如何在 Haskell 中编写 Deque 数据类型

Question

如何在 Haskell 中编写双端队列（“deque”）。数据结构应该有函数emptyDeque、front、back、removeFront、removeBack、addFront、addBack和isEmpty，然后在->;之间显示双端队列。和<-。

这是相同的，但对于队列：

module Queues (Queue, emptyQueue, front, remove, add, isEmpty)
   newtype Queue a = Queue [a]
   emptyQueue = Queue []
   front  (Queue (x:xs)) = x
   front (Queue []) = error("No front of empty queue")
   add (Queue xs) x = Queue (xs ++ [x])
   remove (Queue (x:xs)) = Queue xs
   remove (Queue []) = error("Nothing on queue to remove")
   isEmpty (Queue []) = True
   isEmpty (Queue (x:xs)) = False
   showElems [] = ""
   showElems (x:xs) = " " ++ (Prelude.show x) ++ (showElems xs)
   showQueue (Queue a) = "<" ++ (showElems a) ++ " >"
   instance (Show a) => Show (Queue a) where show = showQueue

我想出的是正确的吗？

module Deques (Deque, emptyDeque, front, back, removeFront, removeBack , addFront, addBack, isEmpty)
newtype Deque a = Deque [a]
emptyQueue = Queue []
reverses (x:xs) = (reverses xs) ++ [x]
front (Deque (x:xs)) = x
front (Deque []) = error("No front of empty Deque")
back (Deque a) = front(reverse(a))
back (Deque []) = error("No back of empty Deque")
addBack (Deque xs) x = Deque (xs ++ [x])
addFront (Deque xs) x = Deque ([x] ++ xs)
removeFront (Deque (x:xs)) = Deque xs
removeFront (Deque []) = error("Nothing on Deque to remove")
removeBack (Deque a) = reverses(removeFront(reverses(a))
                 `

Answer 1

使用列表来实现双端队列效率不是很高，但它可以工作。一些注释

除了类型错误之外，您似乎正在以其他语言的风格编写函数应用程序

front(reverse(a))

在 Haskell 中，括号只是为了分组。编写该行的更 Haskell 方式可能是

front (reverse a)

或者甚至

front $ reverse a

另一个注意事项：在 Haskell 中向列表前面添加内容是非常典型的

[x] ++ xs -- The weird way
x : xs -- The canonical way

，但是向列表后面添加内容很难看。

xs ++ [x] -- No better way for normal lists. This is inefficient

您已经有了一个相当好的开始，但是您应该尝试熟悉 Haskell 独特的范例和风格。 学习 Haskell 的前几章在这方面做得很好。

Answer 2

实际上这里是我的 Final Deque 实现，效果很好

module Deques (Deque, emptyDeque, front, back, removeFront, removeBack, addFront, addBack, isEmpty) where

    newtype Deque a = Deque [a]

    backwards (Deque []) = Deque []

    backwards (Deque a) = Deque( reverse a )

    emptyDeque = Deque []

    front (Deque (x:xs)) = x
    front (Deque []) = error("No front of empty Deque")

    back (Deque a) = front( backwards (Deque a))
    back (Deque []) = error("No back of empty Deque")

    addBack (Deque xs) x = Deque (xs ++ [x])
    addFront (Deque xs) x = Deque (x : xs)

    removeFront (Deque (x:xs)) = Deque xs
    removeFront (Deque []) = error("Nothing on Deque to remove")

    removeBack (Deque a) = backwards( removeFront( backwards (Deque a) ))
    removeBack (Deque []) = error("Nothing on Deque to remove")

    isEmpty (Deque []) = True
    isEmpty (Deque (x:xs)) = False

    showElems [] = ""
    showElems (x:xs) = " " ++ (Prelude.show x) ++ (showElems xs)
    showDeque (Deque a) = "<" ++ (showElems a) ++ " >"

    instance (Show a) => Show (Deque a) where show = showDeque