Multiple type parameters in type class?

Question

Lets assume I have a type class Stack with one instance List:

class Stack a where
    push :: a -> Integer -> a
    pop :: a -> a
    last :: a -> Integer

data List = Empty | Element Integer List
instance Stack List where
    push list value = Element value list
    pop Empty = error "No elements"
    pop (Element _ list) = list
    last Empty = error "No elements"
    last (Element value _) = value

How Stack has to be defined in order for List to not be limited to Integer values?

-- class Stack (?) where ...
data List a = Empty | Element a (List a)
-- instance Show (List a) where ...

Answer 1

Consider using a higher-kinded class variable. Thus:

class Stack s where
    push :: s a -> a -> s a
    pop  :: s a -> s a
    last :: s a -> a

data List a = Empty | Element a (List a)

The instance remains exactly as you wrote it (though List now has kind * -> * instead of *):

instance Stack List where
    push list value = Element value list
    pop Empty = error "No elements"
    pop (Element _ list) = list
    last Empty = error "No elements"
    last (Element value _) = value

This approach is pure Haskell 2010 -- it requires no extensions.

Also, consider making your failures observable; for instance by changing the type of pop and last to return Maybe (s a) and Maybe a, respectively.