Show function for polymorphic type

问题

I'm trying to define the Show function for the polymorphic Tree type. Could anyone help me?

import Char

data Tree t =
    NilT |
    Node t (Tree t) (Tree t)

class Mar t where
    maior :: t -> String

instance Mar Tree where
    maior (NilT) = "a" 
    maior (Node t a b) = "b"

instance Show Tree where
    show = maior

Thanks a Lot!

---

Solution (given by ivanm):

import Char

data Tree t =
    NilT |
    Node t (Tree t) (Tree t)

class Mar t where
    maior :: t -> String

instance Mar (Tree t) where
    maior (NilT) = "a" 
    maior (Node t a b) = "b"

instance Show (Tree t) where
    show = maior

回答1:

Is there any particular reason that you aren't using deriving Show? The Show and Read classes are meant to provide really basic serialisation/deserialisation which (usually) produce valid Haskell code.

But for what you want, I think the error is going back to your Mar class. As defined, the instance is for kind * -> * (e.g. Maybe as opposed to Maybe Int). What you probably meant is to have instance Mar (Tree t) where ....

回答2:

data Tree t =
    NilT |
    Node t (Tree t) (Tree t)
    deriving Show

来源：https://stackoverflow.com/questions/7478878/show-function-for-polymorphic-type