[Haskell-cafe] function unique

[Philip Armstrong]

On Wed, Jul 11, 2007 at 10:31:36PM +0200, Hugh Perkins wrote:
>   Ok so I played with the tweaked problem (Unix 'uniq'), and getting it to
>   be lazy.  This seems to work:

It's slightly worse than that: unix uniq only eliminates *consecutive*
identical lines from its input. If you want to eliminate all
duplicates, you have to pipe your input through sort first.

>   testunique :: Eq a => [a] -> [a]
>   testunique list = testunique' list []
>      where testunique' :: Eq a => [a] -> [a] -> [a]
>            testunique' [] elementssofar = []
>            testunique' (x:xs) elementssofar | x `elem` elementssofar =
>   (testunique' elementssofar xs)
>                                             | otherwise = x : ( testunique'
>   xs (x:elementssofar))

I suspect the author upthread had something more like this in mind:

uniq :: Eq a => [a] -> [a]
uniq []     = []
uniq (x:xs) = x : filter (x/=) (uniq2 xs)

Obviously a true 'unique' function has to traverse the entire list
before it creates any output, as has been pointed out upthread.

Phil

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