以下 haskell 类型类实例有什么问题？

Question

data II = I Int Int deriving (Show)
instance II Show where
  show I a b = show (a+b)

showt.hs:3:2: show' is not a (visible) method of class II'

Answer 1

The class name should come before the type in the instance declaration. You also need to remove the deriving clause, since you're providing your own instance instead of using one that's automatically derived. You also need to add parenthesis around the single argument to show , otherwise it looks like 3 arguments to the parser.

data II = I Int Int
instance Show II where
    show (I a b) = show (a+b)