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a5/apl.hs

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+module Main (main) where
+
+import APL.Eval (Val (..), eval, runEval)
+import APL.Parser (parseAPL)
+import System.Environment
+  ( getArgs,
+    getProgName,
+  )
+import System.Exit (ExitCode (..), exitWith)
+import System.IO (hPutStrLn, stderr, stdout)
+
+stringVal :: Val -> String
+stringVal (ValBool b) = show b
+stringVal (ValInt x) = show x
+stringVal ValFun {} = "#<fun>"
+
+main :: IO ()
+main = do
+  args <- getArgs
+  case args of
+    [fname] -> do
+      s <- readFile fname
+      case parseAPL fname s of
+        Left err -> hPutStrLn stderr err
+        Right e -> case runEval (eval e) of
+          Left err -> hPutStrLn stderr $ show err
+          Right v -> hPutStrLn stdout $ stringVal v
+    _ -> do
+      prog <- getProgName
+      failure $ "Usage: " ++ prog ++ " FILE"
+  pure ()
+  where
+    failure e = do
+      hPutStrLn stderr $ show e
+      exitWith $ ExitFailure 1