feat(day1): Use parsec instead of parsing by hand

day1/Main.hs

@@ -1,33 +1,18 @@
 module Main where
 
 import System.Environment (getArgs)
-import System.IO (Handle, hIsEOF, hGetLine, withFile, IOMode(ReadMode))
-import Text.Read (readMaybe)
 
 import Data.List (sort)
-import Data.Char (isDigit, isSpace)
 
-mapSnd :: (b -> c) -> (a, b) -> (a, c)
-mapSnd f (a, b) = (a, f b)
+import Text.Parsec (parse, many, char, endBy)
+import Text.ParserCombinators.Parsec (GenParser)
+import Text.ParserCombinators.Parsec.Number (decimal)
 
-parseLine :: String -> IO (Int, Int)
-parseLine line = let (l, (_, r)) = (mapSnd (span isSpace). span isDigit) line
-                 in
-                    case readMaybe l >>= \l' -> readMaybe r
-                                     >>= \r' -> return (l', r') of
-                        Just a -> return a
-                        Nothing -> fail "Input file is incorrectly formatted"
+line :: GenParser Char st (Int, Int)
+line = decimal >>= \l -> many (char ' ') >> decimal >>= \r -> return (l, r)
 
-readLists :: Handle -> IO ([Int], [Int])
-readLists h = do
-                  shouldStop <- hIsEOF h
-                  if shouldStop
-                      then return ([], [])
-                      else do
-                          line <- hGetLine h
-                          (l, r) <- parseLine line
-                          (ls, rs) <- readLists h
-                          return (l:ls, r:rs)
+locationIDs :: GenParser Char st [(Int, Int)]
+locationIDs = endBy line (char '\n')
 
 computeDistance :: [Int] -> [Int] -> Int
 computeDistance l r = sum (zipWith (curry (abs . uncurry (-))) l r)
@@ -36,6 +21,9 @@ main :: IO ()
 main = do
     args <- getArgs
     (fp:_) <- return args
-    (l, r) <- withFile fp ReadMode readLists
-
-    putStrLn ("Solution (Part 1) : " ++ show (computeDistance (sort l) (sort r)))
+    content <- readFile fp
+    case parse locationIDs fp content of
+        Left e -> putStrLn ("Input file is incorrectly formatted : " ++ show e)
+        Right ids -> let (l, r) = unzip ids
+                     in putStrLn ("Solution (Part 1) : "
+                         ++ show (computeDistance (sort l) (sort r)))

kalmennAOC2024.cabal

@@ -15,6 +15,9 @@ common warnings
 executable day1
     import:           warnings
     main-is:          Main.hs
-    build-depends:    base ^>=4.18.2.1
+    build-depends:
+                      base ^>=4.18.2.1,
+                      parsec ^>= 3.1.17.0,
+                      parsec-numbers ^>= 0.1.0
     hs-source-dirs:   day1
     default-language: Haskell2010