Haskell：检查Int是否在Int列表中

Question

I am new to Haskell, sorry if this is a basic question.

I currently have a list of Int's and I am trying to create a function that takes a variable x and returns a boolean depending whether the variable exists in the list.

I have had a search and found Data.List find function but this dosent seem to return a boolean.

I am using GHCi.

Thanks,

Answer 1

First find the type of the function you need.

To "Check if" means to return either True or False, a Bool.

So the function takes an Int, a list of Int (aka [Int]) and returns Bool:

Int -> [Int] -> Bool

Now ask hoogle .

elem :: Eq a => a -> [a] -> Bool

Hoogle is a very useful tool. You can integrate it with ghci .

Answer 2

If the standard elem function didn't exist, you could have been on the right track with find .

myElem :: (Eq a) => a -> [a] -> Bool
myElem x = maybe False (const True) . find (== x)

There's lots of other ways to implement it too, like

myElem x = any (== x)
myElem x = or . map (== x)
myElem x = not . null . filter (== x)
myElem x = foldr (\y b -> y == x || b) False

etc.

Answer 3

I'm in my 2 months of trying to learn Haskell during my spare time. Professionally, I do C/C++ for several years. I must say, that the first month of learning Haskell was a headspin experience. I always try to do things on my own if the problem is simple enough rather than using existing APIs like elem . I'm slowly learning the FP way, and below is my solution:

isMember n [] = False
isMember n (x:xs)
    | n == x = True
    | otherwise = isMember n xs

Usage:

isMember 2 [1,9,4,5] -- False
isMember 2 [4,5,2,9] -- True

Answer 4

i did it more simply like this.

l=[1,2,3,4,5]


checkIfElem :: Int -> [Int] ->Bool
checkIfElem x l 
         |x`elem` l =True
         |otherwise=False