make day10 more maybe-oriented

2021/day10.hs

@@ -4,12 +4,12 @@ import Data.Maybe
 import Data.List (sort)
 import Control.Monad (foldM)
 
-closing :: Char -> Char
-closing '(' = ')'
-closing '[' = ']'
-closing '{' = '}'
-closing '<' = '>'
-closing _ = error "Not a bracket"
+closing :: Char -> Maybe Char
+closing '(' = Just ')'
+closing '[' = Just ']'
+closing '{' = Just '}'
+closing '<' = Just '>'
+closing _ = Nothing
 
 score :: Char -> Maybe Int
 score ')' = Just 3
@@ -32,13 +32,13 @@ getInvalidChar :: [Char] -> String -> Char
 getInvalidChar _ [] = ' '
 getInvalidChar st (x: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 `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 ()