Question

Trying to learn Haskell. I am trying to write a simple function to remove a number from a list without using built-in function (delete…I think). For the sake of simplicity, let’s assume that the input parameter is an Integer and the list is an Integer list. Here is the code I have, Please tell me what’s wrong with the following code

areTheySame :: Int -> Int-> [Int]

areTheySame x y | x == y = []
                | otherwise = [y]

removeItem :: Int -> [Int] -> [Int]

removeItem x (y:ys) = areTheySame x y : removeItem x ys

Answer 1

The others are right that the problem is the : operator. I would say that your areTheySame function that returns a list is the wrong approach anyway, though. Rather than switch to the ++ operator, a better implementation of that function would be:

removeItem _ []                 = []
removeItem x (y:ys) | x == y    = removeItem x ys
                    | otherwise = y : removeItem x ys

As you can see, this is a pretty simple implementation. Also, consing like this is much less taxing for your program than appending a bunch of lists together. It has other benefits as well, such as working lazily.