Functions to Polymorphic data types

Question

data Foo a is defined like:

data Foo a where
  Foo :: (Typeable a, Show a) => a -> Foo a
  -- perhaps more constructors

instance Show a =         


        

Answer 1

getFoo :: (Show a, Typeable a) => String -> Foo a
getFoo "five" = fiveFoo
getFoo "false" = falseFoo

If fiveFoo :: Foo Int and falseFoo :: Foo Bool, you're essentially asking for getFoo to return a different type depending on what value you feed it at run-time. You can't do that. In Haskell, all types must be known at compile-time.

If all you want to be able to do is convert the thing to a string, why not just store it as a string in the first place? I'm guessing the answer is that this is actually a simplification of the real problem you're trying to solve...(?)