diff --git a/Data/IVar.hs b/Data/IVar.hs
--- a/Data/IVar.hs
+++ b/Data/IVar.hs
@@ -48,6 +48,7 @@
 import Data.Monoid
 import Control.Monad
 import Data.Unique
+import Control.Exception
 import qualified Data.Map as Map
 
 data State a
@@ -64,7 +65,7 @@
 -- | Write a value to an IVar.  If the IVar already has a value, 
 -- throws an error \"Attempt to write to an IVar twice\".
 write :: IVar a -> a -> IO ()
-write (IVar ref) x = do
+write (IVar ref) x = block $ do
     b <- atomicModifyIORef ref $ \v ->
         case v of
             Value a -> (v, Nothing)
@@ -83,14 +84,17 @@
         Value x -> Left x
         NoValue _ -> Right [LogEntry var (\x -> Reader (return (Left x)))]
 
+-- Actions will be called in a blocked state; they must unblock themselves.
+-- This is because it is impossible to achieve the exception safety of
+-- and action t'other way round.
+
 addAction :: IVar a -> (a -> IO ()) -> IO Unique
 addAction (IVar ref) action = do
     actionid <- newUnique
-    boink <- atomicModifyIORef ref $ \v -> 
+    block . join . atomicModifyIORef ref $ \v -> 
         case v of
             Value x -> (v, action x)
             NoValue blockers -> (NoValue (Map.insert actionid action blockers), return ())
-    boink
     return actionid
 
 deleteAction :: IVar a -> Unique -> IO ()
@@ -102,8 +106,8 @@
                 -- This is strict so that we *actually* clean up
                 -- after ourselves; otherwise ivars that get waited
                 -- on but never filled could cause memory leaks
-                let m = Map.delete actionid blockers in
-                    m `seq` (NoValue m, ())
+                let m = Map.delete actionid blockers
+                in m `seq` (NoValue m, ())
 
 -- Reader is a free monad over (lists of) this functor; i.e. 
 -- it either returns a value or says "here is the variable 
@@ -181,7 +185,7 @@
 primBlocking :: [LogEntry (Reader a)] -> IO a
 primBlocking log = do
     blocker <- newEmptyMVar
-    cleanup <- forM log $ \(LogEntry var action) -> do
+    cleanup <- block . forM log $ \(LogEntry var action) -> do
         ident <- addAction var (\v -> tryPutMVar blocker (action v) >> return ())
         return $ deleteAction var ident
     blocking =<< takeMVar blocker
diff --git a/Setup.hs b/Setup.hs
deleted file mode 100644
--- a/Setup.hs
+++ /dev/null
@@ -1,2 +0,0 @@
-import Distribution.Simple
-main = defaultMain
diff --git a/Setup.lhs b/Setup.lhs
new file mode 100644
--- /dev/null
+++ b/Setup.lhs
@@ -0,0 +1,4 @@
+#! /usr/bin/env runhaskell
+
+> import Distribution.Simple
+> main = defaultMain
diff --git a/data-ivar.cabal b/data-ivar.cabal
--- a/data-ivar.cabal
+++ b/data-ivar.cabal
@@ -3,7 +3,7 @@
     Write-once variables, with the ability to block
     on the first of a set of variables to become 
     available.
-Version: 0.20
+Version: 0.30
 Stability: experimental
 Synopsis: Write-once variables with concurrency support
 License: BSD3