Refactor comments

- Refactor comments to remove redundancy.
- Add explicit case statements for handling branches for ThrowSTM

Author: yogeshsajanikar

io-sim/src/Control/Monad/IOSim/Internal.hs

@@ -935,13 +935,16 @@ execAtomically !time !tid !tlbl !nextVid0 action0 k0 =
           BranchFrame (CatchStmA h) k writtenOuter writtenOuterSeq createdOuterSeq ctl' ->
             {-# SCC "execAtomically.go.BranchFrame" #-} do
             -- Execute the left side in a new frame with an empty written set.
-            -- Rollback `TVar`s written since catch handler was installed,
             -- but preserve ones that were set prior to it, as specified in the
             -- [stm](https://hackage.haskell.org/package/stm/docs/Control-Monad-STM.html#v:catchSTM) package.
             let ctl'' = BranchFrame NoOpStmA k writtenOuter writtenOuterSeq createdOuterSeq ctl'
             go ctl'' read Map.empty [] [] nextVid (h e)
-            --
-          BranchFrame _ _k writtenOuter writtenOuterSeq createdOuterSeq ctl' ->
+
+          BranchFrame (OrElseStmA _r) _k writtenOuter writtenOuterSeq createdOuterSeq ctl' ->
+            {-# SCC "execAtomically.go.BranchFrame" #-} do
+            go ctl' read writtenOuter writtenOuterSeq createdOuterSeq nextVid (ThrowStm e)
+
+          BranchFrame NoOpStmA _k writtenOuter writtenOuterSeq createdOuterSeq ctl' ->
             {-# SCC "execAtomically.go.BranchFrame" #-} do
             go ctl' read writtenOuter writtenOuterSeq createdOuterSeq nextVid (ThrowStm e)
 

io-sim/src/Control/Monad/IOSim/Types.hs

@@ -346,10 +346,9 @@ instance MonadCatch (STM s) where
         Nothing -> throwIO e  -- Rethrow the exception if handler does not handle it.
         Just e' -> h e'
 
-  -- STM actions are always run inside `execAtomically` and behave as if masked
-  -- Another point to note that the default implementation of `generalBracket` needs
-  -- mask, and is part of `MonadThrow`. For STM, we don't need masking because
-  -- async exceptions are handled outside of `execAtomically`.
+  -- Masking is not required as STM actions are always run inside
+  -- `execAtomically` and behave as if masked. Also note that the default
+  -- implementation of `generalBracket` needs mask, and is part of `MonadThrow`.
   generalBracket acquire release use = do
     resource <- acquire
     b <- use resource `catch` \e -> do

io-sim/src/Control/Monad/IOSimPOR/Internal.hs

@@ -1181,13 +1181,16 @@ execAtomically time tid tlbl nextVid0 action0 k0 =
           BranchFrame (CatchStmA h) k writtenOuter writtenOuterSeq createdOuterSeq ctl' ->
             {-# SCC "execAtomically.go.BranchFrame" #-} do
             -- Execute the left side in a new frame with an empty written set.
-            -- Rollback `TVar`s written since catch handler was installed,
             -- but preserve ones that were set prior to it, as specified in the
             -- [stm](https://hackage.haskell.org/package/stm/docs/Control-Monad-STM.html#v:catchSTM) package.
             let ctl'' = BranchFrame NoOpStmA k writtenOuter writtenOuterSeq createdOuterSeq ctl'
             go ctl'' read Map.empty [] [] nextVid (h e)
 
-          BranchFrame _ _k writtenOuter writtenOuterSeq createdOuterSeq ctl' ->
+          BranchFrame (OrElseStmA _r) _k writtenOuter writtenOuterSeq createdOuterSeq ctl' ->
+            {-# SCC "execAtomically.go.BranchFrame" #-} do
+            go ctl' read writtenOuter writtenOuterSeq createdOuterSeq nextVid (ThrowStm e)
+
+          BranchFrame NoOpStmA _k writtenOuter writtenOuterSeq createdOuterSeq ctl' ->
             {-# SCC "execAtomically.go.BranchFrame" #-} do
             go ctl' read writtenOuter writtenOuterSeq createdOuterSeq nextVid (ThrowStm e)