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Question

I\'ve been trying to define a function which, given a list of Integers and an Integer n, returns a Boolean indicating whether n occurs exactly once in the list.

I have t

Answer 1

Here is another version:

once x = not . (\xs -> null xs || x `elem` tail xs) . dropWhile (/= x)

--lambda hater version
import Control.Applicative
once x = not . ((||) <$> null <*> (elem x).tail) . dropWhile (/= x)

Of course it can't deal with infinite lists that contain zero or one x, but at least it terminates in case of more than one occurrence of x.