haskell - 如何解释(等式a)

Question

我需要创建一个包含两个参数的函数，一个 Int 和一个 [Int]，它返回一个新的 [Int]删除所有出现的第一个参数。

我可以通过列表理解和列表递归轻松创建该函数。但是，我使用这些参数来做到这一点:

deleteAll_list_comp :: Integer -> [Integer] -> [Integer]
deleteAll_list_rec :: (Integer -> Bool) -> [Integer] -> [Integer]

但是，对于我的作业，我所需的参数是

deleteAll_list_comp :: (Eq a) => a -> [a] -> [a]
deleteAll_list_rec :: (Eq a) => a -> [a] -> [a]

我不知道如何阅读这个语法。正如 Google 告诉我的，(Eq a) 只是向 Haskell 解释 a 是一种可比较的类型。但是，我不明白这一点，因为所有 Int 自然都是可比较的。我如何使用这些参数来解释和实现这些方法？我的意思是，开始时的参数到底是什么？

<小时/>

@groovy @pelotom

谢谢，这已经很清楚了。我现在明白了，它实际上只要求两个参数，而不是三个。但是，我仍然遇到此代码的问题。

deleteAll_list_rec :: (Eq a) => a -> [a] -> [a]
delete_list_rec toDelete [] = []
delete_list_rec toDelete (a:as) =
        if(toDelete == a) then delete_list_rec toDelete as
        else a:(delete_list_rec toDelete as)

这给了我一个“deleteAll_list_rec的类型签名 缺少随附的绑定(bind)”，这对我来说毫无意义，因为我如何正确绑定(bind)需求，不是吗？根据我的小经验，(a:as) 在提取其中的第一个元素。为什么这会生成错误，但是

deleteAll_list_comp :: (Eq a) => a -> [a] -> [a]
deleteAll_list_comp toDelete ls = [x | x <- ls, toDelete==x]

不是吗？

<小时/>

2/7/13 更新:对于所有将来可能遇到同样问题的人，我已经找到了一些有关 Haskell 的好信息，特别是我的问题，在此链接:http://learnyouahaskell.com/types-and-typeclasses

"Interesting. We see a new thing here, the => symbol. Everything before the => symbol is >called a class constraint. We can read the previous type declaration like this: the >equality function takes any two values that are of the same type and returns a Bool. The >type of those two values must be a member of the Eq class (this was the class constraint).

The Eq typeclass provides an interface for testing for equality. Any type where it makes >sense to test for equality between two values of that type should be a member of the Eq >class. All standard Haskell types except for IO (the type for dealing with input and >output) and functions are a part of the Eq typeclass."

Answer 1

考虑参数的一种方法可能是:

(Eq a) => a -> [a] -> [a]

(Eq a) =>   means any a's in the function parameters should be members of the 
            class Eq, which can be evaluated as equal or unequal.*

a -> [a]    means the function will have two parameters: (1) an element of
            type a, and (2) a list of elements of the same type a (we know that 
            type a in this case should be a member of class Eq, such as Num or 
            String).

-> [a]      means the function will return a list of elements of the same 
            type a; and the assignment states that this returned list should 
            exclude any elements that equal the first function parameter, 
            toDelete.

(*根据 pelotom 的评论进行编辑)