Re: [Haskell-cafe] Re: Combining Wouter's expressions with extensible records

Close - it compiles now! I made a minor change, going to Typeable1 instead of Typeable: instance (Typeable1 f, Typeable1 g) => Typeable1 (f :+: g) where typeOf1 l@(Inl x) = mkTyConApp (mkTyCon "Planning.Wouter.:+:") [typeOf1 x, typeOf1 y] where (Inr y) = undefined `asTypeOf` l typeOf1 r@(Inr y) = mkTyConApp (mkTyCon "Planning.Wouter.:+:") [typeOf1 x, typeOf1 y] where (Inl x) = undefined `asTypeOf` r Except this gives me a runtime error: *WouterTest> getName testNamed "*** Exception: Prelude.undefined The only thing I can think of is to have a class that gives default values to type - ick! -Ron Alford On Thu, Jul 10, 2008 at 4:16 PM, Antoine Latter wrote:

On Thu, Jul 10, 2008 at 2:15 PM, Ron Alford wrote:

...

I'm making progress, but how would I make the following a Typeable instance: data (f :+: g) e = Inl (f e) | Inr (g e) deriving Eq

Here is what I'm using for Expr: data Expr f = In (f (Expr f)) instance Typeable1 f => Typeable (Expr f) where typeOf (In x) = mkTyConApp (mkTyCon "Data.Trie.General.ListGT") [typeOf1 x]

I don't think I can use this for ':+:', because the typeOf instance only has access to a member of one type at a time. This may be similar to a definition of Typeable2 for Either, but I can't find an example to follow for that.

Maybe something like:

instance (Typeable1 f, Typeable1 g) => Typeable (f :+: g) where typeOf in@(InL f) = (some function of 'f' and 'g') where InR g = undefined `asTypeOf` in

typeOf in@(InR g) = (some function of 'f' and 'g') where InL f = undefined `asTypeOf` in

would work?

-Antoine