The typeclass that provides "toString" in haskell is "Show". It defines a few functions, of which the important one is "show", which takes the original type and returns a string. A function that is equivalent to the java-ish example is:
stringify s = show s
No type definition is necessary, because the compiler can correctly infer the correct, most generic type:
stringify :: (Show s) => s -> String
Or, for any s in type class Show, a function that makes s into a String. Note that dispatch is done statically.
Modern type systems eliminate most of the cost of type safety through good generics and type inference. Mainstream statically typed languages are just 20-30 years behind the state of the art.
(note that the example actually can be reduced to: stringify = show)
