haskell - 没有因使用 ‘show’ 而产生的实例(显示 a)

Question

没有因使用“show”而产生(Show a)的实例在‘(++)’的第一个参数中，即‘show a’

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}>>>

Answer 1

在你的行中:

    show (Leaf a) = "{" ++ <b>show a</b> ++ "}"

你调用show a，其中a是一个a类型的元素，但不是说的是该类型 a 是 Show 的实例，因此您需要向 instance 声明添加约束:

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

所以这里我们说 LTree a 是 show given 的实例 a 是 Show 的实例。对于您给定的样本数据，我们将获得:

Prelude> Node (Leaf 1) (Node (Node (Leaf 3) (Leaf 4)) (Node (Leaf 8) (Leaf 7)))
<{1},<<{3},{4}>,<{8},{7}>>>