Advent of Code 2017 01 Captchas in Haskell

2017-01-captcha.hs

#!/usr/bin/env stack
-- stack --resolver lts-9.14 script --package array

{-# LANGUAGE ScopedTypeVariables #-}

import System.Environment
import Data.Array.IArray

captchaPairs :: (IArray a e, Ix i, Num i) => a i e -> ((i, i) -> i) -> [(e, e)]
captchaPairs es getPairIndex =
    let (_, maxIndex) = bounds es
        createPair i = (es ! i, es ! getPairIndex (i, maxIndex + 1))
        pairArray = map createPair (indices es)
        in pairArray

getSum :: [(Int, Int)] -> Int
getSum pairs =
    let equalPairs = filter (uncurry (==)) pairs
        firsts = map fst equalPairs
        in sum firsts

solve :: String -> ((Int, Int) -> Int) -> Int
solve s getPairIndex =
    let (nums :: Array Int Int) = listArray (0, length s - 1) $ map (\c -> read [c]) s
        pairs = captchaPairs nums getPairIndex
        in getSum pairs

main :: IO ()
main = do
    args <- getArgs
    let typeOneSolution = solve (head args) (\(i, l) -> (i + 1) `mod` l)
    putStrLn ("Type one solution: " ++ show typeOneSolution)
    let typeTwoSolution = solve (head args) (\(i, l) -> (i + (l `div` 2)) `mod` l)
    putStrLn ("Type two solution: " ++ show typeTwoSolution)