Martin Sjögren 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 <glynn.clements@virgin.net>