Haskell-递归检查关联列表的元素

Question

Say I have the following associative list:

names = [(6548712, "Charlie Brown"), (27378912, "Linux Van Pelt"), (5831457, "Peppermint Patty")]

I want to check if a given tuple, such as...

 (6548712, "Charlie Brown")

...exists within the associative list.

Here is my function:

check :: (String,a) -> [(String,a)] -> Bool
check val (x:xs)
    | x == val = True
    | otherwise = check val xs

The function does not work. I think the error lies in my first guard, since I do believe my recursive call is correct. I want to be able to do this using recursion, can someone help me out?

Answer 1

1. Your data has Number and String but your function reads String and Number. So that has to be changed to this

    check :: (Int, String) -> [(Int, String)] -> Bool 

2. Your recursion should have a base condition, like this

    check val [] = False 

   It means that if the list is empty (all elements are checked), return False .

So your function should look like this

check :: (Int, String) -> [(Int, String)] -> Bool

check val [] = False
check val (x:xs)
    | x == val = True
    | otherwise = check val xs

Online Demo

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As it is, you can make your function Polymorphic by doing

check :: Eq a => (a, String) -> [(a, String)] -> Bool

To know more about Eq a , please check this excellent answer

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Just for the sake of completion, you can always use elem function, like this

Prelude> elem (6548712, "Charlie Brown") [(6548712, "Charlie Brown"), (27378912, "Linux Van Pelt"), (5831457, "Peppermint Patty")]
True