unique elements in a haskell list

Question

okay, this is probably going to be in the prelude, but: is there a standard library function for finding the unique elements in a list? my (re)implementation, for clarificat

Answer 1

I think that unique should return a list of elements that only appear once in the original list; that is, any elements of the orginal list that appear more than once should not be included in the result.

May I suggest an alternative definition, unique_alt:

    unique_alt :: [Int] -> [Int]
    unique_alt [] = []
    unique_alt (x:xs)
        | elem x ( unique_alt xs ) = [ y | y <- ( unique_alt xs ), y /= x ]
        | otherwise                = x : ( unique_alt xs )

Here are some examples that highlight the differences between unique_alt and unqiue:

    unique     [1,2,1]          = [2,1]
    unique_alt [1,2,1]          = [2]

    unique     [1,2,1,2]        = [1,2]
    unique_alt [1,2,1,2]        = []

    unique     [4,2,1,3,2,3]    = [4,1,2,3]
    unique_alt [4,2,1,3,2,3]    = [4,1]