Advent of Frege code, day 6

Advent6.fr

module Advent6 where

{-
    http://adventofcode.com/2017/day/6
-}

import Data.List

mapAtIndex :: (a->a) -> Int -> [a] -> [a]
mapAtIndex f n xs = take n xs ++ rest (drop n xs) where
    rest (y:ys) = f y : ys 
    rest []     = []       -- index is outside: no mapping
    
incAtIndex   = mapAtIndex (+1)
setAtIndex x = mapAtIndex (const x)     

startBank = [4,1,15,12,0,9,9,5,5,8,7,3,14,5,12,3]

distribute soMany fromIdx xs 
    | soMany == 0 -> 
        xs
    | fromIdx >= length xs -> 
        distribute soMany 0 xs
    | otherwise -> 
        distribute (soMany-1) (fromIdx+1) (incAtIndex fromIdx xs)

allBanks start = iterate step start where
    step bank = 
        let maxVal = maximum bank
            maxPos = unJust $ findIndex (== maxVal) bank 
        in  distribute maxVal maxPos (setAtIndex 0 maxPos bank)

banks      = allBanks startBank
idxBanks   = zip [0..] banks
duplFound  (untilIdx, bank) = elem bank (take untilIdx banks)
sndDuplIdx = length $ takeUntil duplFound idxBanks    

main = do      
    println $ "Number of distribution cycles: " ++ show sndDuplIdx