnewton at<mailto:rrnewton at>); David Luposchainsky; ghc-
> | devs at<mailto:devs at>
> | Subject: Re: GHC 7.8 release status
> |
> |
> | On 4 Sep 2013, at 15:52, Simon Peyton-Jones <simonpj at<mailto:simonpj at>>
> | wrote:
> |
> | >  Edsko/Thomas/Luite: if you want anything for 7.8 it'll have to be
> | jolly soon.  At the moment I don't even know the motivation or design,
> | let alone implementation.  Could you make a wiki page explaining the
> | proposed design?  Is it really important to do this for 7.8?
> |
> | Yes, it is quite important to get this into 7.8.  Not having these
> | features in GHC makes it very difficult for people to adopt GHCJS.  One
> | could argue that GHCJS is not yet production-ready anyway and users that
> | want to try it can figure out building GHC from source to use it, but
> | this doesn't quite apply.
> |
> |  1. GHCJS targets a wider audience than users brave enough to compile
> | GHC from source. In addition, the next chance to get these features into
> | GHC is in a year from now, so when GHCJS becomes more mature then this
> | will be a major hurdle for adoption.
> |
> |  2. These changes are also required for IDE tools which really mustn't
> | require users to build a custom version of GHC.
> |
> |
> | Luite's design is actually very flexible.  It simply allows GHC API
> | users to provide functions that are called instead of (or in addition
> | to) existing functions in GHC.  Instead of calling, say, "genHardCode",
> | the driver now looks up whether the user has specified a hook for
> | genHardCode and calls that instead.
> |
> | Currently we only specify a small number of hooks that are sufficient
> | for our use cases.  Future releases of GHC can be extended to include
> | more hooks, if that is needed.
> |
> | Hooks are stored as an untyped map inside the DynFlags (to avoid issues
> | with circular dependencies).  Each hook is looked up using a single-
> | constructor type and type families are used to make this type safe.
> | There is one use of unsafeCoerce to avoid having to make every hook
> | function an instance of Typeable.
> |
> | Unlike CorePlugins, it is only a GHC API feature, and users cannot
> | specify plugins to be added via command line options.  If we can come up
> | with a good design, then we could add this in GHC 7.10, but it is not
> | necessary at this point.
> |
> | Luite: Do you have a link to your latest patch?

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