模式匹配的数据类型和哈斯克尔

Question

其嵌套的名字，我有：

data Color = Blue | Green | Red | White | Yellow deriving (Eq,Ord) 

然后

data Term = Color | ... 
data Bag = Bag { 
color :: Color 
... 
} 

现在我希望能够匹配，以确保给出的期限是颜色，如果是这样检查它的“价值”（蓝/绿......）。类似这样的：

func :: Term -> Bag -> Bool 
func (c :: Color) bag = (color bag) == c 

但（c :: Color）似乎不起作用。

Answer 1

data Color = Blue | Green | Red | White | Yellow deriving (Eq,Ord) 

data Term = Color Color | Trash 

data Bag = Bag { 
    color :: Color 
} 

func (Color x) bag = (color bag) == x 

-- With the above, a call of func Trash something will fail. 
-- (unexhastive pattern match). You can add 

func Trash bag = False 

-- or 

func _ _ = False 

-- and it will work all time. 

Answer 2

这是一个有点文字的帐号，与sdcvvc的内容没有什么不同，我认为。

 
data Color = Blue | Green | Red | White | Yellow deriving (Eq,Ord,Show) 
data Size = Small | Medium | Large deriving (Eq, Ord, Show) 

data Term = TColor Color | TSize Size | Trash deriving Show 
data Bag = Bag {color :: Color , size :: Size} deriving Show 

请注意，与sdcvvc不同，我使用了“TColor Color”。这是没有什么不同，因为typechecker可以告诉一个是已经建立的类型，另一个是新类型的新构造函数，即使它们拼写相同。这只是少了一点混淆。现在，像“色彩”这样的东西现在不会那么少见，但是在像Bird这样的老书中，“Haskell的FP入门”，他不会做这种事情。这有点像Haskeller对诸如“\ file - > readFile文件”这样的流行风格，它有其优点，但有可能让人困惑，而且看起来很新。以前它只是\ x - > readFile x或其他东西。

 
theMrsThatcher :: Bag 
theMrsThatcher = Bag Blue Large 

theMrsRobinson :: Bag 
theMrsRobinson = Bag {color = Green, size = Small} -- to use the other syntax 

colorCheck :: Term -> Bag -> Bool 
colorCheck (TColor c) b = True 
colorCheck (TSize s) b = False -- as sdcvvc says, this and 
colorCheck Trash b = False  -- the next clause can be replaced by 
           -- colorCheck _ _ = False 

还请注意，在colorCheck袋是无关紧要的。从你说的原因不清楚你为什么需要中间类型术语。

 
colorTest :: Color -> Bag -> Bool 
colorTest c b = color b == c 

colorCheckTest :: Term -> Bag -> Bool 
colorCheckTest (TColor c) b = color b == c 
colorCheckTest (TSize s) b = False -- as above, the last clauses are 
colorCheckTest Trash b = False  -- wordier than need be since any 
             -- but the first pattern is a loser. 

结果：

 
*Main> colorCheck (TColor Blue) theMrsRobinson 
True 
*Main> colorCheck (TColor Blue) theMrsThatcher 
True 
*Main> colorCheckTest (TColor Blue) theMrsRobinson 
False 
*Main> colorCheckTest (TColor Blue) theMrsThatcher 
True 
*Main> colorTest Blue theMrsThatcher 
True 
*Main> colorTest Blue theMrsRobinson 
False