Haskell数据类型列表 [英] Haskell lists of datatypes

问题描述

这可能是另一个简单的Haskell问题。如果我有一些嵌套数据类型，比如在这个例子中的代码：

  data地方=国家
 |州
 | City String 
导出显示
 
数据State = California 
 |纽约
派生显示
 
数据国家=美国
 |加拿大
派生显示
  

我可以合法地列出[美国，加拿大]键入[国家]或类型为[州]的[国家]或[加利福尼亚州，纽约州]或类型为[地点]的[城市a，城市b]。

我需要做些什么才能制作[美国，纽约]等列表？纽约是一个地方的国家，美国是一个地方的国家，但ghci看到美国，所以它假设我正在制作一个国家的名单（纽约是一个国家，所以名单失败）。

我想我需要某种方式将国家或国家投放到某个地方，但是我对如何实现这一点感到茫然。

我试图避免将包含在State和Country中的数据放到Place类型中，我知道它会使其工作，但我拥有相当数量的真实数据，我宁愿不混杂

解决方案

这里有更多的输出让我找出问题所在：

  *主要> [州，州] 
 [州，州] 
 *主要> ：t State 
 State :: Place 
 * Main> ：t NewYork 
 NewYork :: State 
  

这似乎意味着State是Place的有效构造函数，而数据状态指的是仅加利福尼亚或纽约的值。

如果您稍微更改程序，则：

 数据地方=国家国家
 | State State 
 | City String 
导出显示
 
数据State = California 
 |纽约
派生显示
 
数据国家=美国
 |加拿大
派生显示
  

然后您可以创建一个列表，例如[Country USA，State NewYork ]这是正确的类型[地方]。使用与第一个示例中相同的单词两次不会按照我原先认为的方式将State类型绑定在一起。

当然，使用构造方法State状态只是一个偏好问题，如果我喜欢，我可以轻松地在Place类型中做 AmericanState State 。

This is probably another easy Haskell question. If I have some "nested" data types, such as in this example code:

data Place = Country
           | State
           | City String
           deriving Show

data State = California
           | NewYork
           deriving Show

data Country = USA
             | Canada
             deriving Show

I can legally make a list such as [USA, Canada] of type [Country], or [California, NewYork] of type [State], or [City "a", City "b"] of type [Place].

What do I have to do to make a list such as [USA, NewYork]? NewYork is a State which is a Place, and USA is a Country which is a Place, but ghci sees USA so it assumes I am making a list of Countrys (and NewYork is a State, so the list fails).

I think I need some way to cast a Country or State to a Place, but I'm at a loss on how to accomplish this.

I'm trying to avoid throwing the data contained within State and Country into the Place type, which I know would make it work, but I've got a decent amount of real data that I'd rather not jumble up like that.

解决方案

Here's a little more output that led me to figure out the problem:

*Main> [State, State]
[State,State]
*Main> :t State
State :: Place
*Main> :t NewYork
NewYork :: State

This seems to imply that the word "State" is a valid constructor for Place, and that the data State refers to a value of only California or NewYork.

If you change the program slightly, to:

data Place = Country Country
           | State State
           | City String
           deriving Show

data State = California
           | NewYork
           deriving Show

data Country = USA
             | Canada
             deriving Show

then you can make a list such as [Country USA, State NewYork] which is properly of type [Place]. Using the same word twice as in the first example does not "bind" the State type together in the way I had thought it would.

Of course, using the constructor State State is just a matter of preference, I could just as easily do AmericanState State within the Place type if I were so inclined.

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