make day10 more maybe-oriented

- use elem instad of a massive chain of ||
- add explicit error cases

2021/day10.hs

@@ -2,18 +2,21 @@ module Day10 where
 
 import Data.Maybe
 import Data.List (sort)
-closing :: Char -> Char
+import Control.Monad (foldM)
-closing '(' = ')'
-closing '[' = ']'
-closing '{' = '}'
-closing '<' = '>'
 
-score :: Char -> Int
+closing :: Char -> Maybe Char
-score ' ' = 0
+closing '(' = Just ')'
-score ')' = 3
+closing '[' = Just ']'
-score ']' = 57
+closing '{' = Just '}'
-score '}' = 1197
+closing '<' = Just '>'
-score '>' = 25137
+closing _ = Nothing
+
+score :: Char -> Maybe Int
+score ')' = Just 3
+score ']' = Just 57
+score '}' = Just 1197
+score '>' = Just 25137
+score _ = Nothing
 
 closingScore :: Char -> Int
 closingScore ')' = 1
@@ -28,14 +31,14 @@ calcScore xs = foldr (\char score -> score*5 + closingScore char) 0 (reverse xs)
 getInvalidChar :: [Char] -> String -> Char
 getInvalidChar _ [] = ' '
 getInvalidChar st (x:xs)
-    | x == '(' || x == '[' || x == '{' || x == '<' = getInvalidChar (x:st) xs
+    | x `elem` "([{<" = getInvalidChar (x:st) xs
-    | otherwise = if Just x == fmap closing (listToMaybe st) then getInvalidChar (tail st) xs else x
+    | otherwise = if Just x == (listToMaybe st >>= closing) then getInvalidChar (tail st) xs else x
 
 getCompletionString :: [Char] -> String -> String
-getCompletionString st [] = map closing st
+getCompletionString st [] = map (fromJust . closing) st
 getCompletionString st (x:xs)
-    | x == '(' || x == '[' || x == '{' || x == '<' = getCompletionString (x:st) xs
+    | x `elem` "([{<" = getCompletionString (x:st) xs
-    | otherwise = if Just x == fmap closing (listToMaybe st) then getCompletionString (tail st) xs else ""
+    | otherwise = if Just x == (listToMaybe st >>= closing) then getCompletionString (tail st) xs else ""
 
 
 main :: IO ()
@@ -43,7 +46,7 @@ main = do
     input <- lines <$> getContents
 
     putStr "part 1: "
-    let chars = map (score . getInvalidChar []) input
+    let chars = mapMaybe (score . getInvalidChar []) input
     print $ sum chars
 
     putStr "part 2: "