[Haskell-cafe] Idiomatically using lists

[kevin birch]

On 火, 2007-6月-05, at 02:54, Greg Fitzgerald wrote:

> > rotating the fourth element 2 positions would result in: [1, 2,  
> 4, 3, 5]
> Seems odd.  Should that be [4,1,2,3,5]?
>
Yes, I meant to use the 5 element in my second example.  Sorry for  
the confusion.

> > Is there an idomatic way to handle both of these cases in a  
> function?
> Generally people like to see your attempt at a solution before  
> giving the idomatic one so that they are sure it's not a homework  
> question.  What do you have so far?
>
Yeah, I only wish I had gone to a school that would be forward  
thinking enough to each FP.  ;-)

Here is my version:

rotate :: Array Integer Card -> Integer -> Integer -> Array Integer Card
rotate a i n
     | i <= u - n = a // [(i, a ! (i + 1)), (i + 1, a ! (i + 2)), (i  
+ 2, a ! i)]
     | otherwise = a // zip [l..u] (h ++ [a ! i] ++ filter (not . (==  
(a ! i))) t)
     where (l, u) = bounds a
           (h, t) = splitAt (fromInteger ((i - u) + n)) $ elems a

This function is part of my implementation of the Solitaire  
encryption algorithm, so that is why I have the reference to a Card  
data type.  This does what I want, and seems basically idiomatic, but  
perhaps it could be better.

Thanks,
Kevin

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