Improve code

app/VirtualMachine/Interpreter.hs

@@ -13,14 +13,13 @@ import qualified Data.ByteString as B
 
 import VirtualMachine.VM (VM(..), Op, Computation, get, pop, pushS, forward, getPc, isHalted, isDebug)
 import VirtualMachine.Instruction (Instruction(..), Unit(..), instructionByOp)
+import Util (maybeToEither)
 
 
 parseInstr :: [Word8] -> Either String (Instruction, [Word8])
 parseInstr (opCode:rest) = do
   let op = toEnum . fromIntegral $ opCode :: Op
-  instr <- case M.lookup op instructionByOp of
-    (Just i) -> Right i
-    Nothing -> Left "Unknown instruction"
+  instr <- maybeToEither (M.lookup  op instructionByOp) "Unknown instruction"
   let noParams = _noParams instr
   let params = map fromIntegral $ take noParams rest :: [Word8]
   unless (length params == noParams) (Left $ "Expected " ++ show noParams ++ " parameter(s), got " ++ show (length params) ++ " for operator '" ++ show op ++ "'")

app/VirtualMachine/VM.hs

@@ -10,6 +10,7 @@ import qualified Data.Sequence as S
 import qualified Control.Monad.State as ST (get, put)
 import Data.Functor ((<&>))
 import Control.Monad (unless)
+import Util (maybeToExcept)
 
 
 data VM = VM { _pc    :: Int
@@ -85,9 +86,7 @@ isDebug :: Computation Bool
 isDebug = get <&> _debug
 
 stackAt :: Int -> String -> Computation Int
-stackAt index err = get >>= \vm -> case _stack vm S.!? index of
-  (Just i) -> return i
-  Nothing  -> throwError err
+stackAt index err = get >>= \vm -> maybeToExcept (_stack vm S.!? index) err
 
 frameAt :: Int -> (Int -> Int) -> String -> Computation Int
 frameAt index t name = do
@@ -95,9 +94,7 @@ frameAt index t name = do
   fp <- getFp
   unless (fp > -1) (throwError "No active stack frame")
   stackSize <- getStackSize
-  case _stack vm S.!? (stackSize - fp - 1 - t index) of
-    (Just i) -> return i
-    Nothing  -> throwError $ "Cannot determine " ++ name ++ " - index " ++ show index ++ " out of frame bounds"
+  maybeToExcept (_stack vm S.!? (stackSize - fp - 1 - t index)) $ "Cannot determine " ++ name ++ " - index " ++ show index ++ " out of frame bounds"
 
 updateFrameAt :: Int -> Int -> Computation ()
 updateFrameAt index value = do