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
-- ...
However it poses upper build-time limit.
Regards
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