Learning Haskell: How to remove an item from a List in Haskell

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.

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