On 7/11/07, Brent Yorgey <[EMAIL PROTECTED]> wrote:
> 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
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