Simple problem from advent of code 2017. Solved as complicated as possible in as few lines as possible.

day1.hs

import Data.Monoid
import Data.Char (digitToInt)
import Data.Foldable (foldrM)
--- Part One ---

-- Bind using the reader monad:
-- x >>= k = \w-> k (x w) w
-- head >>= foldrM bar = \w -> foldrM bar (head w) w

foo = head >>= foldrM bar

-- The writer monad will sum the first elements
-- the second element will be fed into bar again in the fold.
bar x y | (x==y) = (x,x) 
        | otherwise = (mempty,x)

-- Read a line, read out the digits and wrap them in the Sum monoid.
-- Send the list of Sums to foo.
-- Unwrap the Sum from the Writer.
-- Print it.
main = 
    fmap 
        (getSum . fst . foo
            . map (Sum . digitToInt))
        getLine
    >>= print