How to specify the type of arguments given to Algebraic type in Haskell-CodePudding

If I have an Algebraic Data Type like below

data BinaryTree a = Leaf 
                    | Node a (BinaryTree a ) (BinaryTree a)
                    deriving (Eq,Ord)

Here Leaf is for empty and a is the node and the other two parameters are for subtrees coming from the node.

Is there a way I can specify that the argument a should be deriving Show

I was trying to give my own implementation of Show for BinaryTree , and I started out simple like:

instance Show (BinaryTree a) where
    show Leaf = "x"
    show (Node node left right) =  show node   "\n"    show left   "  "  show right

But show node doesn't work -> No instance for (Show a) arising from a use of ‘show’

CodePudding user response：

You can only work with show node, if node is of a type that is a member of the Show typeclass. You thus should add a type constraint in the head of the instance declaration:

--         ↓ type constraint
instance Show a => Show (BinaryTree a) where
    show Leaf = "x"
    show (Node node left right) =  show node    '\n' : show left    "  "  show right