Generic instance of custom type class in Haskell?

Question

Let's say I have following data structure:

data Dezi = Dezi1 Int | Dezi2 String | Dezi3 [Dezi] deriving(Show)

class TestInterface a where
    testInt :: a -> Dezi

instance TestInterface Int where
    testInt 0 = Dezi1 0
    testInt _ = Dezi2 "Nie nula"

instance Dezi a =>  TestInterface [a] where 
    testInt xs = Dezi3 $ map (\x -> testInt x) xs

In last statement I'm trying to create generic instance for my type class I asume that type 'a' is Int or String, but compiler is not happy:

`Dezi' is applied to too many type arguments
In the instance declaration for `TestInterface [a]'

I'm beginner and still in learning process.

Thanks!

Answer 1

Dezi is a datatype, not a typeclass. Types aren't "instances of Dezi". Instead you might say something like

instance TestInterface a => TestInterface [a] where
  testInt xs = Dezi3 $ map testInt xs

This reads like "to make a list of as an instance of TestInterface, look up the instance for a and use that."