copied solution from github

day01/CHANGELOG.md

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+# Revision history for day01
+
+## 0.1.0.0 -- YYYY-mm-dd
+
+* First version. Released on an unsuspecting world.

day01/app/Main.hs

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+module Main where
+
+import Day01 (day01Part1, day01Part2)
+
+main :: IO ()
+main = do
+  input <- readFile "./data/input.txt"
+  putStrLn $ "Part 1: " <> day01Part1 input
+  putStrLn $ "Part 2: " <> day01Part2 input

day01/day01.cabal

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+cabal-version:      3.6
+name:               day01
+version:            0.1.0.0
+
+-- A short (one-line) description of the package.
+-- synopsis:
+
+-- A longer description of the package.
+-- description:
+
+-- A URL where users can report bugs.
+-- bug-reports:
+
+-- The license under which the package is released.
+-- license:
+author:             Andrew Maier
+maintainer:         andrew@maier.name
+
+-- A copyright notice.
+-- copyright:
+-- category:
+extra-source-files: CHANGELOG.md
+
+executable day01
+    main-is:          Main.hs
+
+    -- Modules included in this executable, other than Main.
+    -- other-modules:
+
+    -- LANGUAGE extensions used by modules in this package.
+    -- other-extensions:
+    build-depends:    base ^>=4.14.3.0
+                    , day01
+    hs-source-dirs:   app
+    default-language: Haskell2010
+
+library
+    build-depends:    base ^>=4.14.3.0
+    hs-source-dirs:   lib
+    exposed-modules:  Day01
+    default-language: Haskell2010
+

day01/lib/Day01.hs

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+module Day01 where
+
+import Data.Maybe (fromMaybe)
+import Text.Read (readMaybe)
+
+day01Part1 :: String -> String
+day01Part1 s = case depths s of
+    Right ds -> show . countIncreases . changes $ ds
+    Left  e  -> e
+
+depths :: String -> Either String [Int]
+depths s = case maybeDepths s of
+    Just ds -> Right ds
+    Nothing -> Left "Error processing data"
+    where
+        maybeDepths :: String -> Maybe [Int]
+        maybeDepths = mapM readMaybe . lines
+
+data ChangeType = Increase | Decrease | Level deriving (Show, Eq)
+
+changes :: [Int] -> [ChangeType]
+changes s = zipWith zipper (tail s) s
+    where
+        zipper :: Int -> Int -> ChangeType
+        zipper n2 n1 | n1 < n2 = Increase
+                     | n1 > n2 = Decrease
+                     | otherwise = Level
+
+countIncreases :: [ChangeType] -> Int
+countIncreases = sum . map (\ch -> if ch == Increase then 1 else 0)
+
+day01Part2 :: String -> String
+day01Part2 s = case depths s of
+    Right ds -> show . countIncreases . changes . averages $ ds
+    Left  e  -> e
+
+averages :: [Int] -> [Int]
+averages s = zipWith3 (\a b c -> a + b + c) (drop 2 s) (tail s) s

project.cabal

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+packages: ./day*/*.cabal