While we're at it, the second pattern match is superfluous. Once you take out nonempty lists from the list type, the only thing left is an empty one! unique :: Eq a => [a] -> [a] unique (x:xs) = x : (unique . filter (/= x) $ xs) unique _ = [] One question left: would the second definition be more efficient if written: unique e = e Inquiring minds want to know... Dan Dan Weston wrote:
Now that I mention it, the base case is much less often called than the recursive case, so I would in hindsight flip the order of the two (mutually exclusive) partial function definitions:
unique :: Eq a => [a] -> [a] unique (x:xs) = x : (unique . filter (/= x) $ xs) unique [] = []
This should be marginally more efficient. I doubt that GHC would automatically detect that a) they are mutually exclusive and b) the second is called less often than the first!
Dan
Dan Weston wrote:
Close. Actually, the author upstream (i.e. me) had in mind:
uniq :: Eq a => [a] -> [a] uniq [] = [] uniq (x:xs) = x : uniq (filter (/= x) xs)