Haskell 中的强制多参数类型
我有一个类型
class IntegerAsType a where
value :: a -> Integer
data T5
instance IntegerAsType T5 where value _ = 5
newtype (Num a, IntegerAsType n) => PolyRing a n = PolyRing [a]
我的主要问题是：如何在特定的 PolyRing 中定义变量？
应该是这样的：（
x = [1, 2, 3] :: Integer T5
我认为） 问题是：:: 之后的正确语法是什么？
我收到错误
Couldn't match expected type `PolyRing Integer T5'
with actual type `[t0]'
In the expression: [1, 2, 3] :: PolyRing Integer T5
In an equation for `x': x = [1, 2, 3] :: PolyRing Integer T5
此外，我正在寻找更好的方法来实现此目的。特别是，我真的希望从列表元素的类型推断出类型 a ，而必须指定 IntegerAsType n （它不应该依赖于列表的长度，即使这是可能的）。
到目前为止我尝试过的事情：
x = [1,2,3] :: PolyRing (Integer, T5)
x = [1,2,3] :: PolyRing Integer T5
I have a type
class IntegerAsType a where
value :: a -> Integer
data T5
instance IntegerAsType T5 where value _ = 5
newtype (Num a, IntegerAsType n) => PolyRing a n = PolyRing [a]
My main question is: how do I define a variable in a particular PolyRing?
It should be something like:
x = [1, 2, 3] :: Integer T5
(I think)
The question is: what is the correct syntax after the ::?
I'm getting the error
Couldn't match expected type `PolyRing Integer T5'
with actual type `[t0]'
In the expression: [1, 2, 3] :: PolyRing Integer T5
In an equation for `x': x = [1, 2, 3] :: PolyRing Integer T5
Also, I'm looking for a better way to implement this. In particular, I'd really like for the type a to be inferred from the type of list elements, while the IntegerAsType n must be specified (it shouldn't depend on the length of the list, even if that is possible).
Things I've tried so far:
x = [1,2,3] :: PolyRing (Integer, T5)
x = [1,2,3] :: PolyRing Integer T5
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评论（2）
首先注意
数据类型上下文，例如：
通常是一个坏主意，并且已从该语言中退役。
忽略这一点
要构造一个实例，您必须使用数据构造函数
PolyRing：但这还不够，到目前为止的类型推断将为
(IntegerAsType n) = > PolyRing 整数 n。您的最终类型签名将完成此操作let x = PolyRing [1,2,3] :: PolyRing Integer T5。回到第一个注释
不过，也许您想要：
每个构建或使用多环的函数都可以强制执行所需的约束：
First Note
Data type contexts, such as:
are generally a bad idea and have been retired from the language.
Ignoring That
To construct an instance you must use the data constructor
PolyRing:But that isn't enough, the type inference so far will be
(IntegerAsType n) => PolyRing Integer n. Your final type signature would finish this uplet x = PolyRing [1,2,3] :: PolyRing Integer T5.Returning To the First Note
Still, perhaps you wanted:
And every function that builds or consumes a polyring can enforce the needed constraints:
newtype不仅仅是一个同义词，而且是一种创建在类型级别上不同的类型的方法（尽管稍后相同）。也就是说 - 您需要使用数据构造函数显式包装它。此外，上下文也没有影响。您仍然必须到处输入它。A
newtypeis not just a synonym but a way to create a type that is distinct at the type level (though identical later). That said - you need to wrap it explicitly using your data constructor. Also, the context has no effect. You still have to type it everywhere.