Martin Sj�gren wrote:
> I have a problem with Data.Dynamic. The problem is probably that I don't
> understand it. From my understanding, the following program should work:
>
> -8<------------------------
> import Data.Dynamic
>
> data Foo = Foo { x :: Int }
> deriving Show
>
> instance Typeable Foo where
> typeOf _ = mkAppTy (mkTyCon "Foo.Foo") []
>
>
> main = do
> let dynObj = toDyn $ Foo 42
> print dynObj
> let Just obj = fromDynamic dynObj :: Maybe Foo
> print obj
> -8<------------------------
>
> But when I compile it (ghc Foo.hs) and run it (./a.out) I get:
>
> <<Foo.Foo>>
>
> Fail: Foo.hs:13: Irrefutable pattern failed for pattern (Data.Maybe.Just
> obj)
>
> Which indicates that fromDynamic returned Nothing. What is the problem
> here? Do I have to employ special trickery to use Dynamic with records?
You have to ensure that the TyCon is unique. A comment in Dynamic.hs
says:
-- | Builds a 'TyCon' object representing a type constructor. An
-- implementation of "Data.Dynamic" should ensure that the following holds:
--
-- > mkTyCon "a" == mkTyCon "a"
--
-- NOTE: GHC\'s implementation is quite hacky, and the above equation
-- does not necessarily hold. For defining your own instances of
-- 'Typeable', try to ensure that only one call to 'mkTyCon' exists
-- for each type constructor (put it at the top level, and annotate the
-- corresponding definition with a @NOINLINE@ pragma).
If you use:
fooTc = mkTyCon "Foo.Foo"
instance Typeable Foo where
typeOf _ = mkAppTy fooTc []
or compile with "-O", your program works.
--
Glynn Clements <[EMAIL PROTECTED]>
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