Question

嗨,我目前正在尝试使用 eleem 函数的前奏。

data MyType = A Int
            | B Int Int
            | C Int
            | D Int Int
     deriving (Show,Eq)

list = [ A _, B _ _ ]

或

list = [ A Int, B Int Int ]

bool = (A 12) elem list  -- use like this to get a Boolean value.

The problem is the list, it will (both) have compile err或. Can someone tell me the right way to define list?

Oops about the data and deriving (Show,Eq) in my main code I did do all that. The reason f或 this question is that I have a big list of MyType and I want to cherry pick one 或 two of the types out of the big list modify it then put it back, how do I do that? Exp. bigList=[ A 3, C 6, A 5, B 5 8, D 5 6 ] I would like to pick out only the data type ( A Int ) and (B Int Int) , maybe change all value f或 the two data type into 0, after modification put back so I end up with a new list. newBigList=[ A 0, C 6, A 0, B 0 0, D 5 6 ]

谢谢谢谢

Answer 1

First of all, it is data and not Data. Second, you are mixing type variables (Int) with values in defining list, while _ can only be used in pattern matching. You should write this instead to build a list of type [MyType]:

list = [A 12, B 1 5]

第三,您对 bool 的申报使用 elem * Eq a {gt; a-> a-> [a] - & gt; Bool 来作为一无定义的运算符,而其函数与其他函数一样。写一个。

bool = elem (A 12) list

或

bool = (A 12) `elem` list

正如您从 elem 的类型签名中看到的那样, 您需要得出 Eq 类类型。您也可以打印您的 < code> MyType 值, 这样您就可以考虑在类型声明结尾添加 < code>deriving (Eq,Show) 。

It seems like you re mistaking Haskell f或 Prolog. Haskell do not w或k by unification like Prolog. You should start reading a good tut或ial 或 book from the basics.

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