小编rgr*_*lma的帖子

在（（++））的第一个参数中，“ show a”的使用不会导致（Show a）的实例。

data LTree a = Leaf a | Node (LTree a) (LTree a)
instance Show (LTree a) where
    show (Leaf a) = "{" ++ show a ++ "}"
    show (Node fe fd) = "<" ++ (show fe)++ "," ++(show fd)++ ">"

Node (Leaf 1) (Node (Node (Leaf 3) (Leaf 4)) (Node (Leaf 8) (Leaf 7)))

我应该得到：

<{1},<<{3},{4}>,<{8},{7}>>>