On 7/11/07, Alexteslin <alexteslin@yahoo.co.uk> wrote:

Oh, I am lost now - for now anyway.
I am attempting to do next exercise in the book to define reverse function
using primitive recursion and pattern matching on lists.  But getting stack
because when i con in front of xs (xs:x) i get en error, which i thought i
would be getting anyway.  I tried to define a helper function and cons there
in front of xs and i get type errors again.

I know these are easy and boring questions but i would appreciate a hint.

Thank you

Neil Mitchell wrote:
>
> Hi
>
>> unique = unique' []
>>
>> unique' _ [] = []
>> unique' history (x:xs) = if x `elem` history
>>   then next
>>   else (x:next) where next = (uniq' (x:hist) xs)
>
> You can express this more neatly:
>
> unique' _ [] = []
> unique' history (x:xs) = [x | x `notElem` history] ++ unique' (x:history)
> xs
>
> Thanks
>
> Neil
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>
>

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