haskell怎么用while

Haskell是一种纯函数式编程语言，它不支持像while这样的循环控制结构。相反，Haskell鼓励使用递归和高阶函数来实现迭代和循环。

例如，以下是一个使用递归来实现while循环的示例：

while :: (a -> Bool) -> (a -> a) -> a -> a
while p f x
  | p x       = while p f (f x)
  | otherwise = x

这个while函数接受一个条件函数p、一个转换函数f和一个初始值x作为参数。它将重复应用转换函数f到初始值x上，直到条件函数p返回False。返回值是最后一个应用转换函数f的结果。

例如，要计算1到10的和，可以使用以下代码：

sum1to10 = while (\x -> x <= 10) (\x -> x + 1) 0

这个代码将不断将x加1，直到x大于10为止，并返回1到10的和。

总之，虽然Haskell没有while这样的循环控制结构，但是可以使用递归和高阶函数来实现类似的迭代和循环。

相关问题

用ruby和haskell，分别写一个21点游戏。

以下是 Ruby 的实现：

class Card
  attr_reader :value

  def initialize(value)
    @value = value
  end
end

class Deck
  FACES = %w[2 3 4 5 6 7 8 9 10 J Q K A].freeze
  SUITS = %w[♠ ♥ ♦ ♣].freeze

  def initialize
    @deck = []
    SUITS.each do |suit|
      FACES.each do |face|
        @deck << Card.new("#{face}#{suit}")
      end
    end
  end

  def shuffle
    @deck.shuffle!
  end

  def draw
    @deck.pop
  end
end

class Game
  POINTS_MAP = { 'J' => 10, 'Q' => 10, 'K' => 10, 'A' => [1, 11] }.freeze

  attr_reader :player_hand, :dealer_hand

  def initialize
    @deck = Deck.new
    @deck.shuffle
    @player_hand = []
    @dealer_hand = []
    2.times do
      @player_hand << @deck.draw
      @dealer_hand << @deck.draw
    end
  end

  def hit
    @player_hand << @deck.draw
  end

  def play_dealer
    while points(@dealer_hand) < 17
      @dealer_hand << @deck.draw
    end
  end

  def points(hand)
    hand.map(&:value).reduce(0) do |sum, value|
      face = value[0..-2]
      if POINTS_MAP.key?(face)
        if POINTS_MAP[face].is_a?(Array)
          sum + POINTS_MAP[face][1]
        else
          sum + POINTS_MAP[face]
        end
      else
        sum + face.to_i
      end
    end
  end

  def winner
    player_points = points(@player_hand)
    dealer_points = points(@dealer_hand)
    if player_points > 21
      :dealer
    elsif dealer_points > 21
      :player
    elsif dealer_points > player_points
      :dealer
    elsif player_points > dealer_points
      :player
    else
      :tie
    end
  end

  def display
    puts "Player has: #{@player_hand.map(&:value).join(', ')}. Total: #{points(@player_hand)}"
    puts "Dealer has: #{@dealer_hand.first.value}, [hidden]"
  end
end

game = Game.new
game.display

while game.points(game.player_hand) < 21
  puts "Do you want to hit or stay? (h/s)"
  input = gets.chomp
  break if input == 's'

  game.hit
  game.display
end

if game.points(game.player_hand) > 21
  puts "You busted!"
else
  game.

Describe and contrast overloading in Java and Haskell.

Overloading in Java and Haskell are similar in concept but differ in implementation.

In Java, method overloading allows multiple methods to have the same name but with different parameters. The compiler determines which method to call based on the number and type of arguments passed. This allows for more flexibility and readability in code.

For example:

public int add(int x, int y) {
    return x + y;
}

public double add(double x, double y) {
    return x + y;
}

In Haskell, function overloading is achieved through the use of type classes. Type classes define a set of functions that can operate on a particular type. This allows for polymorphic behavior and avoids the need for explicit type annotations.

For example:

class Addable a where
    add :: a -> a -> a

instance Addable Int where
    add x y = x + y

instance Addable Double where
    add x y = x + y

In this example, the `Addable` type class defines the `add` function for types that are instances of the type class. The `instance` keyword is used to define specific implementations of the function for different types.

Overall, overloading in Java and Haskell serve similar purposes but differ in syntax and implementation. Java uses method overloading with different parameter types, while Haskell uses type classes to provide polymorphic behavior.