Haskell type system exercises
Haskell的类型系统是强类型的，并且没有任何强制类型转换。所有的类型检查均在编译器做检查。定义新的数据类型之后，即使它们内在的数据结构完全一样，也是完全不同的数据类型，不能混用。
Maybe 类型
Maybe类型是有一种基于已有类型的二次封装类型，用于表述可能为空的抽象类型，用于支持类似于 C++/Java 中的模板机制，不过Haskell的类型系统表达能力比一般语言中的模板强大很多。
Maybe a = Just a | Nothing
新类型定义
-
可以采用
data关键字来定义，=后边第一个标志符是其构造函数，用于构造新类型的实例对象。也可以用type来建立类型别名。不同的类型别名对用的新类型是不能互相转换的。 -
类型定义中可以用
|来指定多个构造函数，这些构造函数最终都会生成同样类型的实例。 -
类型定义中，可以用record语法来提高可读性。
练习题解法
toList 的实现：
data List a = Cons a (List a)
| Nil
deriving (Show)
fromList (x:xs) = Cons x (fromList xs)
fromList [] = Nil
-- my solution --
toList (Cons x (xs)) = x : toList xs
toList Nil = []
运行结果：
Prelude> :load ListADT.hs
[1 of 1] Compiling Main ( ListADT.hs, interpreted )
Ok, modules loaded: Main.
*Main> let alist = fromList [1..12]
*Main> alist
Cons 1 (Cons 2 (Cons 3 (Cons 4 (Cons 5 (Cons 6 (Cons 7 (Cons 8 (Cons 9 (Cons 10 (Cons 11 (Cons 12 Nil)))))))))))
*Main> toList alist
[1,2,3,4,5,6,7,8,9,10,11,12]
用Maybe类型实现的模板树
已经给出的实现代码：
module Tree where
data Tree a = Node a (Tree a) (Tree a)
| Empty
deriving (Show)
simpleTree = Node "parent" (Node "left child" Empty Empty)
(Node "right child" Empty Empty)
- 采用Maybe的实现一：
data MyTree a = MyNode a (Maybe (MyTree a)) (Maybe (MyTree a))
deriving (Show)
mySimpleTree = MyNode (Just "parent") (Just (MyNode (Just "left child") Nothing Nothing))
(Just (MyNode (Just "right child") Nothing Nothing))
- 采用Record语法提高可读性:
data MyTreeRecord a = MyTreeRecord {
parentNode :: a
, leftChild :: Maybe (MyTreeRecord a)
, rightChild :: Maybe (MyTreeRecord a)
} deriving (Show)
anotherSimpleTree = MyTreeRecord {
parentNode = "parent"
,leftChild = Just MyTreeRecord {
parentNode = "left child",
leftChild = Nothing,
rightChild = Nothing
}
,rightChild = Just MyTreeRecord {
parentNode = "right child",
leftChild = Nothing,
rightChild = Nothing
}
}
运行结果：
Prelude> :load Tree.hs
[1 of 1] Compiling Tree ( Tree.hs, interpreted )
Ok, modules loaded: Tree.
*Tree> simpleTree
Node "parent" (Node "left child" Empty Empty) (Node "right child" Empty Empty)
*Tree> mySimpleTree
MyNode (Just "parent") (Just (MyNode (Just "left child") Nothing Nothing)) (Just (MyNode (Just "right child") Nothing Nothing))
*Tree> anotherSimpleTree
MyTreeRecord {parentNode = "parent", leftChild = Just (MyTreeRecord {parentNode = "left child", leftChild = Nothing, rightChild = Nothing}), rightChild = Just (MyTreeRecord {parentNode = "right child", leftChild = Nothing, rightChild = Nothing})}
*Tree>
Leave a Comment
Your email address will not be published. Required fields are marked *