gistfile1.hs

{-# LANGUAGE ViewPatterns #-}
-- problem: given a list of numbers and a counter in a structure C [Int] Int
-- write a function step :: Int -> C -> C which
--   change first occurrence of a multiple of a given number in a list in its successive 
--   and subtract it to the counter 
--      or 
--   subtract 1 to the counter and add 1 to all numbers in the list.
-- step 3 (C [1,2,3,5,6] 7) == C [1,2,4,5,6] 4
-- step 3 (C [1,2,4,5,7] 16) == C [2,3,5,6,8] 15 


import Control.Arrow (second)

data C = C [Int] Int deriving Eq

select :: (a -> Bool) -> [a] -> Maybe (a,a -> [a])
select f [] = Nothing
select f (x:xs) 
        | f x = Just (x,(:xs))
        | otherwise =  fmap (second $ fmap (x:)) $ select f xs

step :: Int -> C -> C
step n (C (select ((==0). (`mod` n)) -> Just (x,g)) y) = C (g $ succ x) (y - x)
step _ (C xs y) = C (map succ xs) $ pred y