Haskell: add error handling and try*/catch*.

Achieve self-hosting!

1 files changed, 26 insertions, 20 deletions

diff --git a/haskell/step3_env.hs b/haskell/step3_env.hs
index 6f65afd..3bd3b19 100644
--- a/haskell/step3_env.hs
+++ b/haskell/step3_env.hs
@@ -1,5 +1,7 @@
-import Control.Monad (when, mapM)
-import Control.Monad.Error (throwError)
+import System.IO (hFlush, stdout)
+import Control.Monad (mapM)
+import Control.Monad.Error (runErrorT)
+import Control.Monad.Trans (liftIO)
 import qualified Data.Map as Map
 import qualified Data.Traversable as DT
 
@@ -10,11 +12,11 @@ import Printer (_pr_str)
 import Env (Env, env_new, env_get, env_set)
 
 -- read
-mal_read :: String -> IO MalVal
+mal_read :: String -> IOThrows MalVal
 mal_read str = read_str str
 
 -- eval
-eval_ast :: MalVal -> Env -> IO MalVal
+eval_ast :: MalVal -> Env -> IOThrows MalVal
 eval_ast sym@(MalSymbol _) env = env_get env sym
 eval_ast ast@(MalList lst m) env = do
     new_lst <- mapM (\x -> (eval x env)) lst
@@ -27,37 +29,37 @@ eval_ast ast@(MalHashMap lst m) env = do
     return $ MalHashMap new_hm m
 eval_ast ast env = return ast
 
-let_bind :: Env -> [MalVal] -> IO Env
+let_bind :: Env -> [MalVal] -> IOThrows Env
 let_bind env [] = return env
 let_bind env (b:e:xs) = do
     evaled <- eval e env
-    x <- env_set env b evaled
+    x <- liftIO $ env_set env b evaled
     let_bind env xs
 
-apply_ast :: MalVal -> Env -> IO MalVal
+apply_ast :: MalVal -> Env -> IOThrows MalVal
 apply_ast ast@(MalList (MalSymbol "def!" : args) _) env = do
     case args of
          (a1@(MalSymbol _): a2 : []) -> do
             evaled <- eval a2 env
-            env_set env a1 evaled
-         _ -> error $ "invalid def!"
+            liftIO $ env_set env a1 evaled
+         _ -> throwStr "invalid def!"
 apply_ast ast@(MalList (MalSymbol "let*" : args) _) env = do
     case args of
          (a1 : a2 : []) -> do
             params <- (_to_list a1)
-            let_env <- env_new $ Just env
+            let_env <- liftIO $ env_new $ Just env
             let_bind let_env params
             eval a2 let_env
-         _ -> error $ "invalid let*"
+         _ -> throwStr "invalid let*"
 apply_ast ast@(MalList _ _) env = do
     el <- eval_ast ast env
     case el of
          (MalList ((Func (Fn f) _) : rest) _) ->
             f $ rest
          el ->
-            error $ "invalid apply: " ++ (show el)
+            throwStr $ "invalid apply: " ++ (show el)
 
-eval :: MalVal -> Env -> IO MalVal
+eval :: MalVal -> Env -> IOThrows MalVal
 eval ast env = do
     case ast of
          (MalList _ _) -> apply_ast ast env
@@ -70,15 +72,15 @@ mal_print exp = show exp
 
 -- repl
 add [MalNumber a, MalNumber b] = return $ MalNumber $ a + b
-add _ = error $ "illegal arguments to +"
+add _ = throwStr $ "illegal arguments to +"
 sub [MalNumber a, MalNumber b] = return $ MalNumber $ a - b
-sub _ = error $ "illegal arguments to -"
+sub _ = throwStr $ "illegal arguments to -"
 mult [MalNumber a, MalNumber b] = return $ MalNumber $ a * b
-mult _ = error $ "illegal arguments to *"
+mult _ = throwStr $ "illegal arguments to *"
 divd [MalNumber a, MalNumber b] = return $ MalNumber $ a `div` b
-divd _ = error $ "illegal arguments to /"
+divd _ = throwStr $ "illegal arguments to /"
 
-rep :: Env -> String -> IO String
+rep :: Env -> String -> IOThrows String
 rep env line = do
     ast <- mal_read line
     exp <- eval ast env
@@ -91,9 +93,13 @@ repl_loop env = do
         Nothing -> return ()
         Just "" -> repl_loop env
         Just str -> do
-            out <- catchAny (rep env str) $ \e -> do
-                return $ "Error: " ++ (show e)
+            res <- runErrorT $ rep env str
+            out <- case res of
+                Left (StringError str) -> return $ "Error: " ++ str
+                Left (MalValError mv) -> return $ "Error: " ++ (show mv)
+                Right val -> return val
             putStrLn out
+            hFlush stdout
             repl_loop env
 
 main = do