[project @ 2005-12-13 15:57:49 by simonmar]

Raise the (new) exception NestedAtomically when atomically is nested
(using unsafePerformIO).  This is a small improvement over crashing.

diff --git a/GHC/IOBase.lhs b/GHC/IOBase.lhs
index f0ce8de..112268e 100644 (file)
--- a/GHC/IOBase.lhs
+++ b/GHC/IOBase.lhs
@@ -602,6 +602,10 @@ data Exception
        -- ^The current thread was waiting to retry an atomic memory transaction
        -- that could never become possible to complete because there are no other
        -- threads referring to any of teh TVars involved.
+  | NestedAtomically
+       -- ^The runtime detected an attempt to nest one STM transaction
+       -- inside another one, presumably due to the use of 
+       -- 'unsafePeformIO' with 'atomically'.
   | Deadlock
        -- ^There are no runnable threads, so the program is
        -- deadlocked.  The 'Deadlock' exception is
@@ -740,6 +744,7 @@ instance Show Exception where
   showsPrec _ (AsyncException e)        = shows e
   showsPrec _ (BlockedOnDeadMVar)       = showString "thread blocked indefinitely"
   showsPrec _ (BlockedIndefinitely)     = showString "thread blocked indefinitely"
+  showsPrec _ (NestedAtomically)        = showString "Control.Concurrent.STM.atomically was nested"
   showsPrec _ (NonTermination)           = showString "<<loop>>"
   showsPrec _ (Deadlock)                 = showString "<<deadlock>>"
 
@@ -759,6 +764,7 @@ instance Eq Exception where
   AsyncException e1   == AsyncException e2   = e1 == e2
   BlockedOnDeadMVar   == BlockedOnDeadMVar   = True
   NonTermination      == NonTermination      = True
+  NestedAtomically    == NestedAtomically    = True
   Deadlock            == Deadlock            = True
   _                   == _                   = False