advent-of-code/2020/day06/part1.hs

import Data.List


main :: IO ()
main = do
    input <- readFile "input"
    print . sum . map length . groups $ input


-- Collect each group's answers
groups :: String -> [String]
groups = map (foldr1 union . lines) . dlfsplit

-- Split a string into a list across all the double newlines in it
dlfsplit :: String -> [String]
dlfsplit str = a : case b of
        [] -> []
        b -> dlfsplit b
    where (a,b) = dlfsplitonce str

-- Split a string into two across the first double newline found
dlfsplitonce :: String -> (String, String)
dlfsplitonce [] = ([], [])
dlfsplitonce (ca:cb:rest)
    | ca == '\n' && cb == '\n' = ([], rest)
    | otherwise                = (ca:before, after)
        where (before, after) = dlfsplitonce (cb : rest)
dlfsplitonce c = (c, [])