awful lot of error handling

app/Main.hs

@@ -4,10 +4,18 @@ import Lox.Interpreter
 import System.IO
 
 run :: String -> IO ()
-run source = print result
-    where result = eval expr
-          expr = parse tokens
-          tokens = scanTokensFromSource source
+run source = do 
+    let tokensMaybe = scanTokensFromSource source
+    case tokensMaybe of
+        Left UnexpectedCharacterError -> putStrLn "Unexpected character"
+        Right tokens -> do
+            let exprMaybe = parse tokens
+            case exprMaybe of
+                Left ExpectedExpressionError -> putStrLn "Expected expression"
+                Left MismatchedParenthesesError -> putStrLn "Mismatched parentheses"
+                Right expr -> do
+                    result <- eval expr
+                    print result
 
 main :: IO ()
 main = putStr ">> " >> hFlush stdout >> getLine >>= run