为什么 `newtype` 不适用于这种模式匹配？

Question

我通过将数据实现为Enum 来尝试以下代码。 而且，我有一个问题，只是将Enum 替换为newtype。

这是我的测试代码。

newtype Fruit = Fruit Int deriving (Eq)

apple = Fruit 0
banana = Fruit 1
grape = Fruit 2
orange = Fruit 3

instance Show Fruit where
  show apple = "Apple"
  show banana = "Banana"
  show grape = "Grape"
  show orange = "Orange"
  show _ = "Fruit"

test =
  map show [apple, banana, grape, orange, (Fruit 5)]

我预计评估的test 将是["Apple","Banana","Grape","Orange","Fruit"]，但它是["Apple","Apple","Apple","Apple","Apple"]

我认为 it 变量可能无法很好地与模式匹配一​​起使用，但我又通过case 尝试了一次，例如：

-- Fails: everything is "Apple"
instance Show Fruit where
  show g = case g of
    apple -> "Apple"
    banana -> "Banana"
    grape -> "Grape"
    orange -> "Orange"
    _ -> "Fruit"

为了确认我的临时代码，我再次尝试了下一个代码。

-- Fails: Works only for apple/banana. Variable does not work!
instance Show Fruit where
  show (Fruit 0) = "Apple"
  show (Fruit 1) = "Banana"
  show grape = "Grape"
  show orange = "Orange"
  show _ = "Fruit"

我可以得到["Apple","Banana","Grape","Grape","Grape"]

好吧，我可以用Enum 编写代码来实现，但我只是想了解为什么这些代码不能正常工作？

Answer 1

这段代码

instance Show Fruit where
  show apple = "Apple"
  show banana = "Banana"
  show grape = "Grape"
  show orange = "Orange"
  show _ = "Fruit"

等价于

instance Show Fruit where
  show x = "Apple"
  show y = "Banana"
  show z = "Grape"
  show w = "Orange"
  show _ = "Fruit"

因为它声明了名为 apple, ... 的新变量，这些变量会影响全局定义的变量，而不会以任何方式与这些变量相关。

试试吧

instance Show Fruit where
  show x | x == apple = "Apple"
         | x == banana = "Banana"
         | x == grape = "Grape"
         | x == orange = "Orange"
         | otherwise = "Fruit"

要记住的经验法则是：在一个模式中，所有变量都是由模式定义的局部变量。

我建议使用 -Wall 打开警告，因为这会指出局部变量对全局变量的影响。

Answer 2

您似乎正在尝试做的另一种方法是使用PatternSynonyms。