允许数据构造函数的多个声明 [英] Allowing multiple declarations of data constructors

问题描述

我目前正在试验数据类型，但遇到了涉及数据构造函数的多个声明的问题.

I am currently experimenting with data types and I've ran into an issue involving multiple declarations of data constructors.

data DBPosition = Unknown
                | Omega Integer
                | Delta Integer
                deriving (Show, Eq, Ord)

data DBGeometry = Unknown | Cis | Trans
                deriving (Show, Eq, Ord)

data DoubleBond = DoubleBond DBPosition DBGeometry
                deriving (Show, Eq, Ord)

如果我要输入-let bond = DoubleBond Unknown Unknown之类的值，则可以推断出第一个Unknown具有DBPosition类型，而第二个Unknown具有DBPosition类型.不幸的是，情况并非如此:

If I was to make a value such as - let bond = DoubleBond Unknown Unknown, then it could be inferred that the first Unknown has a type of DBPosition while the second Unknown has a type of DBPosition. Unfortunately this is not the case:

test.hs:6:27:
Multiple declarations of `Unknown'
Declared at: test.hs:1:27
             test.hs:6:27
Failed, modules loaded: none.

有没有可用于解决此问题的语言扩展?

Are there any language extensions that can be used to get around this?

推荐答案

正如Carsten所指出的，您的定义不起作用，因为您有两个名称相同的构造函数.您需要使用例如UnknownDBPosition和UnknownDBGeometry.但是，我认为更好的解决方案是通过识别:

As Carsten pointed out above, your definition does not work because you have two constructors with the same name. You'd need to use e.g. UnknownDBPosition and UnknownDBGeometry. However, I'd argue a better solution arises from recognising:

• 无论您谈论的是双键位置，几何形状还是其他任何事物，未知值的概念都以完全相同的方式起作用；和
• Unknown实际上不是各种双键几何形状或位置.

• That the concept of an unknown value works in precisely the same way no matter if you are talking about double bond positions, geometries, or whatever else; and
• That Unknown is not actually a variety of double bond geometry or position.

既然如此，我建议您同时删除Unknown和使用Maybe来指定知识不足.

That being so, I recommend that you remove both Unknown and use Maybe to specify lack of knowledge.

data DBPosition = Omega Integer
                | Delta Integer
                deriving (Show, Eq, Ord)

data DBGeometry = Cis | Trans
                deriving (Show, Eq, Ord)

data DoubleBond = DoubleBond (Maybe DBPosition) (Maybe DBGeometry)
                deriving (Show, Eq, Ord)

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