Standarize GHC.Prim.Any

[Maciej Piechotka]

On Sat, 2009-09-05 at 23:35 -0400, Isaac Dupree wrote:
> Maciej Piechotka wrote:
> > On Thu, 2009-08-20 at 19:59 +0200, Maciej Piechotka wrote:
> >> Sorry for delay in responding.
> >>
> >>>> Any from GHC.Prim makes unsafeCoerce much useful and safe. Can it be
> >>>> included in Unsafe.Coerce module?
> >>>>
> >>>> Pros:
> >>>> - unsafeCoerce is much more useful with Any (it's a safe placeholder for
> >>>> any value and therefore can be passed simply in/out FFI).
> >>> Yes it's a safe placeholder, but so is an existential, I believe...
> >>> data ContainsAny = forall a. ContainsAny a
> >>> to put in the container:
> >>> "ContainsAny x"
> >>> to remove from the container:
> >>> "case ... of ContainsAny x -> unsafeCoerce x"
> >>> Albeit, there might be a slight performance penalty for the extra box, 
> >>> which can't be removed in all haskell implementations.
> >>>
> >>> Also, what do you mean about "FFI"?  I don't think you can pass the 
> >>> "Any" type to foreign functions through the FFI...
> >>>
> >>> Also, can/do all compilers that implement unsafeCoerce implement a safe Any?
> >>> Hugs can do it with just "data Any = Ignored" I believe, not sure about 
> >>> nhc, yhc or jhc...
> >>>
> >>> -Isaac
> >> Well. May be I have one specific problem which seems to not be possible
> >> to be implemented in portable way (I'm not sure if it is possible
> >> outside ghc). Sorry for mentioning FFI without further explanation.
> >>
> >> The problem arise if one would like to implement GClosure from glib.
> >> Once it was implemented in gtk2hs in C using RTS API. I also have my own
> >> implementation in Haskell which is doing something (removed IO monad for
> >> clarity):
> >> applyValue :: Any -> Value -> Any
> >> applyValue f v =
> >> 	case funsamentalTypeOf v of
> >> 		Type1 -> (unsafeCoerce f :: HaskellType1 -> Any) $ get v
> >> 		...
> >>
> >> Such trick is (looks for me?) safe as a -> b -> ... -> d can be
> >> represented by Any. However I cannot safely cast to function a -> Any.
> >>
> >> To/from FFI it is passed in Ptr (StablePtr Any).
> >>
> >> Regards
> >> PS. I assume that it is not possible as it was done in importable was in
> >> gtk2hs. 
> > 
> > With any known from the beginning number of parameters function and GADT
> > one can write:
> > 
> > data Closure where
> > 	Closure0 :: IO a
> > 	Closure1 :: a -> IO b
> > 	Closure2 :: a -> b -> IO c
> > 	-- ...
> 
> That isn't GADT syntax... results of "Constructor :: ..." must be 
> "Closure" in this case. 

Sorry. To much delete:
data Closure where
	Closure0 :: IO a -> Closure
	Closure1 :: (a -> IO b) -> Closure
	Closure2 :: (a -> b -> IO c) -> Closure
	...

> What are you trying to do here?  An alternative 
> for "Ptr (StablePtr Any)"?  Is "Ptr (StablePtr Dynamic)" sufficient, 
> where Dynamic from Data.Dynamic is represented in an 
> implementation-specific way equivalent to
> data Dynamic = forall a. Typeable a => DynamicConstr a
> a.k.a.
> data Dynamic where DynamicConstr :: Typeable a => a -> Dynamic
> ?
> The main limitation would be if you want to include a type that's not in 
> Typeable.  Is that common, to have types that aren't in Typeable?
> 

Point taken.

Regards
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