Question

我对这个问题存有疑问。问题是编写haskell函数跟踪（包括答案中的类型声明行），该跟踪以Path p和BTree作为输入，通过跟踪路径返回您到达的子树树。请注意，（迹线p空）应始终返回空。到目前为止，这是我的解决方案

username, joinDate, organization, friends, location.

然后我得到

data BTree a = Empty 
             | BNode a ( BTree a ) ( BTree a ) deriving (Show)
data Dir = Lft |Rght
type Path = [Dir]

tree = BNode 1 (  BNode 2 (BNode 4 (Empty) (Empty)) (BNode 5 (Empty) (Empty)) ) ( BNode 3 Empty Empty)

tree1 = BNode 1 (  BNode 2 (BNode 4 (Empty) (Empty)) (BNode 5 (Empty) (Empty)) ) (  BNode 2 (BNode 4 (Empty) (Empty)) (BNode 5 (BNode 3 Empty Empty) (Empty)) )

-- 1.
trace :: Path -> BTree a -> BTree a
trace  p Empty =  Empty 
trace [] a =  a 
trace  (x:xs) (BNode b left right) = if x == Lft then trace xs left else trace xs right

Answer 1

运算符==是type class Eq的成员，因此，如果要在==类型上使用运算符Dir，则必须派生Eq为其输入类：

data Dir = Lft | Rght deriving Eq

但是，更好的方法是使用模式匹配而不是与==进行比较：

trace  p Empty =  Empty 
trace [] a =  a 
trace (Lft:xs) (BNode _ left _) = trace xs left 
trace (Rght:xs) (BNode _ _ right) = trace xs right

通过给定的跟踪Hakell查找子树

1 个答案: