Or carry an instance in along with a type parameter, using existentials or GADT.
Brandon Moore
Do you know of an example that would apply to my situation? I think I neglected to state part of my problem. I am storing the root nodes of btree indexes in a heterogeneous list using Typeable. When they come out of the list they need to be unwrapped I think this will clarify my situation because I've been doing a poor job of explaining: import Data.Dynamic data Foo a = FVal a deriving (Show, Typeable) type FooStr = Foo String type FooInt = Foo Integer main = do fooType <- getLine fooVal <- getLine let foo = toDyn (FVal fooVal) fs = [foo] (f:_) = fs Just dynFoo = fromDynamic f dostuff dynFoo dostuff :: (Show a) => Foo a -> IO () dostuff (FVal x) = print x This fails with: Ambiguous type variable `a' in the constraints: `Typeable a' arising from use of `fromDynamic' at /Users/weeksie/workspace/haskell/Main.hs:243:20-30 `Show a' arising from use of `dostuff' at /Users/weeksie/workspace/haskell/Main.hs:247:2-8 Probable fix: add a type signature that fixes these type variable(s) However, changing main: main = do fooType <- getLine fooVal <- getLine let foo = toDyn (FVal fooVal) fs = [foo] (f:_) = fs Just dynFoo = fromDynamic f if fooType == "str" then dostuff (dynFoo::FooStr) else dostuff (dynFoo::FooInt) Fails with: Couldn't match `Integer' against `String' Expected type: FooInt Inferred type: FooStr In the expression: dynFoo :: FooInt In the first argument of `dostuff', namely `(dynFoo :: FooInt)' I'm probably going about this in the wrong way. I'd love some advice on how to either do it better or weave some Type Magic to achieve the same effect.