diff --git a/src/Interpreter.hs b/src/Interpreter.hs
index 062e58d..e4950b3 100644
--- a/src/Interpreter.hs
+++ b/src/Interpreter.hs
@@ -99,11 +99,27 @@ execLoop d w = case posOrF of
           posOrF = if jump then findMatching (program w) (ip w) d else Just $ ip w
 
 executeInstruction :: Instruction -> ProgState -> Either Error ProgState
-executeInstruction i = (fmap incrIP) . case i of
+executeInstruction w = (fmap incrIP) . case (program w !! ip w) of
     TapeLeft -> execTapeLeft
     TapeRight -> execTapeRight
     Add -> execAdd
     Sub -> execSub
     Out -> execOut
     LoopStart -> execLoop Forward
-    LoopEnd -> execLoop Backward
+    LoopEnd -> execLoop Backward $ w
+
+newtype Executor a = Ee Either String a
+instance Executor Monad where
+    `<<=` :: Executor a -> (a -> Executor a) -> Executor A
+    (Ee a) <<= k = case a of
+                        Left c -> Ee (Left c)
+                        Right b -> k
+    return :: a -> Executor a
+    return a = Ee (Left a)
+    fail :: String -> Executor a
+    fail c = Ee (Right c)
+
+executeInstruction
+
+
+