Day 2, parts 1 & 2: Initial solutions.

Day2/Part1.hs

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+#! /usr/bin/env stack
+-- stack --resolver lts-12.20 --install-ghc script
+module Main where
+
+import Control.Arrow ((&&&))
+import Data.List (group, sort)
+
+main :: IO ()
+main = interact $ show . \input ->
+   let codes  = lines input :: [[Char]]
+       counts = map (map (head &&& length) . group . sort) codes :: [[(Char, Int)]]
+       twos   = length $ filter (any ((== 2) . snd)) counts
+       threes = length $ filter (any ((== 3) . snd)) counts
+   in (twos, threes, twos * threes)

Day2/Part2.hs

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+#! /usr/bin/env stack
+-- stack --resolver lts-12.20 --install-ghc script
+module Main where
+
+loopRight :: b -> (b -> Either a b) -> a
+loopRight b f = case f b of
+   Left  a  -> a
+   Right b' -> loopRight b' f
+
+main :: IO ()
+main = interact $ show . \input ->
+   let codes = lines input :: [[Char]]
+   in loopRight ([], codes) $ \(seen, c:cs) ->
+         case filter (`isSimilarTo` c) seen of
+            []    -> Right (c:seen, cs)
+            (x:_) -> Left (c, x, foldMap (\(a,b) -> if a == b then [a] else [])
+                                         (zip c x))
+
+isSimilarTo :: (Eq a) => [a] -> [a] -> Bool
+(x:xs) `isSimilarTo` (y:ys)
+   | x == y    = xs `isSimilarTo` ys
+   | otherwise = xs == ys
+_ `isSimilarTo` _ = False

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