refactor: Handle file read error

src/Day1.hs

@@ -4,6 +4,7 @@ module Day1
   )
 where
 
+import Control.Exception (try)
 import qualified Data.Bifunctor as Bifunctor
 import Data.Either
 import Data.List (sort)
@@ -12,14 +13,22 @@ import Prelude hiding (id)
 
 type LocationID = Int
 
-data Error = ParseLocationError String String | ParseLineError String
+data ParseError
+  = ParseLocationError String String
+  | ParseLineError String
+  deriving (Show)
+
+data Error
+  = ReadFileError String
+  | ParseInputError [ParseError]
   deriving (Show)
 
 partOne :: String -> IO (Either Error Int)
 partOne filepath = do
-  contents <- readFile filepath
+  result <- tryReadFile filepath
-  let score = calculateScore <$> parseInput contents
+  case result of
-  return score
+    Left readError -> return (Left readError)
+    Right contents -> return (calculateScore <$> parseInput contents)
   where
     -- To calculate the overall score we just need to
     -- sort the lists and calculate the distances between each element
@@ -29,9 +38,10 @@ partOne filepath = do
 
 partTwo :: String -> IO (Either Error Int)
 partTwo filepath = do
-  contents <- readFile filepath
+  result <- tryReadFile filepath
-  let score = calculateScore <$> parseInput contents
+  case result of
-  return score
+    Left readError -> return (Left readError)
+    Right contents -> return (calculateScore <$> parseInput contents)
   where
     -- To calculate the overall score we need to sum all the similarity scores
     -- of each element from the left list applied to the right list
@@ -42,31 +52,30 @@ partTwo filepath = do
     count x = length . filter (== x)
 
 parseInput :: String -> Either Error ([LocationID], [LocationID])
-parseInput input = do
+parseInput input = Bifunctor.first ParseInputError $ unzip <$> parseLines (lines input)
-  parsedLines <- parseLines (lines input)
-  return (unzip parsedLines)
 
-parseLines :: [String] -> Either Error [(LocationID, LocationID)]
+parseLines :: [String] -> Either [ParseError] [(LocationID, LocationID)]
-parseLines xs = case take 1 errors of
+parseLines xs =
-  [parseError] -> Left parseError
+  if null errors
-  _ -> Right parsedLines
+    then Right parsedLines
+    else Left errors
   where
     errors = lefts $ map (parseWords . words) xs
     parsedLines = rights $ map (parseWords . words) xs
 
-parseWords :: [String] -> Either Error (LocationID, LocationID)
+parseWords :: [String] -> Either ParseError (LocationID, LocationID)
 parseWords [left, right] = do
   leftLocation <- parseLeft
   rightLocation <- parseRight
   return (leftLocation, rightLocation)
   where
-    parseLeft :: Either Error LocationID
+    parseLeft :: Either ParseError LocationID
     parseLeft =
       Bifunctor.first
         (ParseLocationError left)
         (readEither left :: Either String LocationID)
 
-    parseRight :: Either Error LocationID
+    parseRight :: Either ParseError LocationID
     parseRight =
       Bifunctor.first
         (ParseLocationError right)
@@ -75,3 +84,9 @@ parseWords [left, right] = do
 -- If the line does not contain exactly two words,
 -- it is considered an error
 parseWords xs' = Left $ ParseLineError (unwords xs')
+
+tryReadFile :: String -> IO (Either Error String)
+tryReadFile filepath = Bifunctor.first (const $ ReadFileError filepath) <$> io
+  where
+    io :: IO (Either IOError String)
+    io = try (readFile filepath)