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Let's say I'm creating a data type in haskell, and this data type accepts multiple constructors. Is there an easy way to determine later in my code which one it was created as?

EDIT:

Example, I'm using the dataType

data LogicValue =  CloseAnd (Int, Int) (Int, Int)
            | CloseXor (Int, Int) (Int, Int)
            | FarAnd LogicValue LogicValue 
            | FarXor LogicValue LogicValue

Is there an easy way to determine if something is a CloseAnd for instance?

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2 回答 2

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当然，只需对构造函数名称进行模式匹配！

f :: LogicValue -> Ret
f (CloseAnd a b) = ...
f (CloseXor a b) = ...

当然，同样的事情也可以用一个case开关来完成。

由于这个问题一直出现在我的通知框中（我的回答有投票，如果我说实话，可能不太值得）我想补充一点，你的问题与“确定类型”没有任何关系. a 的不同构造函数data 都具有相同的结果类型，即LogicValue. 您可能正在考虑“子类型” CloseAnd，CloseXor...，就像您在 OO 语言中可能拥有的那样。Haskell 变体类型与 OO 类层次结构有一些相似之处，但它们仍然是一个不同的概念。

于 2013-09-26T05:59:56.530 回答
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您为此使用模式匹配：

logictype :: LogicValue -> [Char]
logictype (CloseAnd _ _) = "It is a closeAnd."
logictype (CloseXor _ _) = "It is a closeXor."
logictype (FarAnd _ _)   = "It is a FarAnd."
logictype (FarXor _ _)   = "It is a FarXor."

您还可以匹配参数：

logictype (CloseAnd (a,b) (c,d)) = "it is a closeAnd with parameters " ++ show [a,b,c,d]
于 2013-09-26T06:00:11.037 回答