On 7/11/07, Brent Yorgey
take 5 (unique [1..])
I don't think this is possible. Perhaps you misread the original problem description? The unique function is supposed to return a list of those elements which occur exactly once in the input list, which is impossible to determine for an infinite input list (the only way to prove that a given element occurs only once in a list, in the absence of any other information, is to examine every element of the list). Of course, a function that behaves like the unix utility "uniq" ( i.e. returning only one copy of every list element) is possible to implement lazily in the manner you describe.
Why wouldn't this work? (I haven't tested it, sorry) unique = unique' [] unique' _ [] = [] unique' history (x:xs) = if x `elem` history then next else (x:next) where next = (uniq' (x:hist) xs) Whether or not it's a good idea is a separate issue. Antoine