试图了解Haskell的=>与定义类型 [英] Trying to understand Haskell's => vs defining the types

问题描述

在Haskell中，为什么要定义具有类型约束的函数:

In Haskell, why would you define a function with a type constraint:

ghci> :t (==)  
(==) :: (Eq a) => a -> a -> Bool

而不是定义它的类型是:

Rather than defining it so it's type was:

ghci> :t (==)  
(==) :: Eq -> Eq -> Bool

推荐答案

您不会执行第二个版本，因为会遇到编译错误. Eq不是类型，它是类型类.您不能在需要类型的地方使用类型类.

You wouldn't do the second version because you'd get a compile error. Eq is not a type, it's a typeclass. You can't use a typeclass in places where a type is required.

如果您确实定义了自己的类型MyEq，然后定义了类型为MyEq -> MyEq -> Bool的函数==，则表达式"hello" == "hello"将无效，因为"hello"是String类型而不是type类型的值MyEq.由于haskell中没有子类型，因此不能同时将String和MyEq类型的值赋值.

If you did define your own type MyEq and then define a function == with the type MyEq -> MyEq -> Bool, the expression "hello" == "hello" would be invalid because "hello" is a value of type String and not of type MyEq. Since there is no subtyping in haskell a value couldn't be of type String and of type MyEq at the same time.

因此，如果要定义一个可以接受满足特定条件的不同类型的值的函数，则需要类型类.

So if you want to define a function that can accept values of different types which meet certain conditions, you need type classes.

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