[Haskell-beginners] a simple problem

[Markus Läll]

Hi Anonymous W!

As list is a recursive datastructure, you need a recursive (or
folding) function to check for duplicates. Right now your function
really does accept only two-element lists.

The function would look something like this:

maybeA list@ ((a1,b1) : (a2,b2) : rest) =
   if hasUniqueKeys list
      then Just (a1, b1, b2)
      else Nothing
   where
      hasUniqueKeys xs = ... check the list ...

You also need to decide what to do, if the list is empty or contains
only one pair.


Markus Läll

On Thu, Nov 4, 2010 at 12:31 AM, Anonymous W <w_anonymous at ymail.com> wrote:
> Hi,
> I'm having trouble with this small exercise.
>
> A function maybeA takes a list of pairs. A pair has a variable 'a' and a
> value 'b'. If any first element of a pair equals to any first element of
> another pair, the function returns Nothing. If they aren't equal, the
> function returns a tuple containing the first variable, its value and the
> value of the second variable.
> maybeA :: [(a,b)] -> Maybe (a,b,b)
>
>
> For example:  [(a1,b1),(a2,b2),(a3,b3)..] returns Nothing if a2 = a3
> and returns Just (a2,b2,b3) if  a2 /= a3
>
>
>
> I'm currently have :
>
> maybeA [(a1,b1),(a2,b2)]
>
>     |a1 == a2 = Nothing
>
>     |a1 /= a2 = Just (x1,y1,y2)
>
> but it only takes 2 pairs. Any helps?
>
>
>
> Thanks
>
>
>
>
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