Fix a bug and allow runtime errors without dying

1 files changed, 10 insertions, 5 deletions

diff --git a/calculator.hs b/calculator.hs
index a59f0fb..767845d 100644
--- a/calculator.hs
+++ b/calculator.hs
@@ -4,6 +4,7 @@ import Text.Parsec.Token
 import Text.Parsec.Language
 import Text.Parsec.Expr
 import Control.Monad.State
+import Control.Monad.Error
 import qualified Data.Map as M
 
 data Expression = Constant Double
@@ -94,7 +95,8 @@ parseInput = do
 
 type StoredVal = Either Double FunctionBody
 
-type Calculator a = StateT (M.Map String StoredVal) IO a
+type CannotFailCalculator = StateT (M.Map String StoredVal) IO
+type Calculator a = ErrorT String CannotFailCalculator a
 
 interpretExpression :: Expression -> Calculator Double
 interpretExpression (Constant n) = return n
@@ -140,7 +142,7 @@ interpretExpression (FunctionInvocation fn e) = do
       n <- interpretExpression e
       modify (M.insert argname (Left n))
       r <- interpretExpression expr
-      modify (M.delete argname)
+      put ctx
       return r
 
 interpretStatement :: Statement -> Calculator ()
@@ -160,15 +162,18 @@ defaultVars = M.fromList
               , ("phi", Left ((1 + (sqrt 5)) / 2))
               ]
 
-calculate :: String -> Calculator ()
+calculate :: String -> CannotFailCalculator ()
 calculate s =
   case ret of
     Left e -> liftIO $ putStrLn $ "error: " ++ (show e)
-    Right n -> interpretStatement n
+    Right n -> do res <- runErrorT $ interpretStatement n
+                  case res of
+                    Left e' -> liftIO $ putStrLn $ "run error: " ++ e'
+                    Right _ -> return ()
   where
     ret = parse parseInput "" s
 
-calculator :: Calculator ()
+calculator :: CannotFailCalculator ()
 calculator =
   liftIO getContents >>= (mapM_ calculate) . lines