I am trying to create an instance of a class for String types. I have the following: class Foo a where mkFoo :: a -> String instance Foo String where mkFoo x = x and receive the following error: test.hs:9:0: Illegal instance declaration for `Foo String' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Foo String' Failed, modules loaded: none. I see that in general I am not able to create an instance of a class for any type [a], where a is a specific type. I can work around it by wrapping my Strings in some other data type but I'd like to understand why it's not possible. Any insight would be appreciated. -Adam