fixed calculator

arithmetic/Main.hs

@@ -46,23 +46,23 @@ opPriority (Number _) = 3
 transform:: [Maybe Token] -> [Token]
 -- tokens - stack - output
 transform ts = transform' ts [] [] where
-    transform' [] [] q = q
-    transform' [] s q =
-        if head s == ParenOpen 
+    transform' [] [] out = out
+    transform' [] st out =
+        if head st == ParenOpen 
             then error "Mismatched parentheses" 
-            else transform' [] (tail s) (q ++ [head s])
-    transform' (x:xs) s q = case x of
+            else transform' [] (tail st) (out ++ [head st])
+    transform' (t:ts) st out = case t of
         Nothing -> error "Illegal tokens"
-        (Just (Number n)) -> transform' xs s (q ++ [Number n])
-        (Just ParenOpen) -> transform' xs (ParenOpen:s) q
-        (Just ParenClose) -> transform' xs s0 q0 where
-            s0 = tail $ dropWhile (/= ParenOpen) s
-            q0 = q ++ takeWhile (/= ParenOpen) s
-        (Just o1) -> transform' xs s1 q1 where
-            cond o2 = isOp o2 && (opPriority o1 < opPriority o2)
-            spl = span cond s
-            s1 = o1 : snd spl
-            q1 = q ++ fst spl
+        Just (Number n) -> transform' ts st (out ++ [Number n])
+        Just ParenOpen -> transform' ts (ParenOpen:st) out
+        Just ParenClose -> transform' ts st0 out0 where
+            st0 = tail $ dropWhile (/= ParenOpen) st
+            out0 = out ++ takeWhile (/= ParenOpen) st
+        Just o1 -> transform' ts st1 out1 where
+            cond o2 = isOp o2 && (opPriority o1 <= opPriority o2)
+            spl = span cond st
+            st1 = o1 : snd spl
+            out1 = out ++ fst spl
 
 eval :: [Token] -> Token
 eval ts = head $ eval' [] ts where
@@ -74,8 +74,8 @@ eval ts = head $ eval' [] ts where
         rem = tail . tail
         in case t of
             Number n -> eval' (t : st) ts
-            AddOp -> eval' ((Number (fstNum st + sndNum st)) : (rem st)) ts
-            MulOp -> eval' ((Number (fstNum st * sndNum st)) : (rem st)) ts
+            AddOp -> eval' ((Number (sndNum st + fstNum st)) : (rem st)) ts
+            MulOp -> eval' ((Number (sndNum st * fstNum st)) : (rem st)) ts
             DivOp -> eval' ((Number (sndNum st / fstNum st)) : (rem st)) ts
             SubOp -> eval' ((Number (sndNum st - fstNum st)) : (rem st)) ts
     eval' st [] = st