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)
Again, this is solving a different problem than what the OP stated. Using your definition: Prelude> :l unique [1 of 1] Compiling Main ( unique.hs, interpreted ) Ok, modules loaded: Main. *Main> unique [1,2,3,1] [1,2,3] *Main> ...which behaves like the Unix utility 'uniq'. But the problem described originally is to write a function which produces [2,3] when given the same input; 1 is not included in the output since it occurs more than once. -Brent