You are trying to assign two distinct types to dynFoo; that's a no-no. You need to move the usage of the polymorphic function out of the let so that the use at each distinct type doesn't get unified.
{-# OPTIONS -fglasgow-exts #-} 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 if fooType == "str" then dostuff (fromDyn f undefined :: Foo String) else dostuff (fromDyn f undefined :: Foo Int)
dostuff :: (Show a) => Foo a -> IO () dostuff (FVal x) = print x
BTW, this little example always stuffs a string into the FVal, so trying to get anything else out will fail.
Thanks for the example it makes a bit of sense now, I really appreciate you taking the time to help me on this. Cheers, Scott