Day 5, parts 1 & 2: Initial solutions.

Day5/Part1.hs

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+#! /usr/bin/env stack
+-- stack --resolver lts-12.20 --install-ghc script
+{-# LANGUAGE FlexibleContexts, LambdaCase #-}
+module Main where
+
+import Data.Function (fix)
+import qualified Data.Char as C
+import qualified Data.List as L
+
+main :: IO ()
+main = interact $ show . react . takeWhile C.isAlpha
+
+react input =
+   let scan (x:y:zs) = if x `reactsWith` y then scan zs else (x : scan (y:zs))
+       scan zs = zs
+       input' = scan input
+       result = (input', length input')
+   in if input' == input then result else react input'
+
+x `reactsWith` y = (C.isUpper x && C.toLower x == y) ||
+                   (C.isLower x && C.toUpper x == y)

Day5/Part2.hs

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+#! /usr/bin/env stack
+-- stack --resolver lts-12.20 --install-ghc script
+{-# LANGUAGE FlexibleContexts, LambdaCase #-}
+module Main where
+
+import Data.Function (fix, on)
+import qualified Data.Char as C
+import qualified Data.List as L
+
+main :: IO ()
+main = interact $ \input -> show $ L.minimumBy (compare `on` snd) $
+   [ react $ filter (\c -> C.isAlpha c && c /= r && c /= C.toUpper r) input
+   | r <- ['a'..'z']
+   ]
+
+react input =
+   let scan (x:y:zs) = if x `reactsWith` y then scan zs else (x : scan (y:zs))
+       scan zs = zs
+       input' = scan input
+       result = (input', length input')
+   in if input' == input then result else react input'
+
+x `reactsWith` y = (C.isUpper x && C.toLower x == y) ||
+                   (C.isLower x && C.toUpper x == y)