[Haskell-cafe] Ambiguous types for collection keys

[Scott Weeks]

>
> 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