[Haskell-cafe] function unique

[Alexteslin]

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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