如何输入演员表 [英] How to type cast

问题描述

C＃：

static int F(object x)
{
 return x is string ? 1 : 2;
}

Haskell吗？
在我看来，Haskell没有根类型 object 。

编辑：我不在乎转换为字符串。我想知道如何进行类型转换（例如，查看对象是客户还是订单。

Edited: I do not care about converting to string. I want to know how to typecast (for example to see if an object is a Customer or an Order.

推荐答案

在Haskell中，所有允许转换为字符串的类型都实例化 Show 类型类，该类型类提供了

In Haskell, all types that allow a conversion to a string instantiate the Show typeclass which provides

show :: Show a => a -> String

所以您的整个代码不过是

So your whole code is nothing but

f x = show x

或

f = show

::显示一个=> a->字符串（所有可转换为字符串的类型 a ，取该类型的值并返回

with the same generic type f :: Show a => a -> String (Forall types a that are conversible to a string, take a value of this type and return a string).

请注意，您不必像C＃中那样进行显式的运行时类型检查；通用模板是在编译时解析的。您不需要多态的根类型-实际上，像C＃这样的类型转换会有些复杂，并且会违反l痛苦的概念。

Note that you don't have to do an explicit, run-time type-check like in C#; the generic template is resolved at compile-time. You don't need a polymorphic root type - A cast like in C# would in fact be somewhat complicated and against the language's conception. Instead of allowing arbitrary casts between types, it defined typeclasses for certain meaningful conversions.

请注意，在编译时检查兼容性：

Note that compatibility is checked at compile-time:

-- Working
f 1
f "Hallo"
f (1, 2)
f [1, 2, 3]

-- Not working
f (\x -> x + 1) 

针对您已编辑的问题：

正如我之前说过的，Haskell不允许任意转换（无非常非常不安全的代码）。而且由于Haskell不是面向对象的，因此不存在需要任何强制转换的继承关系。根本就没有没有意义的 object 值需要运行时检查/广播。为了表达替代方案，您必须定义一个联合类型，类型类或使用 Ething 类型。

As I said before, arbitrary conversions aren't allowed in Haskell (without very very unsafe code). And since Haskell is not object-oriented, there is no inheritance relationship that required any cast. There simply aren't meaningless object values that needed runtime-checking/casting. For expressing alternatives, you'll have to define a union type, a typeclass or use the Either type.

在哪种情况下，您遇到的对象是 Customer 还是 Order ？该类型的值简直是荒谬的。请再次说明。

In what case do you encounter an object that is a Customer or an Order? A value of that type is simply nonsensical. Please clarify again.

以您的记录器示例为例：
您将需要一个类型类：

As to your logger example: You'll need a typeclass:

class Loggable a where
    writeToLog :: a -> IO ()

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